D 20418 F

s e l e c t e d

a i i i m M X i i M B O f ffor Malmo and six other sites

The ATCR-2T is an all weather high power, L band, surveillance radar with a revolutionary digital Mil - video integrator. It is normally supplied with 2 channels whichcan operate simultaneously in frequency diversity. A dual beam arrangement reduces'angels' phenomena and enhances MTI performance without degrading low coverage.

S O U T H A F R I C AH A S O R D E R E DT H R E EA T C R - 2 T R A D A R S

- i

SELENIA INDUSTRIE ELETTRONICHE ASSOCIATE SpA RADAR DIVISION ROME ITALY

Corporation Membersof the Internat ional Federat ionof Air Traffic Controllers' Associations

AEG-Telefunken, Ulm/Donau, GermanyThe Air Transport Association,Washington D. C., U.S.A.Wolfgang Assmann GmbH., Bad Homburg v.d.H.Compagnie Generale de Telegraphie sans FilMaiakoff, Paris, FranceCossor Radar and Electronics Limited,Harlow, EnglandThe Decca Navigator Company Limited, LondonELLIOTT Brothers (London) LimitedBorehamwood, Herts., EnglandFERRANTI L im i t edBracknell, Berks., EnglandGlen A. Gilbert & Associates,Washington D. C., U.S.A.IBM World Trade Europe Corporation,Paris, FranceInternational Aeradio Limited,Southall, Middlesex, EnglandITT Europe Corporation, Brussels, Belgium

Jeppesen & Co. GmbH, Frankfurt, GermanyThe Marconi Company Limited Radar DivisionChelmsford, Essex, EnglandN.V. Hollandse SignaalapparatenHengelo, NetherlandsN.V. Philips Telecommunicatie IndustrieHilversum, HollandThe Plessey Company LimitedChessington, Surrey, EnglandSelenia - Industrie Elettroniche Associate S.p.A.Rome, ItalyThe Solartron Electronic Group, Ltd.Farnborough, Hants., EnglandTexas Instruments Inc., Dallas 22, Texas, USAWhittaker Corporation,North Hollywood, California, USA

The International Federation of Air Traffic Controllers' Associations would like to invite all corporations, organizations, and institutions interested in and concerned with the maintenance and promotion of safety in air traffic to join their organization as Corporation Members.

Corporation Members support the aims of the Federation by supplying the Federation with technicalinformation and by means of an annual subscription. The Federation's international journal "The Controller" is offered as a platform for the discussion of technical and procedural developments in thefie ld o f a i r t r a f fic con t ro l .

Marconi air traffic contro

so cm radarsS650 and S670

Tw o a d v a n c e d 5 0 c m B a n d r a d a r sto sat is fy a l l requi rementsf o r T e r m i n a l A r e a a n dLong Range survei l lance

Long wave lengthf o r i n h e r e n t w e a t h e rclutter protection

C r y s t a l - c o n t r o l l e dd r i v e n t r a n s m i t t e r

for MTI stabilityK l y s t r o n o u t p u tw i t h t . w . t . d r i v e

f o r f a i l - s o f tpe r fo rmance

C o m p r e h e n s i v es i g n a l p r o c e s s i n g

A l l s o l i d - s t a t efor outstanding reliability

S65G gives full TMA cover—130 miles range on a 3-squaremetre target with 6 second datar e n e w a l r a t e

S670 gives Long Range cover—160 miles range on a 3-squaremetre target using larger aerial

for improved auto-extraction

M A R C O N I R A D A R S Y S T E M S L I M I T E DA GEC-Marconi Electronics Company

Crompton Works, Chelmsford, Essex, EnglandTelephone : Chelmsford 67111. Telex; 99108

T W O M O R E A I R T R A F F I C C O N T R O L R A D A R S F R O M T H E S 6 0 0 S E R I E S L T D / S 7 4

Marconi air traffic centre

Chcillenaer

A n e w a d v a n c e d m o d u l a r

secondary surve i l lanceradar to satisfy the totalSSR requirement provid ingc o m p r e h e n s i v e 3 D a e r o s p a c econtrol for the supersonic age.Maximum system flexibility.System expansion without

redundancy.Inherent reliability, stability

and simplified maintenance.Operates with all ICAO

specified transponders.

S u m a n d d i f f e r e n c e a e r i a l w i t h

improved sidelobe, suppressionper fo rmance .

Integral control and interrogatea e r i a l s .

A m p l e p o w e r — 5 k w —fora l l app l ica t ions.

Can operate all on civil andmilitary modes.

Interlacing in any combinationo f m o d e s .

A c t i v e a n d P a s s i v e f u n c t i o n

d e c o d i n g .

Active and Passive Heightdecoding.

Automatic decoding facilities.Digi tal defrui t ing.Processes both 64 and 4096

i n f o r m a t i o n c o d e s .Degarbling and code

c o r r e l a t i o n f a c i l i t i e s .R e m o t e c o n t r o l f a c i l i t i e s .A l l s o l i d - s t a t e

Bu i l t - in au tomon i to r ing .Easily integrated with all

s u r v e i l l a n c e r a d a r s .

M A R C O N I R A D A R S Y S T E M S L I M I T E D

A GEC-Marconi Electronics Company

R a d a r D i v i s i o n

Crompton WorksChelmsford, Essex, EnglandTelephone : Chelmsford 67111Te l e x : 9 9 1 0 8

Airspace Control DivisionElstree Way, BorehamwoodHerts, EnglandTelephone : 01 -953 2030T e l e x : 2 2 7 7 7 l t d / s 6 8

P r e c i s i o n A p p r o a c h R a d a r

P

Precision approach radar PAR-T4 at Frankfurt/M a i n

I t I s o u r b e l i e f t h a t s a f e t ym e a s u r e s i n a i r t r a f fi c c a n n e v e rbe exaggerated. Recent statistics show that the majority of airt ra ffic acc iden ts happen dur ingthe landing approach phase. Thisis why we have always held and

are still holding that a completeG C A ( G r o u n d C o n t r o l l e d A pproach) System is of advantageto any airport. And a completeG C A S y s t e m i n c l u d e s a PA R( P r e c i s i o n A p p r o a c h R a d a r ) .T E L E F U N K E N P A R ' S a r e k n o w n

and held in high esteem as gen u i n e p r e c i s i o n e q u i p m e n t a tn u m e r o u s a i r p o r t s i n m a n yc o u n t r i e s .A L L G E M E I N E E L E K T R I C I T A T S - G E S E U L S C H A F TA E G - T E L E F U N K E NExport Department7 9 U l m ■ E l i s a b e t h e n s t r a R e 3

Germany

N 1 9 4 1 . 8 2 7

Air traffic controlsystemsf r o m A E G - T E L E F U N K E f J

I F A T C A J O U R N A L O F A I R T R A F F I C C O N T R O L

T H E C O N T R O L L E RFrankfurt am Main, January/March 1970 Volume 9 • No. 1

P u b l i s h e r : I n t e r n a t i o n a l F e d e r a t i o n o f A i r T r a f fi c C o ntrollers' Associations, S. C. 11; 6 Frankfurt am MainN.O. 14, Bornheimer Landwehr 57a.

Officers of IFATCA: M. Cerf, President; J. R. Campbell,First Vice President; G. Atterholm, Second Vice President; G. W. Monk, Executive Secretary; H. Guddat,Honorary Secretary; B. Ruthy, Treasurer; W. H, End-lich. Editor.

Editor: Walter H. Endlich,3, rue Roosendael,Bruxelles-Forest, BelgiqueTelephone: 456248

Publishing Company, Production and Advert ising SalesOffice: Verlag W. Kramer & Co., 6 Frankfurt am MainN014, Bornheimer Landwehr 57a, Phone 434325,492169,Frankfurter Bank, No. 3-03333-9. Rate Card Nr. 2.

Printed by: W.Kromer&Co., 6 Frankfurt am Main NO 14,B o r n h e i m e r L a n d w e h r 5 7 a .

Subscription Rate: DM 8,— per annum (in Germany).

Contributors are expressing their personal points of viewand opinions, which must not necessarily coincide witht h o s e o f t h e I n t e r n a t i o n a l F e d e r a t i o n o f A i r T r a f fi c

Control lers' Associations (IFATCA).

IFATCA does not assume responsibility for statementsmade and opinions expressed, it does only accept responsibil i ty for publishing these contributions.

Contributions ore welcome as are comments and crit icism. No payment con be made for manuscripts submittedfor publication in "The Controller". The Editor reservesthe right to make any editorial changes in manuscripts,which he believes will improve the material withoutaltering the intended meaning.

Written permission by the Editor is necessary for reprinting any part of this Journal.

Advertisers in this Issue: Decca/HARCO (Back cover);ELLIOTT Space and Weapon Automation Limited (Insideback cover); IFATCA 70 (14, 15); Ferranti Limited (13);Marconi Ltd. (2, 3); Standard Elektrik Lorenz (30); Se-lenia S.p.A. (Inside Cover).

Picture Credit: NASA (24, 25, 26); Plessey Radar Ltd. (16);Ratcliffe (20); Vickers (11, 12, 23, 25, 27).

C O N T E N T S

Automat ion in A i r Tra ffic Cont ro lS. Ratcliffe, B. Sc.

Passive Horn May Clean Up Radar DisplaysTirey K. Vickers

IFATCA 70

Lasers to Determine Visibility at AirportsR. L. Burr and E. T. Hill

Mathemat ica l Models for the Pred ic t iono f A i r Tra ffic Con t ro l l e r Work load

S. Ratcliffe, B. Sc.

Book Review

The Improvement of Wet-Runway OperationsTirey K. Vickers

Terminal Area Approach Control SequencingE. Crewe and T. E. Foster

I F A T C A A d d r e s s e s

A u t o m a t i o n i n A i r T r a f fi c C o n t r o i

Paper to the United Kingdom Symposium "Electronics for Civil Aviation", 1969*

S. Ratcliffe, B. Sc.Royal Radar Establishment,Malvern, Worcestershi re

I n t r o d u c t i o n

It is easy for aviation and electronic engineers to lookon air traffic planning as a tradition-bound organisationthat will adapt itself to exploit technological advancesonly with reluctance and under extreme pressure fromgrowing traffic. Members of this school of thought pointout that most features of the present day air traffic systemare dictated by the weaknesses of various electronic aidswhich were developed without clear guidance from theATC authority as to the precise operational need that wasto be met, and which were often intended primarily forsome quite different application.

Basically, the difficulty stems from the standards ofreliability which are expected of civil aviation in generaland air traffic control in particular. These are very high,and it has been argued (1) that with the growth of trafficthey should become even higher. Electronics designers mustface the serious implications of this demand. It is easyproudly to point to some brave new device which avoidsthe well-known drawbacks of an established technique, butthe hard-bitten air traffic planner may be justified in hisrefusal to exchange the "devil that he knows" for the less-well-tested and therefore dangerous novelty.

Aviation and electronics techniques have grown uptogether and have persistently exercised a significant influence on each other. Despite everything said above, electronic developments tend to match the operational requirements of civil aviation. The willigness of the military toadopt new techniques and to use them on a large scale hasmade an invaluable contribution to the building-up of confidence In many innovations.

When we come to discuss the applications of computersto air traffic control, the situation is very different. Theexplosive growth of computer technology has produced asituation where adequate reliability statistics are onlyavailable for machines using obsolete techniques, andwhere air traffic control represents only a small corner ofthe potential market for digital devices, and cannot hopesignificantly to influence the broad lines of either hardwareor software development.

There are advantages as well as disadvantages in thissituation. The existence of a broad-based market for computers helps to keep down the cost of developing bothhardware and some of the more basic software. Systemweaknesses which lead to subtle and not readily detectedhazards in ATC may prove much more conspicuous in otherapplications of the same machinery.

Another striking difference between the digital computer and nearly all other aids to air traffic control is in

' Reprinted with kind permission of the author and the U.K. Ministryof Technology

the much greater flexibility of the device. The advent ofAutoland, for example, now faces the planners with onlyone major decision, whether to fit. If a decision is taken tointroduce a digital computer into an air traffic process,there are an immense range of roles that the device couldplay. In practice, the choice of the role for the computer isinextricably bound up with arguments about reliability. Itis proposed, therefore, to discuss this topic before proceeding further.

ReliabilityA computer is only worth installing if it is to serve a

useful purpose. The more useful the purpose, the greaterthe penalty when the machine fails. In most ATC applications, a significant hazard will then arise. If it is proposedto overcome this difficult by "reversion to manual", threecondit ions must be fulf i l led: —

(i) There must be available at all times an adequate number of control staff with the necessary training andadequate recent experience in the appropriate arts.

(ii) These staff must always be familiar with the currents i t u a t i o n .

(iii) The computer must not be allowed to use its powers togenerate a traffic situation which the human controllers cannot safety unscramble.

Since such a system contains the staff who are capableof dispensing with the computer, the simplest way ofensuring that they preserve their skills and stay in touchwith the traffic situation is not to install a computer at all.If this solution is not adopted, it Is necessary to call for acomputer system with a probability of failure so low thatone can tolerate a relatively hazardous situation duringthe down time of the system, or, at least, until the operatingrules can be changed to ease the load on the defectivesystem. The flight plan processor now being installed atLATCC West Drayton, for example, has to meet a specification that only once in five years allows a loss of servicefor more than 30 sees, or an undetected error in a singlec h a r a c t e r .

Such a stringent requirement poses serious problems —not only for the equipment designer. Consider, for example,the problem facing the authority conducting the acceptance tests on the equipment of the previous paragraph.Even if it were possible to build a single non-redundantcomputer having an adequate reliability, a successful acceptance test could hardly take less than ten years or so.In practice, the proposed system is not based on a singlecomputer, there are several operating channels, enoughcomparison mechanisms to detect a failure, and enoughredundancy to enable the system to survive one or more

6

equipment failure. This approach may be much quicker andless expensive than one that requires the development ofcomponents to reliability standards far higher than "goodcommercial practice". If the customer is prepared to acceptarguments based on the independence of failure mechanism, it is possible now to calculate the probability of asystem failure from the much higher, and therefore measurable, figures for components failures.

The failure-survival redundancy is commonly applied atcomputer level (2). Whereas in automatic landing, say, itis necessary to triplicate virtually the entire control system,advantage is often token of the flexibility of the stored-programme digital computer to enable a spare machineto sand-in for more than one main-line computer. It ispossible, to some extent, to re-apportion roles between thesurviving apparatus to ensure that if the number of faultsbuilds up to the point where operational efficiency mustsuffer, the blow will fall first on the least important functions of the system.

The weakness of this approach lies in the complexity ofthe data highways, the switching system, and the rest of thehardware and software involved in transferring data andprogrammes between computers. In these areas, amongstothers, there are possibilities of systematic failures whichcan invalidate the assumptions on which the probability ofa system failure were calculated.

An experimental "reliable" computer exists in Vv'hichthe redundancy is applied at a lower level. Error-correcting codes are used to check that words are stored andtransmitted correctly. Any failure should result in a fairlyprecise indication of the location of the fault, since boththe logical element and the faulty digit can be specified.Since there is no known way of performing arithmeticoperations on an error-correcting coded number whilst preserving the error-correcting property, this technique cannotbe used throughout the computer, and other techniques,such as triplication, are used in limited areas. This schemeoffers the possibility of building a reliable computer withless than twice the number of components that would berequired in its "unreliable" counterpart. As always, thedanger lies in the risk of several non-independant andnear-simultaneous failure, such as might be triggered by afault which caused a small fire.

A separate problem is the risk of a flaw in the programme logic. In a complex on-line system it is nearly impossible to test all combinations of circumstances, andseveral years is needed to establish confidence in the software. It is improbable that, in practice, such a period couldelapse before changing circum.stances made it necessaryto modify the programme, each modification bringing anew risk of error. In the present state of the art it may bethat this problem sets a bound to the confidence one canhave in an automated system.

Choice of area to be aufomafedIf computers and their associated programmes were

foolproof, possible reasons for introducing computers intoair traffic control might be: —(i) to save manpower by taking over some of the tasks

presently performed by human controllers or theirassistants,

(ii) to eliminate human errors,(iii) to perform tasks which are beyond the capability of a

team of human controllers working under time pressu r e .

Given the inevitable residual uncertainty about thelong-term reliability of such systems as are within the present state of the art, the case for introducing computers intothe system may be more subtle. Certainly there is noevidence to show that any saving in m.anpower has, so farresulted from the introduction of computers into ATC, andso long as decisions are taken by human controllers andcommunication with the aircraft is by voice, there is stillplenty of scope for controller errors, together with contributions from the machinery and the programmers.

There are, however, other grounds for introducing computers into ATC. These include the need to build-up operational experience of automated systems before rising trafficmakes them essential.

For one reason or another, many ATC authorities arenow using or experimenting with computer systems. Thetendency has been to begin by introducing a machine toprint flight progress strips. This is a relatively easy task,there is no obvious difficulty in reverting to manual operation, and input of flight plans to the computer is a necessarystep towards most more advanced proposals. One possibleline of further develepment is to delay print-out of flightprogress strips until a departure message is received, oreven until nearer the ETA at the appropriate reportingpoint, to give on indication of procedural conflict withother aircraft, or even to suggest modifications to resolvethe confliction (3).

Experience has taught that the superficial simplicity ofthis task is deceptive. Particularly troublesome mechanicalproblems ore involved in delivering a legible strip, loadedin its holder, silently and rapidly to the working position.

The requirement for delayed printing of the strip leadsto a demand for very high integrity in the flight planstorage system. In the Flight Plan Processing System atWest Drayton the specification lays down that a failureinvolving damage to a single character or a break in service lasting more than 30 seconds must not occur moreoften than once in 5 years. To meet this specification it isproposed to duplicate most features of the installation andto triplicate the central facilities such as the computersystem. There are also programming complexities whicharise from the need to detect faults, to reconfigure thefaulty system, and to retrieve the damage data.

At least one attempt has been made (4) to extend theautomation process to provide a travelling printing headwhich can update flight progress strips after they havebeen placed in position in front of the controller. To preserve the controllers' freedom to rearrange his strips, it isnecessary to code the strip or its holder to enable thecomputer to sense the location of a given strip on the display.

An alternative approach to the problem, is to replacethe printed paper strips by a multiplicity of electromechanical indicators. This lends itself more easily to numerousrevision of the flight plan data, but in practice, controllersfound it difficult to read the strips (5) or to input revisions,and rearrangement of flight progress strips is a rathercumbersomie process.

Both these schemes are open to the criticism that in ATCsystems which are, in practice if not in theory, radar based,it is not worth introducing this degree of complexity intomechanising the procedural data processing alone. TheApollo scheme (3) escapes this criticism because the U. K.authorities chose to begin their experiments in the Shanwickcentre controlling N. Atlantic traffic, not because this pro-

blem was most acute, but because the low data rate andthe absence of radar minimised the complications.

Systems based on printed strips or electromechanicalindicators have the m.erit that even a major electroniccatastrophe is unlikely to damage more than a very smallamount of displayed data. The advent of the touch-wiredisplay technique (6) and advances in CRT character-writing (9) have led to a weakening of the objections to the useof CRT's for the display of procedural data. CRT displayshave the added advantage of being compatible with radardata display systems, and the two sets of data can even bemerged on a single display.

Just as the automation of a procedural system startswith computer storage of flight plans, so the automation ofa radar-based system starts with computer storage of aircraft tracks. Although manual tracking techniques are usedin certain limited applications (7) the manpower requiredto track large numbers of aircraft usually drives designersto the conclusion that it is necessary to provide automatictracking. The problems involved in this process are dealtwith in another contribution to this symposium (8) and areonly indirectly relevant to the present paper.

The starting point for the exploitation of the radar trackdata is its "association" with flight plan data. The computersystem can be used to drive a "labelled radar display" inwhich one or more computer-driven symbols are superimposed on the raw radar display, or, alternatively, thecomputer may drive a "synthetic display" based entirely oncomputer-driven symbols (9). Because of doubts about theintegrity of the data and the reliability of the apparatus,most designers prefer the "labelled radar display", onwhich the controller can personally check the credibility ofthe computer tracks. For similar reasons, most designersavoid both automatic association of tracks with flight plandata and the use of flight plan data to aid the automatictracking process.

Up to this point, the case for introducing computers hasrested on the intuitive argument that automation is boundto be needed in the end and that we must begin to getexperience of the operational problems. The processes ofgetting the data into the computer are inescapeable, but atsome stage In the process a large number of alternativepaths open up before the system designer, and it is necessary to choose which of the controllers many tasks canprofitably be automated.

In an ideal world, there might be a formula which madepossible the prediction of the workload which a given ATCsystem imposed on each controller. It might then be possible to compare the costs of various control system configurations having different degrees of mechanisation, andhence to perform a convincing cost/benefit exercises. Inreality, quantitative prediction of controller workload isnot possible (10), and such modelling and operationalgaming techniques as are available (11) are commonly usedmainly as props to support an intuitive decision, previouslyarrived at, rather than to make a more objective choice ofthe approach to the problem.

The main tasks facing a controller can be broken downinto the following headings; —(i) voice transfer of data between pilot and controller,(ii) data transformation, extrapolation, conflict detection,(iii) data transfer controller-controller,(iv) planning, including conflict resolution.

In the present state of the art, there is no immediatepossibility of mechanising (i) unless the voice link is re

placed by a digital automatic link. The use of such a linkfor routine messages is technically feasible but may besome way off for other reasons.

We have already discussed the processes for gettingbasic ATC data into the computer. Detection of proceduralconflicts presents no particular problem. Simple programmes for the automatic detection of radar conflicts arenot very helpful, even when height information is availableto the computer. Unless adequate "intention" data can alsobe provided, the conflicts search must be based on worst-case assumptions, which lead to a prohibitive false-alarmrate. This is precisely the problem that has for many yearsheld back the provision of an airborne collision warningd e v i c e .

In the absence of automation, most of the operationsinvolved so far, including elementary conflict checks are, orcould be, performed by an ATC assistant. Since the controller needs to be familiar with the situation, however,there is a case for involving him in at least some of theclerical operations, as a means of impressing the data onhis memory. Automation has, so far, done little to ease thecontrollers* lot, therefore.

The problems of data transfer between controllers areinteresting. In a system of only moderate complexity, thelayout of the control centre mimics the route structure, sothat controllers of adjacent sectors sit shoulder to shoulder.This enables them easiliy to converse or to consult, oramend even, each others' data. In a large TMA, for example, where traffic from several airways may be mergingat a common holding point from which each aircraft isreleased to one of a number of destinations, such a convenient layout is no longer possible, and recourse must behad to telephones, closed circuit television, or other datatransmission techniques. Similar problems occur at thein ter faces between d i f fe ren t cont ro l cent res and betweendifferent control agencies.

Telephone communication between controllers presentscertain problems. Each controller must maintain a listeningwatch on his R/T channel. Such channels are quite frequentlyloaded up to 50% of theoretical capacity. If an attempt isthen made to phone the controller, there is an even chancethat he cannot answer at once because of the R/T. For apair of controllers working under these conditions, there isonly a 25% probability that both will be free at a giveninstant. Controllers can, to some extent, evade this difficulty by passing messages through their assistants, whoact as buffers to hold the message till they can capture thecontrollers' attention. In a mechanised system, the moreroutine messages, such as estimates or offer or acceptanceof a handover, can be puffered in a computer.

Inter-controller liaison problems are a crucial factor indecisions to automate. With the growth of air traffic, allthe problems concerned with aircraft one at a time can behandled either by increasing the control staff or, possibly,by automation. The problems of testing a given aircraftfor conflict are proportional to the traffic density in thevicinity, and the problems of resolving any conflict rise alsowith the number of control lers involved. The control lers onwatch at a given time already tend to outnumber the aircraft under their control, and there is clearly an upper limitto the number of controllers who can share the tasks in a

given sector.It is possible to organise the control system so as to

minimise the need for inter-controller liaison, by dividingthe airspace and runways into reasonably watertight

8

compartments. For example, all traffic inbound to Heathrow from the North might use runway 28R, and all trafficfrom the South might use 28L. Similarly, at an intersectionsuch as Dunsfold, traffic inbound to Gatwick might be confined to 2,000 ft., Gatwick outbounds to 3,000 ft., Heathrowoutbound traffic to 4,000 ft., and traffic inbound to Heathrow via Epsom to 5,000 ft., or above. By constraints of thistype, one can minimise the need for inter-controller liaison,but only at the expense of some loss in capacity.

Delays in an ATC system, assuming it is not hopelesslyoverloaded, arise because of the irregularities in thedemand for certain facilities. If the traffic is split up into anumber of separate compartments, these irregularities, andhence the delays, are aggravated. For the Heathrow inbound traffic quoted above, a more regular demand on thelanding runways would ensue if arriving aircraft could beassigned to either runway on a flexible basis. Simulationstudies at the Royal Radar Establishment and by the Aviation Operational Research Branch, Board of Trade, suggestthat a suitable strategy could increase the landing capacityat Heathrow by at least 10%, with a smaller but significantreduction in departure delays.

It does not seem likely that controller/controller liaisoncon be much improved by further automation of mostexisting systems. It is sometimes agreed that the role ofautomation is to relieve the controller of his more humdrum

tasks, thus freeing him to spend his time on the decision-taking processes. Since the computer cannot remove theneed for the controller to familiarise himself with the data,the saving in time may be quite small. In any event, it isd i f ficu l t t o be l i eve tha t wha t amoun ts to a commi t tee o fabout 50 controllers could plan movements through a largeterminal area at a rate of, say, 160 movements/hr. except byvirtue of on extensive set of constraints on the possiblesolut ions.

An alternative approach is to attempt to use a computerto devise the brood overall plan for the traffic movementsin the next ten minutes or so, leaving the controllersresponsible for the safe implementation of the computer'splan. The possibility of using computer planning has beendemonstrated in the course of work on large scale fast-time computer models of air traffic systems. The problemsinvolved in this approach will form the main topic of therest of the present paper.

Automated planningMuch of the fundamental work on planning by computer

takes the form of attempts to programme a machine toplay games whose rules and objective are explicitly formulated. Some of the more highly developed computerprogrammes can play games quite well. There exists adraughts playing programme (12) which is nearly up to thestandard of a minor champion.

Another problem — not a game — which has receivedattention in recent years is the "Travelling Salesman". It isrequired to find the round-trip path through a sucession ofpoints which will minimise the total cost, the cost of travelbeween each pair of points being provided as input data.This problems bears a very close resemblance to that ofdeciding on the best order in which a waiting queue ofarriving and departing aircraft should be allowed to use agiven runway, "best" being defined, for example, as thatorder which minimises the total delay to all the aircraft. Atechnique known as "backtrack" or "branch and bound"

(13, 14) has been used to solve both this runway problemand a more generalised version where each aircraft hasa choice of two runways (15).

In the digital computer simulation of large ATC systems(11), it becomes necessary to provide automatic solutionsto all, or nearly all, of the conflict problems that arise. It istoo much to claim that these programmes produce perfectsolutions, but there is evidence that some of these pro-gram.mes are, at least, com.parable with those producedby human controllers working under time pressure.

Techniques for solving the planning problems con besplit into two categories. The first, here termed the algorithmic method, conducts on exhaustive search of all the possibilities with a view to finding the optimum solution. Thesecond, termed the heuristic method, uses some rules ofthumb, based on human experience, to find on adequate,even if not optimum, solution.

The difficulty with the algorithmic approach is the numi-ber of possible olternotivs. Exhaustive enumeration is outof the question. Consider, for example, the runway problemmentioned above. If the programme looks only 12 movesahead, there are 2^^ 12! or about 2 X 10^^ possible solutions.By the "branch and bound" technique, it is possible to findthe optimum solution without exploring more than a verysmall fraction of the possibilities in detail. By suitable programme organisation and by storing intermediate resultsit is usually also possible to ovoid calculating the behaviourof each sequence a b initio. The experimental airportprogramme, using these techniques, can run at about realtime on a medium speed computer whilst exploring thesituation to a depth of 8 moves ahead. Some increase inspeed is no doubt possible, but It seems unlikely that a programme of this type will ever be capable of handling theplanning problems of a whole terminal area.

The "heuristic"approach to this problem, commonlyused in large-scale computer simulation of ATC systems,uses a set of rules based on controller experience. It isusually necessary to go through a protracted process ofprogramme testing and modification as the controller isconfronted with the consequences of what he formerly believed to be the rules on which he acted. Eventually, suchprogrammes can give quite satisfactory results and arefast in operation. There ore two drawbacks to this technique. Firstly, there is no yardstick by which the efficiency ofthe process con easily be judged, and, secondly, there isa danger that the laborious process of programme development will have to be repeated whenever there is achange in route structure or mode of operations. Suchchanges are relatively frequent in ATC, and it will probablyprove essential to modify the programme in the field.

In an algorithmetic programme such as the runwayprogramme quoted above, the rules of operation are allincorporated into a data array. Since the actual problemsolving method mokes no use of "common sense", it isimprobable that a change in the rules will invalidate theproblem solving mechanism, though it may change thespeed with which the programme can run.

The discussion in this section of the paper has, so farbeen confined to the solution of idealised problems, wherethe "rules of the game" are precisely defined, where thesituation is deterministic, i. e. the future can be preciselypredicted from on adequate knowledge of the present, andwhere there is some convenient yardstick which indicatesthe re la t ive mer i t o f two a l ternat ive so lu t ions. In the rea lworld, it is difficult to do more than approximate to these

9

conditions. The runway programme, for example, whilstproducing a very whortwhile reduction in the total delay,had an irritating tendency to impose a quite preposterousdelay on some isolated aircraft, a light aircraft in asequence of jets, for example. If automated planning isto play a part in live ATC, it seems clear that provisionmust be mode for human intervention, if only to cope withemergencies and other situations where the account mustbe taken of factors beyond the comprehension of theplanning programme. Consideration of the problems ofintroducing automated planning into ATC suggests that,even if the above argument did not apply, it would benecessary, until the necessary experience had been gainedand confidence built up, to start with a system in which thedecision-taking task was shared in some flexible manner,between the computer and the controller.

Controllers and computers

A c e r t a i n a m o u n t o f n o n s e n s e i s t a l k e d a b o u t ' ' m a nversus machine". As ref. 17 points out, the principle involved in "automatic" control is that part of the control task isperformed by a human being planning in advance, by acomputer programmer for example. The remainder of thecontrol task is performed by the controller working "online". Viewed in this light, the advantage of introducingcomputers into the planning process, for example, is thatwe can bring human effort to bear on a co-ordinationproblem on a scale and at a speed that would be otherwiseimpossible. The main drawback, as Majendie (18) pointsout, is that the system may have been intended for a tasko t h e r t h a n t h e o n e w i t h w h i c h i t i s f a c e d .

The real difficulty is to formulate a sufficiently comprehensive operational requirement and to arrive at aneconomical division of effort between the computer programmer and the on-watch controller.

Reference 16 describes some experiments on theTravelling Salesman and other problems in which a compar ison was made between unaided human operators ,operator-computer systems, and a fully automatic, heuristic, system. The heuristic programme nearly alwaysproduced the best solution, eventually, but a dramatic increase in speed is possible if use is mode of a man's abilityto produce at least a rough solution nearly instantaneously.The authors point out that the Travelling Salesman problemis one which has been the subject of considerable research,and that for most problems the man-machine system islikely to prove far more satisfactory.

For our real life problems, this argument applies withirresistable force. It may be helpful to think of the controller as playing a game on the computer. The game is basedon a possibly rather idealised version of the real-l ifesituation, so that the result of the game is regarded as aguide to action rather than a mandatory decision. Work isin progress on a Computer Aided Approach Sequencingsystem which is Intended to incorporate some of theseideas. In particular, it is planned to provide computerguidance as to the best sequence of runway events.

In the experiments of ref. 16 only one man was conversing with the computer. In the CAAS system, about fourcontrollers will eventually be involved, although the interactions between them need be extensive. If an attempt ismade to extend the technique to more general planningproblems, we must face a situation in which the controllers'

interactions take place mainly, if not exclusively, throughthe computer system. This is an area where considerableexper imental work is needed.

AcknowledgementContributed by permission of the Director R.R.E. Copy

right Controller H.M.S.O.

References1. Lundberg, B. K. O. "The Allotment of Probability Shares

Method, a Guidance for SafetyMeasures in Av ia t ion" .

Proceedings of Symposium on CivilAviation Safety. Swedish Society ofA e r o n a u t i c s .

Stockholm April 1966.2. McLachlan, W. L. "Redundancy in Ground Data [Hand

ling Equipment for Air Traffic Cont r o l "

Symposium on "The Use of Redundancy in System Design".Society of Instrument Technology,London February 1964.

3. Cherry, fH. "Evaluation of Computer Facilitiesfo r Ocean i c Con t ro l "3 rd I n te rna t i ona l Av ia t i on R&DSymposium,Federal Aviation Agency.Atlantic City, November 1965.

4. Moraski, J. J. "ATC Data Processing Central"Journa l o f A i r Tra ffic Con t ro l

April 1959.5. Smit, J.S. "Survey-of Experiences with the

SATCO-System"3rd Aviation R&D Symposium,Federal Aviation Agency.Atlantic City, November 1965.

6. Johnson, E. A. "Touch Display — A ProgrammedM a n - M a c h i n e I n t e r f a c e "

Ergonomics 10 2 pp. 271—277,March 1967.

7. Martin, D. A. "Application of a CAAS System"17th lATA Technical Conference.Lucerne October 1967.

8. Ord, G. "Automatic Tracking in ATC"This Symposium.

9. Evans, D. R. "Displays for ATC"This Symposium.

10. Ratcliffe, S. "Mathematical Models for the Pred ic t ion o f A i r Tra ffic Cont ro l le rW o r k l o a d "This Symposium.

11. Laite, P. J. "Computer Simulation in ATC Sys t e m s "

This Symposium.12. Samuel, A. L. "Some Studies in Machine Learning,

Using the Game of Checkers"I.B.M.J. Res. Dev. 3, 210—229,July 1959Cont inued in :I.B.M.J. Res. Dev. 11, 601—617,November 1967.

1 0

13. Little, J. D. C. et al. "An Algorithm for the TravellingS a l e s m a n P r o b l e m "

Operations Research 11, 972—989,N o v e m b e r 1 9 6 3

14. Golomb, S. W. "Backtrack Programming"X Ass. Computer Machinery 12,516—524, October 1965.

15. Ratc l i f fe , S. "Automat ic Solut ion of IMA ATCAssignment Problems" 17th lATA

T e c h n i c a l C o n f e r e n c eLucerne October 1967.

Michie, D.

15. Ratcliffe, S.Majendie, A. M. A.

"A Comparison of Heuristic, Interactive, and Unaided Methods ofSolving a Shortest-Route Problem"Machine Intelligence 3.Edinburgh University Press 1968." H u m a n E r r o r a n d A c c i d e n t s "

Design No. 116 August 1958.(Council of Industrial Design)." T h e H u m a n v e r s u s t h e A u t o m a t i c

Navigator"J. Inst. Navgn. 13, 1, January 1960.

Passive Horn May Clean Up Radar Displaysby TIrey K. VickersS e n i o r C o n s u l t a n tJames C. Buckley, Inc.

On June 20, 1969 the US FAA signed a contract withRaytheon Corporation for the modification of an ASRantenna, utilizing the passive horn principle. The main objective of this development is to eliminate most groundclutter from the radar returns which are fed to the receiver.Thus it should greatly assist the MTI circuitry in providinga clean, clutter-free radar picture.

With a conventional radar antenna, as shown in Fig. 1,the radar energy from the transmitter proceeds through awaveguide to a feed horn, which bounces it off a reflectorscreen, to focus it into the desired radar beam (Pattern Ain Fig. 2). Part of this energy bounces off aircraft andground targets, and returns to the reflector screen, whichfocuses it into the feed horn. From there it travels backdown the waveguide and into the receiver.

A characteristic problem of conventional radar installations is to get the beam low enough to provide good coverage of the lower altitudes at maximum range, without reflecting too much energy off the terrain and obstructionsclose to the antenna. Such reflections can produce verystrong returns at short ranges, as shown in Fig. 2. Some ofthese returns may be too strong to be cancelled out by theMTI circuitry. The resulting ground clutter may obscure aircraft targets at short ranges.

If the tilt of the radar beam is raised to the point wherethe ground clutter is eliminated, then the nose of the beamis well above the altitudes used by air traffic. This has theeffect of reducing the maximum range of the radar for airtraffic control purposes. Consequently, the beam tilt is usually adjusted to some compromise angle (usually 3 to 5degrees) where the low altitude coverage is considerablyless than ideal, but the ground clutter can still be tolerated,or cancelled out by the MTI circuitry.

F E E D H O R N

R E C E I V E R M - f C I R C . H — f T / R

T O D I S P L A Y S

S Y N C H R O NI Z E R

4 T R A N S M I T T E R

Figure 1 Simplified block diagram of a conventional radar.

Reprinted from ATCA Journal with kind o f t h e E d i t o r

11

V E F f Y S T R O N G G R O U N DR E T U R N S F R O M T H I S A R E A

Figure 2 Typical transmitting/receiving paftern of airport surveillancer a d a r .

I SWITCHOVER POINT

Figure 5 Portions of receiving patterns utilized.

1 2

How then can better coverage be obtained, withoutgreatly increasing the ground clutter? This is where thepassive horn comes in. It will be mounted on the antennaassem.bly below the feed horn, and looking upward at aslightly higher angle, as shown in Fig. 3. This will providea receiving pattern similar to Pattern B in Fig. 4. The posit ion of the horn in relat ion to the reflector screen wi l l beadjusted so that Pattern B barely grazes the ground without strongly illuminating any nearby objects which couldcause objectionable clutter.

The passive horn will not be connected to the transmitter. Instead, as shown in Fig. 3, it will be connected througha diode switch, to the radar receiver. The diode switch willconnect the normal feed horn o r the passive horn alternately to the receiver. The switch will be adjustable, toflip automatically at desired points in the duty cycle, asexplained below.

As soon as the transmitting pulse is fired, the passivehorn is switched on to receive returns coming from nearbytargets within Pattern B, but blocking returns from PatternA. As Pattern B will be relatively free from close-in groundreturns, this portion of the radar sweep will be relativelyfree from ground clutter.

At a time equivalent to 10 to 15 miles range (depending on the switch adjustment) the diode switch flips automatically to the alternate position. This blocks the returnsf rom Pat te rn B bu t a l lows the re tu rns f rom Pat te rn A tofeed through the normal (feed) horn to the receiver. As theground retuns contained in this portion of Pattern A, fromthe switchover point out to maximum range, are muchweaker than those close to the antenna, the outer portionof the sweep will be relatively free of ground clutter. Fig. 5shows the parts of the patterns actually used.

Using this combination of receiving patterns (Pattern Bfrom zero range out to the switchover point, and Pattern Afrom the switchover point out to maximum range) it is expected that the number and strength of ground returnsreaching the receiver will be drastically reduced. Thisshould make the job of the MTI circuitry much easier, ineliminating objectionable clutter, particularly in instanceswhere the clutter is complicated by precipitation.

A by-product of the passive horn may be an improvement in radar coverage at higher angles or altitudes closeto the station. It is also expected that, since the close-inreturns of Pattern A will not be used anyway, it will bepractical in many cases to lower the elevation angle (tilt)of the antenna to obtain better coverage of the lower altitudes at long ranges. This in turn should provide bettertargets from small aircraft at greater distances from thea n t e n n a .

The passive horn project is part of a continuing in-service improvement program for FAA radars. The projectis under the direction of the Radar Systems Section of FAA'sSystems Research Development Service, and will requireabout a year for completion.

If the passive horn development is successful, its retrofitto ASR antennas in the field will be complicated somewhatby the fact that each installation will be a hand-tailoredjob. Radar waves behave like light waves, so improving aradar antenna pattern is basically a matter of optics. Eachfitting is a unique job tailored to that specific installation,like fitting a pair of spectacles. Optically, this one is equiva lent to b i foca ls !

T K V

Pity the Air Traffic Controllerwhen they come in like this

An exaggerated picture perhaps but notfor long. Air Traffic is increasing so fastthat the controller's j oh needs an entirelynew appraisal. And one of the thingswe've got to look at is the method oftraining control lers. Is i t adequate tom e e t t h e d e m a n d s o f t h e S e v e n t i e s ?The flex ib i l i t y o f the Fer ran t i RadarSimulator provides the answer—now andfor the future. It gives the trainee cont ro l le r p rac t ice in A i r Tra ffic Cont ro lu n d e r c o n d i t i o n s s o r e a l i s t i c t h a t w h e nh e t a k e s o v e r c o n t r o l o f r e a l a i r c r a f t h e ' l ln o t o n l y b e f u l l y t r a i n e d h u t c o n fid e n t t o o .

Digital techniques readily permit modifications to accommodate changes in aw ide range o f parameters , i nc lud ingaircraft type and speed, radar and geog r a p h i c a l d a t a . R a w r a d a r o r f u l l ysynthetic output can be provided to driveany type of display. The system cant h e r e f o r e s i m u l a t e n e w a i r c r a f t a n dprocedural techniques not even envisaged at this stage.Ferrant i have the capabi l i ty and experience to design and develop a systemto suit any individual requirements. I fyou have an ATC training or evaluationproblem talk to Ferranti.

F E R E W ^ IATC training systemsF e r r a n t i L i m i t e d ,D i g i t a l S y s t e m s D e p a r t m e n t , B r a c k n e l l , B e r k s h i r e , E n g l a n d . R G 1 2 I R A

D S 2 1 / 2

i t sM§caToM O N T R E A L , M A Y 1 9 7 0T h e I n t e r n a t i o n a l F e d e r a t i o n o f A i r T r a f fi c C o n t r o l l e r s

A s s o c i a t i o n , w i l l h o l d i t s 1 9 7 0 a n n u a l c o n f e r e n c e i n t h eQ u e e n E l i z a b e t h H o t e l a t M o n t r e a l o n M a y 11 - 1 4 .

I t p r o m i s e s t o b e t h e b e s t c o n f e r e n c e y e t . T h e p r o g r a m m e w i l l h a v e a n

e x c e l l e n t b l e n d i n g o f s e m i n a r s o n a l l AT C a d v a n c e m e n t s a n d p r o b l e m s , w h i l eo n d i s p l a y w i l l b e t h e l a t e s t e l e c t r o n i c e q u i p m e n t a v a i l a b l e , m a n u f a c t u r e d b yt h e w o r l d s ' l e a d e r s . A l l t h i s , c o u p l e d w i t h t h e o f f h o u r s o f e n j o y m e n t t h a t

o n l y M o n t r e a l c a n o f f e r .I f you a re i n t e res ted i n Jo i n i ng t he con fe rence as a de l ega te o r obse rve r, t henfi l l o u t a n d m a i l i n t h e a t t a c h e d f o r m .

IFATCA 70 is actively being supported by:A . C . S I M M O N S & S O N L T D . • A I R

V I S I O N I N D U S T R I E S LT D . • AT L A N T I CAV I AT I O N LT D . • AV I AT I O N E L E C T R I CL T D . . C A N A D I A N A I R L I N E P I L O T SA S S O C I AT I O N . C A N A D I A N W E S T I N G -H O U S E C O . L T D . . C O M P U T E R D E

V I C E S O F C A N A D A L T D . • E . S . J O H NS O N C O M PA N Y • F E D E R A L D E PA R TM E N T O F T R A N S P O R T • F E R R A N T IL T D . - U . K . . G E N E R A L A V I A T I O N L T D .• L T V E L E C T R O S Y S T E M S I N C . . M A R

C O N I E L L I O T T C O M P U T E R S Y S T E M SLT D . . N . V. H O L L A N D S E S I G N A A L -A P PA R AT E N . P H I L I P S E L E C T R O N I CEQUIPMENT . PLESSEY CANADA LIMITE D • P L E S S E Y E L E C T R O N I C G R O U P. S O L A R T R O N E L E C T R O N I C S G R O U PL T D . . T E X A S I N S T R U M E N T S I N C . •

3 M C O M P A N Y « U N I T E D A I R C R A F TC O M P A N Y L T D . » A V I T A T L T D .

i i o B c a v oP.O. BOX 513 AMF, MONTREAL INTERNATIONAL AIRPORTD O R V A L , Q U E . , C A N A D A

G e n t l e m e n ,I am Interested In receiving ttie attendance registration form andi n f o r m a t i o n k i t .

(Please Print).

NAME

A D D R E S S ^ C I T Y

STATE, PROV

COUNTRY

1 5

Lasers to Determine Visibility at Airports

Aircraft and airports are extremy costly to rund. Closure of major airports and aircraft diversions cost millionsof pounds every year and are mainly attributable to fog.To minimise this loss, aircraft tend to operate in the worstpossible visibility conditions commensurate with the stipulated safety regulations. These safety regulations are verystringent but even so accidents con still occur.

The recognised solution to this problem is InstrumentLanding System (ILS), and with the ultimate instrument,safe, fully automatic landings in zero visibility will be possible. However the ultimate ILS instrumentation is not yetavailable and for the next ten years or so, the majority ofILS landings will be semi-automatic with ILS guidance downto 100 ft followed by manual control to touch-down. Theessential features of a runway equipped for semi-automaticlandings are shown in Fig. 1.

The International Civil Aviation Organisation has evolved three categories of ILS equipments, these being outlinedin Fig. 2.

For categories other than III (C), accurate measurementsof Runway Visual Range (RVR) and cloud base are essential for category classification.

Prior to complete blind landing systems being available,there is increasing pressure on airport authorities wheretraffic densities are high, to progress rapidly to Category IIoperations. It is mandatory to have automatic RVR equip-

by R. L. Burr and E. T. HillRadar Equipment Division,Plessey Radar Limited.

ments at all airports where Category II conditions are nowoperative. However, monitoring RVR alongside the runway is no precise guide to the visibility conditions existingalong the glide path itself. Comparatively clear conditionson or near the ground can be accompanied by poor visibility tens of feet above the ground and the converse canalso occur. Visibility can decrease as the pilot loses altitude resulting in complete loss of visual contact with theground below his decision height — typically 200 feet up.

The need to monitor Slant Visual Range (SVR) from theground is equally, if not more important than RVR. Unfortunately the latter is easier to achieve than the former.

Current Category II ILS systems are only reliable downto 100 ft, where the pilot is committed to the landing andmust be able to maintain visual contact with the groundfor azimuth guidance. The pilot must know before committing himself to the final landing that he will be able to seea sufficient landing light pattern during the final approachstage to land safely. In particular, during the final 300 feetof descent he must know the heights at which his descentwill take him into fog and impair his vision and at whichhe will see sufficient ground or guidance lights. A pre-knowledge of adequate RVR is also essential to ensuresatisfactory roll-out and final taxiing to the disembarkation point. RVR is also of vital concern during the takeoff phase.

A i r c r o f t d e c i s i o n h e i g h t

^t. I ceiling RVR 800

^prox.limit of reliable ILS guidance. Pilot takes cantral of azimuthpainting in autaflare

Pilot takes over from" -^^wCat. II ceiling 400 m RVR Autoland ,after touchdown

8 0 0 m R V R C o t l

^F lo reout s ta r ts

Threshold / 3° T o u c h d o w n

3 0 0 m ( 1 0 0 0 f t . )I 4 0 0 m R V R C o t . I

▶ 6 0 0 m

Approach l ights

T o u c h d o w n

C l o u d b a s em o n i t o r

F l o r e o u tR V R M o n i t o r s ISOm^ J

Runway lights

Figure 1 A typical instrumented runway.

1 6

To date, RVR has been estimated by trained observersat the threshold counting the number of runway lights visible to them. As a method it is subject to human error anddoes not lend itself to being automated as is required byICAO for Category II operations.

Several automated methods of measuring RVR havebeen proposed including the use of closed circuit TV cameras, but it is now generally accepted that the transmissio-meter is the preferred instrument. The transmissiometer usesa light source at one end of a baseline and a detector atthe other end. The loss of light flux per unit length betweenthe t ransmi t te r and the rece iver i s known as the ' ' a tmospheric extinction coefficient". This loss is primarily dueto scattering at visible wavelengths.

The ability of a pilot to see the lights Is a function of theeye's sensitivity and the minimum contrast it can discernbetween the apparent brightness of the light source and thebrightness of the background. The problem is made moredifficult due to the considerable variations in backgroundbrightness and hence visibility which occurs between dayand night even for identical fog conditions. In practice RVRis defined as the maximum range at which the runway lightscan be just discerned by the pilot.

RVR is a function of the atmospheric extinction coefficient, the sensitivity of the human eye under the prevailingconditions, the level of the background illumination andthe power of the runway lights. These four parameters arelinked together by Allard's Law. In an instrumented system,the extinction coefficient is the only parameter measured,the other three being pre-set. Thus RVR can be determinedand passed to the pilot.

Two basic types of transmissiometer for RVR measurement exists. The first uses a light source and a light receiverseparated by a straight baseline. The second uses a comparison technique, the light source and detector beingmounted close together in a common unit and the baseline folded back by an optical reflector. This arrangementallows a reference signal to pass directly between thetransmitter and detector to provide a continuous monitorof light output and detector sensitivity. This obviates theproblem of maintaining highly stable light outputs anddetector sensitivities necessary in the first system.

If accurate measurements of RVR are to be obtainedover the operational range from 50 to 1500 metres a doublebaseline transmissiometer is necessary, utilising baselinesof approximately 15 and 150 metres. Fairly complex data

computations are required to convert the measured extinction coefficient to the RVR values required by the pilot.

Although the assessment of RVR is perhaps fairly involved, the optical instrumentation is relatively simple. Toassess SVR, the optical instrumentation now becomes difficult. An RVR type system using a mast on which are mounted a number of light sources at various heights could beused. Receivers on the ground then measure the verticalprofile of the extinction coefficient. For obvious reasons,the most cannot be placed on the glide path nor con itsheight be greater than about 100 feet. For stable and horizontally homogenous fog conditions, vertical profiles upto 100 feet can be obtained and extrapolated to 300 feet.This data in conjunction with cloud base data etc. cangive good results in terms of contact height and visualsequence. However, during patchy fog or the lifting andformation stages of fog, forecasting becomes unreliable.Other systems such as flares, burning on the glide path,extendable masts, lights on balloons etc. have been considered but hardly seem to offer safe and dependablec h a r a c t e r i s t i c s .

Indirect methods involving the measurement of bock-scatter from optical beams are now being investigated. Itis known that the passage of light through most unpollutedfogs is attenuated almost entirely by multiple scattering.The main problem is to establish a relationship betweenthis scattered light and the atmospheric extinction coefficient. The light bock-scattered can be monitored by a singleended instrument combining transmitter and receiver. Instruments of this type exist using pulsed light sources, butin general the light sources ore not powerful enough.

Effort is now being directed to the improvement ofthese systems and in particular towards the use of laserradars (lidars). Lidars can be used in a variety of ways, themost promising enabling the lidar to scan a volume of fogand determine its spatial distribution. Precautions need tobe taken however to ensure that these high power laserbeams do not illuminate any aircraft on final approach because of the possible eye hazard to the crew. This protection can be achieved using a simple radar which inhibitsthe l idar when aircraf t are in the hazard zone.

The possibility of using a single scanning lidar systemto obtain RVR, SVR and cloud base data makes the laseran extremely attractive proposition. A significant saving indata handling equipment should also result from such aconcentrat ion of funct ions wi th in the one inst rument .

Ceiling B r e a k o f f

h e i g h t

8 0 0 m e t r e s

4 0 0 m e t r e s 3 0 m

A 2 2 0 m e t r e s

5 0 m e t r e s

R e m a r k s

Equivalent to noninstrumented operations

Important, requiresRVS, SVR and

cloud base monitoring

Fully automatic landing

Fully automatic landingsufficient to taxi

Fully automatic landingtoo poor to taxi

Type of landing

I S L a s s i s t e d t o 6 0 ms i m i l a r t o n o n

assisted landing

I S L a s s i s t e d t o 3 0 m

then manual contro lto touchdown

Fully Automatic

Fully Automatic

Fully Automatic

Figure 2 Categories of ILS Installations — ICAO.

7

Mathematical Models for the Prediction ofAir Traffic Controller WorkloadPaper to the United Kingdom Symposium "Electronics for Civil Aviation", 1969*

Royal Radar Establishment,

SummaryMuch effort and expenditure has gone into the auto

mation of ATC, but we lack quantitative data about thecontroller workload which automation is meant to relieve.It is unreasonable to expect more than a rough measureof workload, however defined. A "mathematical model"tnkes the form of one or more algebraic expressions whichpurport to predict the workload. One way to construct sucha model is to break down the controllers' task into a number of rudimentary components, to measure the time spenton each sub-task, to multiply these times by suitable weighting factors and add to obtain the total load. Alternatively,a workload equation may be arrived at intuitively, and thecoefficients adjusted to fit the observed facts.

The paper is a critical review of models so for suggested, and of techniques for measuring controller workload.The author has little confidence in any of the publishedresults. When faced with a serious overload situation, thecontroller normally preserves air safety and his own sanityby slowing down the traffic demand, by one means oranother. In such a situation, tests on controller loading maybe an insensitive method of measuring the traffic delayswhich are of primary importance.

I n t r o d u c t i o n

Given the scale of effort and expenditure that has goneinto the mechanisation of air traffic control, it is perhapssurprising that there is relatively little quantitative dataabout the nature of the nature of the workload whichautomation is meant to relieve. If there existed on adequatetool for the prediction of the various components of thiscontroller workload, it might be a much easier matter tocompare the economics of various possible control configurations which might be adopted to deal with a givent a s k . . .

It is a formidable task to get even an approximate solution to this problem. Controllers differ markedly in theamount of work they manufacture for themselves in agiven situation; and in their ability to deal wit it Interactions between different members of the control teammay considerably confuse any simple arithmetical approach. "Workload" is not defined with any rigour andsubjective estimates of work difficulty are confused by thevariability of controller ability. For the purposes of thepresent paper, the author is prepared to accept any definition of "workload" that is not in conflict with commonEnglish usage and which lends itself to measurement.

The scientific approach to the prediction problem is toset up a mathematical expression or expressions into whichare substituted the appropriate values of the parameters

Reprinted with Irind permission of the outhor and the U.K. Ivlinistryof Technology

which are deemed to characterise the problem, and whichyield a predicted value for the ensuing workload. Thealgebra should be of the minimum complexity and theparameters of the minimum number necessary to give anadequate fit to the observed data.

Published papers (1, 2, 3, 4) treat the number of aircraft under control as the main parameter and expressthe workload by an expression of the general form: —

L = a + b N + c N ^ 0 )where a, b and c vary with the nature of the control taskand the organisation. Equation 1 can undoubtedly bearrived at by arguing that it is convenient to use a powerseries expansion, that a and b are certainly non-zero, thata linear model is inadequate, and that if more than oneadditional term is added the experimental evidence (if any)will be inadequate to determine values for the coefficients.

Techniques, other than pure intuition (2, 4), for the construction of workload models will be classified, in thepresent paper as "synthetic" or "analytic". A syntheticmodel is arrived at by assembling, by one method or another, a list of the various tasks which a controller mustperform, e. g. "conflict search", breaking each task downinto manageable components, and determining the amountof work involved in each component, and adding theresults, with suitable weighting, to determine the total work.Basically, this is the technique xysed in motion and timestudy (6). The "analytic" approach takes the actual ATCsituation as a whole, and attempts to determine thecontribution to the total workload due to each factor ofinterest by analysis of variance or other market researchtechniques. An alternative terminology would describe the"analytic" approach as "descriptive" — giving an accountof the behaviour of the system; the "synthetic" approachbeing described as "prescriptive" — giving an account ofhow the system should behave.

The next two sections of this paper will consider thesetechniques in greater detail.

SynthesisIf the controllers' task is to be studied as an assembly

of sub-tasks, the first need is for a breakdown of the jobinto its components, and for the construction of a flowdiagram showing the sequence of events required to dealwith a known task. This is a coarser scale version of theproblem facing a computer programmer who attempts tomechanise some aspect of ATC. It is not enough to categorise the various problems, it is necessary to know thestrategy employed to solve them. Basically, the method isto interrogate one or more controllers. An extension of thistechnique, termed "instigated introspection" by somepsychologists, requires the subject to solve a series ofproblems whilst simultaneously giving a running commentary on his mental processes. The difficulty here is that thecommentary introduces unreality into the situation, thecontroller may describe, not what he usually does, but what

1 8

h e t h i n k s h e d o e s o r e v e n w h a t h e t h i n k s h e s h o u l d d o .

Further, there is the temptation to spend more time in describing the easy processes and less on those, common inATC, which ore extremely difficult to explain to an outsider.

Leplat and Bisseret (5) used the technique of the previousparagraph, followed by a series of trials under laboratoryconditions. Their study was confined to a particular controller task, the test for procedural conflicts. In the laboratory trials, the controller was faced with a flight progressboard depicting a traffic situation at a fixed time. He wasthen faced with new aircraft wishing to join the system, orwith requests for a level change. Measurements were madeof the time needed to solve each problem. The trial wasrepeated for on adequate sample of controllers and for aset of problems designed to stimulate each branch of theflow diagrams.

If this technique is to provide an overall measure ofcontroller workload, it is necessary to have a measure ofthe relative frequency with which each sub-category oftask will face the controller. It may be possible to measurethese frequencies by analysis of the results of normal operation, but, except where there is already extensive mechanisation, the labour involved in data collection andanalysis may be intolerable. In any event, this techniquemay not be applicable to a hypothetical new organisation.An alternative is to use fast-time simulation (10) to determine the magnitude of the various control tasks.

By confining themselves to a procedural system, andby discussing the conflict detection task only, Leplat andBisseret were able to avoid the more subtle situations thatarise when information is arriving by more than onechannel at a time, e. g. by ear and eye, and where thecontroller may be uttering ritual words over the R/T andsimultaneously be thinking of something different. Theclassical techniques of time and motion study have beenthe subject of heavy criticism (7) even when they wereapplied to almost purely manual tasks. The application ofthis technique to a largely cerebral activity will need considerable justification.

AnalysisThe work by Bar-Atid Arad (1) appears to be one of the

earliest attacks on a significant ATC system, and is almostcertainly the most ambitious so far attempted. Arad choseto study the entire contemporary U.S. en-route ATC environment, with traffic loading and sector configuration as themain var iables.

The study breaks down into three components: —(i) formulation of a workload model,(il) experimental determination of coefficients,(iii) effect on workload of changes in sectorisation.

The present section will be concerned with (i) andsection 4 with (ii).

Arad states as an axiom that the control workload consists of three terms: —(i) a "background" load, Lq independent of N, the num

ber of aircraft under control ,(il) a "routine" load, L^, directly proportional to N,(iii) an airspace load, Lj, proportional to N^.

In practice, the Lq term is ignored. The term in is ass u m e d t o b e o f t h e f o r m : —

L , ( 2 )N is the number of aircraft under control, T is the averagetime that aircraft are in the sector, and is the "coeffi

c i e n t o f r o u t i n e l o a d " w h i c h v a r i e s w i t h t h e n a t u r e o f t h e

task and the traffic mix.Traffic was, in fact, broken down into "standard" and

"non-standard" aircraft, and additional allowances madefor traffic in the following four categories: —

— Vertical handoff,— TMA handoff,— Climbing and descending,— "Pop-up" (a/c demanding impromptu admission to

the IFR system).A "specific weight", empirically derived, is then as

signed to each class of traffic and the weighted mean ofthese weights is K^.

The term in Lj is assumed by ref. 1 to be of the form: —

^ 2K,aVN^gS

where a is a parameter fixed by the rules of separation(nm/ac).

V is the average traffic speed (kts)N is the number of aircraft under control (ac)S is the sector size (nm)2Kj is the coefficient of airspace load.g is the "equivalent volumetric flow organisation fac

tor" (see ref. 8 para. 1.6.11.2(g)).Arad defines a unit of work as the load generated by

"one standard aircraft overflying the Sector in straight andlevel flight when no interaction with other aircraft is considered". This is the "Dynamic Element of Work" or DEW.The unit of load is termed the "Dynamic Element of Load"or DEL. One DEL equals one DEW per hour. The units ofL, and L2, are therefore, DEL. The units of and K2 areDEW/a.c. There is a dimensional error in equation (3), since,as shown in ref. 9, Appendix II, the right hand side of theequation does not have dimensions DEL. The reader shouldsee ref. 9 for details of a tidied-up and dimensionallycorrect version of equation (3) which was used in thecomputer programme for the evaluation of the Arad model.It should be noted that the definition of a DEW is apparently local to the Sector under consideration.

it is unfortunate that none of the published papers onthe Arad model d iscuss the axioms on which i t is based.These imply that the L^ term represents work generatedwhen aircraft enters or leaves a Sector, and that this work,presumably communications and data entry of one typeor another, is directly proportional to the number of aircraft entering or leaving and independent of the timewhich the aircraft spends in the Sector. Similarly, the L2term is implicitly taken to be proportional to the numberof conflicts arising between aircraft in a given Sector.Since the difficulty with which a conflict can be resolvedis itself a function of traffic density, it can easily be arguedthat higher order terms are necessary in the workloadequation.

It is a relatively simple matter to measure the timespent on R/T or on inter-controller conversation, andalthough this does not measure workload, in DEW units orotherwise, the results throw an interesting light on the

It is necessary to spend only a little time listening toR/T to realise that the nature of the messages changes withloading. In a busy period, a burst of noise as the R/T keyis flicked replaces the message "good day to you, sir" whichmight well be passed in a slack period. This phenomenonmay well complicate the relationship between the routinework load and the number of a i rcraf t .

1 9

NUMBER OF AIRCRAFT Figure 1 Total speech versus traffic level.

J I I I I I L1 4 1 6 1 8 2 0 2 2 2 4 2 6

A recent study (11) at LATCC, West Drayton, has yielded, amongst other things, figures for the total speech load(expressed as a percentage of the study period) on controllers on Sectors 5 and 11, as a function of the number ofaircraft passing through the Sector during the half-hourperiod over which each loading was measured. To quotefrom ref. 11: — "Sector 5's sphere of operations is a bidirectional airway bounded at its western extremity by aIMA and at the eastern by the convergence of two busyairways. The incumbent is frequently more heavily engagedin liaison than in actual controlling. On the one hand, he isengaged in almost continuous liaison with the Garstonstack controller — on the other, he is liaising with theSector 11 controller, initial descents on inbound aircraft,and cl imbs on outbound. The Sector 11 control ler, in contrast to his neighbour is primarily engaged in active controlling, and since his area of operations is almost entirely-over water, his duties in so far as inbound releases to, andclearances from airports, are concerned ore limited with aconsequent reduction in telephone workload".

2 8 3 0 3 2N-UMBER of aircraft Figure 2 Curve fitting.

The present author has fitted to this data quadraticcurves giving the least-squares best-fit prediction of speechloading as a function of the number of aircraft in the Sector(m a half-hour period). The results are shown in fig. 1. Thecurve for Sector 5, in particular, suggests that there mustbe a maior part of the total speech load which does notdepend on the number of aircraft in the Sector. This con-ciubiun IS, at least partly, supported by the quotation fromref. 11 given in the paragraph above.

The difficulties of curve-fitting are illustrated in fig. 2,based on the data from Sector 5. The best-fit curve is hereshown together with the dots which mark the experimentalresults and elongated "I's" which mark the ±1 sigmalimits about the mean loading for each traffic level. (Sigmawas calculated on the assumption that the scatter aboutthe mean for each level of loading constituted a samplefrom the same population). It will be seen that whilst themain features of the curve are almost certainly correct,sampling errors may be playing a significant role. It wouldbe interesting to have a larger sample of data (say,

2 0

10 times the present size, or about 600 half-hour periods)on which to work. Ref. 11 gave a breakdown of the speechload into various components: — R/T, intercom., GPO lines,and direct liaison. Unfortunately, the sampling errors formost of these components are even greater than those infig. 2. because the samples are smaller, and the presentwriter has achieved no meaningful breakdown of the resu l t s .

What does seem clear, at any rate, is that the Aradassumption that the workload curve passes near zero forzero traffic is seriously inaccurate for the Sector 5 resultsshown in fig. 1.

Using methods which are discussed later in this paper,the F.A.A. mounted a study (9) to check the validity of theArad model. This compared the accuracy with which threedifferent models could predict the average order in whichcontrollers would rank the difficulty of handling givennumbers of aircraft in various sectors.

The three models were: —

(i) the Arad model (termed the "M" model in ref. 9)(ii) a variant of (i) in which only the "routine load" com

ponent was used (the model)(iii) a model which assumed the load to be proportional

to the "equivalent traffic count" i. e. the average number of aircraft under simultaneous control in the sector(the "E" model).

Tests of the predictions of the three models against controller judgment showed that the differences of proximitywere statistically significant, and that the order of closenessw a s : —

(i) the E model(ii) the M model(iii) the L, model.

The difference between the E and L, models is particularly interesting. As was pointed out above, the L, term iscalculated on the assumption that the routine load dependson the number of aircraft entering or leaving a Sector in agiven period. The E model assumes that the load dependson the number of aircraft within a Sector simultaneously.It seems clear that the poor showing of the present Aradmodel could be improved by taking the E model as theroutine load component instead of as presently defined.

M e a s u r e m e n t o f W o r k l o a d

It was pointed out at the start of this paper that "controller workload" has not been rigourously defined. If weare prepared to adjust the definition of workload to suitthe method of measurement, possible techniques include: —(i) External observation of control activities (basically,

the method used by Leplat and Bisseret).(ii) Physiological tests for strain in the controller.(iii) Simulator trials in which traffic levels are pushed up to

the point where the controller is saturated.(iv) Methods of detecting controller overload by measure

ment of his error rate, either in performing his normaltask or in some artificial base-load task involving, say,elementary arithmetic.

(v) Controller judgment as assessed by questionnaire andinterview (the method used by Arad and Jolitz).

Method (i) has been discussed in section 2. Method (ii)does not seem to have been applied with any success. Method (iii) is perhaps capable of being used to form a quali

tative estimate of the relative capacity of two systems,but it is often impossible to define a precise point wherethe control system "breaks down". A controller who isapproaching saturation will progressively adopt more andmore tricks to reduce his workload and postpone his problems, possibly with a significant loss of expedition to thetraffic concerned, until his mode of operation may eventually bear only a very rough relationship to that normallyused. The technique has the added drawback that the process of building-up a saturation situation is necessarily afairly slow one. Since it is clearly necessary in any measurement of workload to take a big enough sample of controllers and of traffic to keep sampling errors down to areasonable level, the simulation process can become veryexpensive in time and effort.

Method (iv) suffers to a somewhat reduced extent fromthe objections to method (iii), but has the added drawbackthat it is not easy to measure controller errors. At best, theprocess is laborious, and the results may be ambiguous.For example, if a busy controller had decided that updatingof a particular piece of data on his display was reallyirrelevant, and omitted to make the revision, would thisconstitute a mistake? If not, how can one be sure that this isnot the explanation of an "error"? The base-load taskavoids this difficulty, but there is now on additional unwanted variable, for controllers will differ considerablyin the priority they accord to this task when the work-loadbegins to build up.

The workers at the FAA (8, 9) apparently decided, inthe face of the above difficulties, to adopt method (v). Itcon be argued that the consensus of controller opinion isbased on a much larger sample of traffic situations thancan be covered in any simulation, and, because the opinionis based on the real world, it avoids the systematic errorsthat are always possible in simulation. It remains to obtaina large enough sample of controllers to reduce effects dueto personal bias to a reasonable level, to devise a suitabletechnique for extracting a quantitative judgment of controlwork-load, and to show that there is a meaningful consensus of opinion.

In the evaluation of the Arad model, Jolitz (9) selectedfive air traffic control centres, and studied a total of 16sectors, each of which hod been worked in common by atleast two experienced controllers. The sectors were selectedto have different functions, but each had "complete radarcapability" and was bounded by other sectors that normally used radar handovers to the sector under study.Further, the selected sectors hod average or above averageactivity.

Subject controllers, having experience on two sectors,A and B say, were faced with questions of the form: —

"How many aircraft under simultaneous control insector A would you judge, on average, create the samel o a d a s N a i r c r a f t i n s e c t o r B ? "

Twenty-four questions were generated by putting N =6, 8, 10 and 12 and by reversing A and B. These were arranged in pseudo-random order and put to each subject.After a lapse of about one week, the questions were rearranged and again administered to each subject. At thetime of the first interview, the subjects were not told toexpect the second.

The FAA project team decided to eliminate from thedata any set of answers which contained "reversals in thejudgmental response". (The example quoted in ref. 9 is asubject who stated that the load due to 6 aircraft in Sector

21

A was equivalent to 4 aircraft in Sector B, but who, inreply to another question, stated that 8 aircraft in Sector Awere equivalent to 10 in Sector B.

No detail is given of the train of reasoning that led tothis decision. It seems intuitively obvious that the workloadin any Sector will increase monotonically, but it is far fromobvious that the curves for two sectors wi l l never cross.Consider, for example, the speech load curves of fig. 1. Acontroller whose answer to questions comparing Sectors 5and 11 reflected the situation depicted in fig. 1. would havehad his responses deleted as inconsistent.

Ref. 9 does not reveal the percentage of the repliesthat were censored out of the data collected, though it ispossible to deduce from local irregularities in the samplesize (in Table III, for example) that at least 5% of theresults were rejected for one reason or another.

C o n c l u s i o n

The objects of air traffic control are stated to be "thesafe and expeditious movement of air traffic". There aredangers in accepting without question the assumption thatreduc t ion o f con t ro l l e r work load i s a use fu l i n te rmed ia testep in the search for more efficient ATC. When faced witha serious overload situation, the controller normally preserves air safety and his own sanity by slowing down thetraffic demand, by one means or another. In such a situation, tests on the controller loading may be an insensitivemethod of measuring the traffic delays which are ofprimary importance. It is unreasonable to expect that"contro l ler workload" can be defined or measured wi th theprecision customary in the physical sciences, but attemptsto date at defining, predicting or measuring controllerworkload can be termed succesful only i f judged byextremely relaxed criteria.

4. Rosenshine, M. "The Application of Automation tot h e S o l u t i o n o f A i r Tr a f fi c C o n t r o lP rob lems" .FA A T h i r d I n t e r n a t i o n a l A v i a t i o nR. & D Symposium — Automation inAir Traffic Contro l . November 1965.

5. Leplat, J. "Analyse des Processus du Troite-B i s s e r e t , A . m e n t d e L ' i n f o r m a t i o n c h e z l e C o n -

troleur de la Navigation Aerienne"Bulletin d'Etudes et Recherches Psy-chologiques XIV no. 1—2 pp. 51—67,1965.

Available in poor English translation in Controller 5 no. 1 pp. 13—22,January 1966.

6. Barnes, R. M. (Ed) "Motion and Time Study"Chapman & Hall 1949.7. Gillespie, J. J. "Dynamic Motion and Time Study"Paul EIek 1947.

9. Jolitz, G. D

8. Arad, Bar-Atid "Notes on the Measurement of Con-trol Load and Sector Design in theEn-route Env i ronment"FAA SRDS June 1964.

9. Johtz, G. D. "Evaluation of a Mathematical Model for use in Computing ControlLoad at ATC Facilities"FAA SRDS Report No. RD-65-69June 1965.

10. General Precision "Contract No. C/38/0/65 for the com-Systems pletion of an Arithmetical (Fast-

Time) Simulation Study".Final Report Vols. I—III.January 1968.

AcknowledgementThe author is indebted to Messrs. C. Dowling and

W. Feison of the FAA who spent some time discussing theArad model and its evaluation, to Mr. M. Rosenshine ofCornell Aeronautical Laboratory, and to ATCEU Hum forpermission to quote from their study of workload at LATCCWest Drayton.

Contributed by permission of the Director, R.R.E. Copyr igh t Con t ro l le r H .M.S.O.

R e f e r e n c e s

1. Arad, Bar-Atid,e t a l .

2 . R a t c l i f f e , S .

3. Chandler, G. A.

"Control Capacity and OptimalSector Design"FAA SRDS Interim Project ReportNo. 102-llR, December, 1963.

"Congestion in Terminal Areas"J. Inst. Navgn. 17, 183 (1964)

"ATC Capacities at Sydney Kings-ford Smith (Mascot) Airport andCont ro l l e r Sa tu ra t ion Leve ls " .J. Inst. Navgn. 18, 42 (1965)

Book Review

StromungsmeStechnik

By W. Wuest, German Languoge, Friedr. Vieweg & Sohn Braunschweig, ,9,9, 93, pp. clothbhlg DMz o , J U . ^

This book intends to give an introduction to the problems of aero-dynam.c testing and the appropriate use of instruments and measurement techniques. The author, a well-known authority in the Beld, givesfirst a description of the various types of wind tunnels from the lowspeed range up to hypersonic velocities. The fundamental techniques toso ve problems in oeroayncmic testing - the measurements of forces,velocities, pressures and temperatures - are then treated in a logicalsequence. Emphasis is on the instruments ond techniques which are usedmore frequently. The more specialized methods are mentioned and theoriginal literature is cited. Two chapters ore devoted to measurementsm VISCOUS flows (boundary layers) and turbulence measurements. Thebook IS completed by the description of the visualization techniques ofwater and air flows and the electrical and hydraulic analysis. The opticalmethods are stressed because of their importance in practical use.

This book IS the result of the experience of the author in the field inwhich he has been working for some twenty-five years and a series oflectures which he has given on the subiect. It is addressed to studentsand new-comers primarily. This nevertheless is very useful to the specialist smce It allows him to find further details easily. The list of olmosf600 reference papers represents another source of informotion.

H . U e b e l h a c k

2 2

The Improvement of Wet-Runway Operation

by Tirey K. VickersS e n i o r C o n s u l t a n t

One factor which con limit the acceptance rate of anairport is the time required for a landing aircraft to decelerate after touchdown to a speed at which it can safelymake a turn and exit from the active runway. This factor,which is sensitive to runway surface conditions, becomesparticularly important at locations where takeoffs andlandings must share the same runway.

When runways are wet, rollout distances and runwayoccupancy times tend to increase. One reason is that thepresence of water on the runway can lead to a conditionknown as hydroplaning, which can greatly degrade thebraking capability and the directional control of the aircraft during the landing rollout. There are three differenttypes of hydroplaning, as described below:

Viscous Hydroplaning (thin-film lubrication) can occurif the runway surface has been polished smooth by repeated landings and the coincidental buildup of repeatedlayers of burned rubber, as well as other contaminantssuch as soot and oil. In this case a very thin film of water,less than .001 inch deep, can keep the tire from contactingthe runway. Instead, the tire skims along the top of this film,where it cannot contribute either to the braking action orthe directional control or stability of the aircraff. This typeof hydroplaning can occur down to relatively low taxispeeds.

Reverted Rubber Hydroplaning can occur during aprolonged skid with a locked wheel. The resulting frictiongenerates heat at the point where the tire contacts therunway. When the rubber reaches a temperature between400 and 600 degrees F, it reverts back to its uncured (sticky)s t a t e .

Until recently it was believed that if there was water onthe runway, the reverted rubber could form a seal whichdelayed exit of the water from the tire footprint area —and that the high temperature within this pocket changedthe water instantly to high-pressure steam which lifted thetire a microscopic distance off the runway surface.

Recent research at the University of Michigan providesa completely different explanation for reverted rubberhydroplaning:

If the runway is wet, the combination of the wet film onthe runway with the reverted rubber forms a highly flexiblebearing surface which changes shape to "flow" over andaround irregularities in the runway surface, with very littlefriction. Even if the reverted rubber subsequently is cooleddown to the ambient temperature, the tire may continue toskid smoothly, down to a speed of five to ten knots.

A tire can produce a cornering or side force for lateralstability or steering only when it is rotating; thus a skid

with a locked wheel reduces its cornering or steering capability to zero.

Dynamic Hydroplaning can occur when there Is a layerof standing water on the runway surface. If the water cannot get out of the way of the speeding tire fast enough, itforms a liquid wedge which lifts the tire off the surface, asshown in Figure 1. Figure 2 shows how this wedge of waterreduces the footprint (runway contact) area of the tire,thereby degrading the braking action and increasing thestopping distance of the aircraft.

Test data incidates that the minimum dynamic hydroplaning speed of conventional aircraft tires, in knots, isabout 8.6 times the square root of the tire pressure, inpounds per square inch. For example, a typical executivejet aircraft uses 135 pounds pressure in the main tires and45 pounds pressure in the nose wheel tire. The calculatedhydroplaning speed for the main tires is approximately100 knots. However, if a high-speed turnoff is anticipated,it should be noted that the calculated hydroplaning speedof the nose wheel tire is slightly less than 60 knots.

If the runway is wet, a tire can hydroplane at any speedabove the value calculated above, which simply representsthe lowest speed at which dynamic hydroplaning can start.Once it has started, however, the condition can be sustain-

D - D I R E C T I O N O F A I R C R A F T; W - WAT E R L AY E R O N R U N WAY;L - L I F T F O R C E F R O M W E D G E O F WAT E R ( S H A D E D A R E A ) ;H - H E I G H T O F T I R E O F F S U R FA C E ( N O T E : H E I G H T E X A G G E R AT E D

F O R C L A R I T Y )

Figure 1 Dynamic hydroplaning

2 3

ed on down to a speed somewhat lower than the minimumstarting speed.

Hydroplaning has been the direct cause of a numberof accidents in which aircraf t have run off the end of therunway. Although there hove been relatively few fatalitiesfrom this cause, aircraft damage has been extensive. Ininstrument weather conditions on overrun of this typeoften wipes out the localizer antenna, which in turn mayclose the airport for o long period.

During hydroplaning, the reduced traction reduces thepilot's directional control of the aircraft. As a result, ahydroplaning aircraft tends to weathercock into any appreciable crosswind, skidding sidewise down the runway— or drifting off the downwind side into the mud, a situation which may require closing of the runway to othertraffic, until the unfortunate aircraft can be retrieved.

Erecting barriers or lengthening runways is not necessarily the answer to the wet-runway problem, as most

hydroplaning incidents occur off the side of the runway. Itwould appear more rewarding to develop a means of preventing hydroplaning, by obtaining more positive drainageof surface water from the pavement itself.

Most present runways are designed with a transversegrade (slope) up to IVj per cent, to drain surface waterto the sides, as shown in Figure 3. To obtain faster drainage, FAA engineers are now considering the possibility ofincreasing the maximum transverse grade to two per cent.

One of the most effective means of reducing hydroplaning problems is the use of runway grooving. The grooving consists of small slots cut across the runway to facilitatewater drainage and to improve tire traction. First tried inEngland in 1956, the concept has been implemented atseveral major U.S. airports with excellent results.

In the United States, most of the research on this subjecthas been conducted or sponsored by NASA. Figure 4 showsNASA's experimental grooved runway at Wallops Island,

Figure 2 Photos looking up through a glass runway, showing the footprint area of a 20 X 4.4 aircraft tire, under partial and total hydroplaningconditions. Vertical load = 500 pounds, tire pressure 30 pounds per

square inch, water depth = Vr inch. Tire motion is from left to right;tufts show direction of water displacement. Speeds in knots: A = 28,B = 5 6 , C = 7 1 , D = 8 8 . — N A S A P h o t o s

2 4

Virginia. Here a number of different groove patterns weretested under damp, flooded, and slushy conductions, andover a speed range up to slightly over 100 knots. Figure 5shows a typical test.

It has been found that transverse grooving tends toreduce the amount of time in which any type of hydroplaning can occur, as the grooving expedites drainage of thesurface water off the runway surface. This effect is quitenoticeable in comparing the amount of standing waterremaining on grooved and ungrooved portions of a runway after a shower; the grooved portions tend to dry offimmediately. This greatly reduces the amount of spraythrown up by aircraft during takeoff or landing.

Runway grooving con prevent viscous hydroplaning byproviding a continuous series of edges on which the tire cangrip, in order to secure positive traction, even though therunway surface may be covered by a thin film of water aswell as other contaminants. Grooving can also reducereverted rubber hydroplaning, as a locked wheel condition

Figure 3 Cross-sect ion o f typ ica l runway showing t ransverse grade(slope) G for drainage.(Drawing not to scale)

is less likely to occur on a runway which has a uniformlyhigh coefficient of friction. The provision of additionalgripping surfaces also tends to start the tire rolling again.

Runway grooving con eliminate dynamic hydroplaningby providing multiple escape paths for the water beneaththe tire, thus preventing buildup of the type of high-pressure wedge shown in Figure 1.

2 5

- - ' • - * ^ a i ' 1

f S f ^Figure 5 Surf's up!

NASA tests made with a Convoir 990 iet transport ongrooved portions of o flooded runway, with 'A inch ofwater above the grooves, and using smooth tires, indicatethat braking is identical to dry-runway conditions at runway speeds below 106 knots. These results have been confirmed by aircraft users at Washington National Airport,Kansas City Municipal Airport, J. F. Kennedy Airport, andChicago Midway Airport.

Comporotive tests made on grooved and ungroovedrunway surfaces hove shown that the grooving tends toproduce more uniform friction (less variation in the friction

— NASA Photo

coefficient) than ungrooved surfaces. This is particularlyimportant in the operation of heavy aircraft, since most ofthem utilize onti-skid systems based on the principle ofreducing the brake pressure (to prevent wheel lock) whenever the system senses an incipient skid condition. The moreuniform friction characteristic of the grooved surfaceallows the braking system to maintain a constant brakingtorque, as opposed to the intermittent torque required fora smooth stop on on ungrooved wet runway. The ability touse constant torque tends further to reduce the stoppingd i s t a n c e .

2 6

Figure 6 Maximum-traction grooving,spacing 1" (25 mm), width Vi" (6 mm),depth 'A" (6 mm). — NASA Photo

The greater the cross-sectional area of the groove, themore wa\er it con hold before overflowing. If the grooveis too wide, however, it will trap stones and small debris,creating a housekeeping problem. The narrower the spacing between adjacent grooves, the faster the runway surface will be dried off. If the spacing between grooves istoo narrow, however, its top surface is more subject toshear damage.

Of the various groove configurations tested so far, theV4 inch wide and V4 inch deep, spaced one inch betweencenters, as shown in Figure 6. Figure 7 compares variousoperational grooving installations.

The cost of grooving existing runways varies over awide range. One variable is the total volume of materialwhich has to be removed. Another var iable relates to therestrictions imposed by aircraft operations. For example,if the work has to be done during night or early morninghours, in order to avoid traffic peaks, the labor cost is likelyto be considerably high than if the work could be doneduring regular daytime hours. Similarly, if the groovingequipment has to be pulled off the runway periodically, inorder to permit resumption of aircraft operations, theadditional labor and waiting time will increase the cost.

Runway 18—36 at Washington National Airport was thefirst operational runway to be grooved by the FAA. The jobrequired thirty-five days of work during the off-peak hoursbetween 2300 and 0700 local time. Diamond saw machineswere used; each machine cut thirteen grooves simultaneously, into the asphalt surface, at a cost of about S 0.09per square foot.

Although the number of aircraft operations which hovebeen mode on this grooved runways is now approachingone million, so far there has been surprisingly little deterioration of the groove pattern. In the touchdown zones,some of the patterns have been distorted slightly, but thishas been due to shifting between the upper and lowerlayers of the pavement, rather than to movement of thegrooved portion relative to the upper layer.

One problem which is involved in cutting grooves intoan existing runway is the need to flish out and dispose ofthe large amount of abrasive waste which is produced bythis process. Otherwise, this material can create a dust (orslurry) problem which can be damaging to jet engines andaircraft wheel-well components. One of the newest typesof grooving machines has an automatic clean-up feature,to take care of this problem.

Ultimately, it may be desirable to develop a means ofcasting or rolling the grooves into new runway surfacesinstead of having to grind them out at some later date.

High-speed photographs of tires encountering groovesindicate that under heavy loads the tire tread pushes downinto the grooves. This allows the tire to get a bite on therunway, for traction purposes; however, it also gives therunway a chance to bite back. As a result, some tires whichhave been subjected to prolonged operation on groovedrunways have shown a pattern of tiny transverse orchevron-shaped cuts on the tread.

It is conceivable that certain grooved configurationswill result in less tire wear than others. Additional research

may be desirable, particularly for those airports whererunway length is not a problem, to determine the optimumcompromise between tire traction and tire wear. Meanwhile, it appears to be the general concensus of the U.S.airline operators that a slight amount of additional tirewear is more than justified by the protection which the

A I R P O R T

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1/4" 1/4" 9 0 *

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JUS us a i r p o r t s .

grooving provides against a catastrophic hydroplaninga c c i d e n t .

Because an aircraft tire may be recapped ten or moretimes during its useful life, there was some initial concernas to whether the vibrations produced in riding over thegrooves would create a fatigue problem with the tire cords.The preliminary data from recent tests indicate that thereis no appreciable difference between the stresses producedin the cords of tires running on grooved and ungroovedr u n w a y s .

Initially, some of the aircraft manufacturers were concerned about another aspect of runway grooving —whether the touchdown conctact of a non-rotating wheelwith, a grooved runway during the landing would producea much greater spin-up drag load on the landing gear, thanif the aircraft were landing on an ungrooved runway(Spin-up drag is the rearward force transmitted to theaxle when the wheel touches down and accelerates to thelanding speed). The preliminary data from a recent testprogram indicates that the difference in the runway surface (grooved or ungrooved) makes no appreciable difference in the spin-up drag.

The overall effect of runway grooving is to permit a wetrunway to approach the braking capability of a dry runway. Up until the present time, the FAA has made noallowance for the presence of grooved runways, as far asrunway length is concerned. The present regulations requirea runway length adequate to allow a full-stop landing(based on the aircraft type certification tests) within60 percent of the effective length of the runway.

Beginning in 1966, Federal Air Regulation 121.195 (d)has required an additional 15 percent runway length foroperation into wet or slippery runways. There is now considerable evidence that the original 15 percent allowancewas not realistic, and should be increased for turbojettransport aircraft landing on smooth wet runways. If thisincrease is made, it is expected that it will apply to ungrooved runways, but not to adequately grooved runwaysu n d e r w e t c o n d i t i o n s .

2 7

Because the elimination of hydroplaning restores theability of aircraft tires to provide side forces for lateralstability and steering, it is hoped that the installation ofgrooving will permit the authorized maximum crosswindcomponent for wet runways to be increased to the limitallowed for the same runways under dry conditions.

Runway grooving offers definite advantages from thestandpoint of airport capacity. With ungrooved runways,braking d istances are increased s ignificant ly in wetweather. High speed runway exits which are ideally placedfor dry weather operation may be 1,000 feet or more tooclose to the touchdown point to be used by the same aircraft in wet weather, because of reduced braking action.In addition, if pilots anticipate hydroplaning conditions,they will not start any turnoff until well below their minimum hydroplaning speed.

All this naturally increases runway occupancy time, soair traffic controllers have to increase approach intervalsaccordingly, when such problems are anticipated. Thisreduces airport capacity and tends to increase trafficdelays.

In surveys made at Washington, Kansas City and Kennedy Airports, the majority of the controllers who werepolled, definitely felt that runway grooving aided mostpilots in controlling their landing run, and that the turn-off point from a wet grooved runway was identical in mostinstances to that for dry operations on the same runway.Controllers reported that this definitely improved runwaytraffic management, and increased the acceptance rateover that possible with the original ungrooved runway inwet condi t ions.

Any accident in which a large aircraft slides off the sideof a runway, or overshoots the end, can cause a criticaltie-up of the runway, and sometimes the entire airport, forlong periods. The disruption caused by a single accidentcan be very costly in terms of airport traffic capacity andr e v e n u e .

The elimination of hydroplaning should practicallyeliminate accidents caused by wet runways. Besides thisobvious advantage for safety, runway grooving can helpcontrollers to maintain airport traffic capacity in wetweather cond i t ions .

Terminal Area Approach Control SequencingPaper to the United Kingdom Symposium "Electronics for Civil Aviation", 1969*

by F. J. Crewe &T. E. FosterAirspace Control DivisionE l l i o t t A u t o m a t i o n L i m i t e d

A major airport could be defined as a place whereseveral airways each ten miles wide and about 18,000' highare compressed into a piece of airspace 300' wide, zerofeet high with a lower surface made of concrete. Anyonewho has spent hours carving out a racetrack in the skyover Southern England, New York, Chicago or any otherbusy terminal control area will agree that there are timeswhen the quart will not fit into the pint pot. There are twoways of eliminating this problem, but each has severepractical limitations. The first is to build more airports —controversial, expensive, often impracticable — and thesecond is to reduce or practically eliminate the separationminima — possible only by the application of area navigation methods to VTOL aircraft.

So the problem will be with us for many years, and wemust think in terms of reducing it rather than removing it.

With modern jet aircraft, the practical limit of separation on final approach is about 3 miles or 90 seconds. If wewish to work on these limits, then accuracy in achievingthem becomes of paramount importance. Ten seconds toolittle and an overshoot may mean the loss of a landing slot.

Reprinted withof Technology

of the authors end the U.K. Ministry

Ten seconds too much and one landing in nine will be lost.Thus the separation standard and the tolerance on it couldbe written as 90 seconds, —0 -f 5. If it were possible tomaintain this separation to these limits, then in a fairlyshort time, great savings in holding time, airline moneyand controller wear and tear could be effected. Unaided,however, the controller cannot work to this accuracy, forreasons both subjective and objective. Subjectively, theneed to avoid an overshoot or potentially dangeroussituation will introduce an element of caution. Objectively,the controller cannot observe deviations from plannedflight path or speeds quickly enough or accurately enoughto guarantee making a five or ten second time slot.

Let us look at the basic problem and the way in whichit must be solved. From some point in the terminal controlarea, an aeroplane has to be directed to touch down toarrive there at a specific, closely defined time. To do thisthe aircraft must fly a flexible stretching flight path at aknown speed, and its progress constantly assessed andcorrections made. This flight path could easily be 40 mileslong in a fairly small TMA and will involve a descentthrough at least 5,000 ft., so that many changes in windvelocity — including shear conditions — could be encountered. Intermediate approach may start from an illdefined point in a holding pattern commencing with a

2 8

large turn. At any one time in a small TMA there maybeup to twelve aircraft on intermediate approach all descending towards ground level and all requiring separation— possibly in IMC.

How could a computer help to solve these problems?The first requirement must be that it has a complete recordof post and present position and some means of identifyingthe aircraft. The most straightforward method of achievingthis will be to use secondary radar, since the carriage oftransponders in this environment will become mandatoryvery shortly. A system of allocating a unique identity codewill be required and all confusion could be avoided if anunallocated mode were utilised for this purpose. Afterbeing decoded in a conventional decoder, the aircraftreplies will be processed in a plot validation unit whichestablishes the validity of a train of replies and the "centreof gravity" of the plot, correlates it with the mode andpasses the data to the computer for processing into thesystem.

The computer will hold in store performance data on allthe most common aircraft using the TMA and the meteorological data, particularly the wind velocity distributionover the area including variations in the vertical plane.

From any point in a terminal control area, and withinthe limits imposed by the dimensions of the area, a flightpath of any length can be drawn to the runway touchdown point. This will consist of two straight legs — intermediate approach and final approach. Thus the time totouchdown of an aircraft of given performance in givenwind conditions is theoretically dependant only on theheading given for the intermediate leg. This determinesthe length of both ports of the flight path. The progress ofthe aircraft can be monitored constantly during the intermediate phase and the heading changed as required bysuch circumstances as change of wind velocity and pilotinaccuracy.

Before discussing in any more detail the system requirements, we must decide how the operational requirementsand procedures can be integrated with the current manualsystem. From these considerations we can derive the man/machine communications requirements and then data anddata arrangement and display needs.

The first assumption is that the system is working to therunway capacity. This means a regulation of flow at somepoint or points such as holding areas either inside or outside the TMA. Consider first the procedures which do notinvolve holding within the TMA. As each aircraft enters thesystem., its position in the sequence is allocated by the No. 1director and the callsign and type entered by means of akeyboard and positionally identified by Rolling Ball. Thecomputer then allocates an SSR identification mode whichis passed to the aircraft, and a landing time slot. The computer then stores the aircraft position derived from its SSRreturns and computes its groundspeed and track. It estimates from stored performance and meteorological data,the flying distances required to make good the time slotand divides this into three sections — continuation on present heading, intermediate approach and final approach.It is absolutely essential that the aircraft be flown veryaccurately with regard to heading and speed so that thistrack distance can be accurately related to landing time.

The computer assumes a mean controller/pilot reactiontime and, on the tabular data display, gives a countdownfrom 30 seconds before displaying a heading change fromone section of the flight path to the next.

For many reasons, the aircraft will deviate from theideal flight path. The computer will be comparing continually the actual and ideal positions and recalculating newflight paths to ensure that the allocated time slot is met.The changes in heading required will be displayed to thecontroller and will flash to attract attention. The computerwill however, be working in terms of accuracy, at limits towhich no pilot could fly. The controller will interpret thecomputer demands and pass reasonable instructions to thepilot. Normally, the aircraft will fly at the speed close totheir performance data held by the computer, which will bedivided into two categories — intermediate approach andfinal approach. It will be possible, however, for the computer to demand speed changes within very narrow limits,if these ore required to maintain the timetable.

It should be noted that although time at touchdown isthe critical factor in the system, the problem is eased somewhat by the fact that the computer need consider the timingonly as far as a "gate" point on final approach probablyabout six miles from touchdown. After this point, the aircraft will fly the ILS localizer course at constant speed.Successive aircraft will be affected by very nearly the samewind effects, so that providing their arrival time at the gateis correct, their landing time will be correct.

No mention has been made so for of aircraft separationor descent. In fact, the computer will arrange the sequencing working in only two dimensions. Since all aircraft willcommence intermediate approach with height separation,descent timing is left to the controller who can ensure thatvertical separation is maintained.

Once an aircraft enters a holding pattern, the maintenance of track and groundspeed records in the computeris not really necessary. Instead the computer will issue aleaving clearance in terms of time and heading with whichthe pilot will be expected to comply. Any errors producedby this fairly difficult piece of flying v/ill be readily absorbed OS they occur very early in the procedure.

In the best of regulated systems, overshoots will occur,and aircraft in emergency will demand to be fitted into thesystem out of turn. Some method of entering a resequenc-ing requirement will therefore be necessary. In the case ofan overshoot, the aircraft can be reintroduced in a fairlyleisurely way at a point which will delay but not requirererouting of other traffic. In other words it can follow thelast aircraft to hove been turned from its heading towardsthe clearance l imit.

For an emergency, however, the controller must firstinform the computer of the point at which he wishes tobreak the current sequence. The aircraft following thispoint will require to have their flight paths stretched, possibly by an orbiting maneouvre. It could well be that thisaircraft in an emergency has just taken off, so that thecomputer has no knowledge of it, and it would hardly bedesirable to bother the pilot with the selecting of an SSRcode. This recovery will therefore be carried out manuallyand the controller can readily make an estimate of thedelay. This suggests the method that can be used to introduce the appropriate delays into the system. Having selected the first of the aircraft to be delayed, the controller willinput with his keyboard, two digits to identify the aircraft(the final two of the SSR identity) three digits to indicatethe new time slot, and a "Delay Until" function key. Thetime slots of all subsequent aircraft will be correspondinglyadjusted and displayed, and the appropriate controlactions required.

29

D V O R S - S y s t e m

The new DVORS-Systemc o m b i n e s t h e m o r e t h a n25 years' experiences of SELin research, development andproduction of navigational aidsw i t h t h e e x c e l l e n t r e s u l t s o f t h e

tube-type DVOR-Systemsachieved in operation, bothnow applied to the neweststate of modern techniques.

Higher ACCURACY andhigher RELIABILITY, the maincomponents of the newgeneration of our groundfacilities, are guaranteed withthe DVORS-System.

Comparison between signalsreceived from a VOR and- af ter modificat ion - f rom aDVOR on same location, course,a l t i t u d e a n d d i s t a n c e .

THE DVORS-SYSTEM IS ANESSENTIAL PART OF THEWORLDWIDE AIR NAVIGATION

For further information contact:Standard Elektr ik Lorenz AGT r a n s m i s s i o n a n dNavigation Division42, Hellmuth-Hirth-Str.7000 Stuttgart 40(West Germany)

I T Tstandard Elektrik Lorenz AG Germany

The In te rna t i ona l Fede ra t i onof A i r Traffic Contro l lers Assoc ia t ions

Addresses and Officers

A U S T R I A

Verband Osterreichischer FlugverkehrsleiterA 1300, Wien Flughafen, Austria, Postfoch 36P r e s i d e n t A . N o g yV i c e - P r e s i d e n t H . K i h rS e c r e t a r y H . B a u e rD e p u t y S e c r e t a r y W. S e i d IT r e a s u r e r W . C h r y s t o p h

B E L G I U M

Belgian Guild of Air Traffic ControllersAirport Brussels NationalZaventem 1, BelgiumPresidentVice-PresidentSecretarySecretary GeneralT r e a s u r e r

E d i t o r

IFATCA Liaison Officer

A . M a z i e r s

M . v a n d e r S t r a a t eC . S c h e e r s

A. DavisterH. CampsteynJ. MeulenbergsJ. Aelbrecht

C A N A D A

Canadian Air Traffic Control Association56, Sparks StreetRoom 305Ottawa 4, CanadaP r e s i d e n t J . D . L y o nF i r s t V i c e - P r e s i d e n t R . M c F a r l a n eSecond Vice-President D. M. DiffleyM a n a g i n g D i r e c t o r G . J . W i l l i a m sT r e a s u r e r A . C o c k r e m

SecretaryT r e a s u r e rI F A T C A L i a i s o n O f fi c e r

F R A N C E

R. JdrvinenH . P u l l i n e n

F. L e h t o

French Ai r Traffic Contro l Assoc iat ionAssociat ion Profess ionnel le de la Ci rcu la t ion Aer ienneB. P. 206, Paris Orly Airport 94F r a n c e

P r e s i d e n t

F i r s t V i c e - P r e s i d e n tS e c o n d V i c e - P r e s i d e n t

General SecretaryT r e a s u r e r

Deputy SecretaryDeputy TreasurerI F A T C A L i a i s o n O f fi c e r

Francis ZammithJ . M . L e f r a n cM . P i n o n

J . L e s u e u r

J. BocardR. PhilipeauM . I m b e r tA . C l e r c

G E R M A N Y

G e r m a n A i r T r a f fi c C o n t r o l l e r s A s s o c i a t i o nVerband Deutscher Flugleiter e. V.3 Hannover-Flughafen, GermanyPostlagerndP r e s i d e n t W . K a s s e b o h mV i c e - P r e s i d e n t H . G u d d a tC h a i r m a n A T C E . v o n B i s m a r c k

C h a i r m a n M I L W . E h r h a r dC h a i r m a n A I S W . K r o n c k eS e c r e t a r y H . J . K I i n k eT r e a s u r e r K . P i o t r o w s k i

C Y P R U S

The Cyprus Air Traffic Controllers AssociationCivil Aviation Dept.Nicosia, Cyprus

D E N M A R K

Danish Air Traffic Contro l lers Associat ion

Copenhagen Airport — KastrupD e n m a r k

C h a i r m a n

V i c e - C h a i r m a n

SecretaryT r e a s u r e r

IFATCA L ia ison Officer

E . T. L a r s e nO . C h r i s t i a n s e n

E. ChristiansenM . J e n s e n

V. F r e d e r i k s e n

F i n l a n d

Associat ion of F inn ish Ai r Traffic Contro l OfficersSuomen Lennonjohtajien Yhdistys r. y.A i r Tra f fic Con t ro lHelsinki LentoF i n l a n d

C h a i r m a n V . S u h o n e nV i c e - C h a i r m a n N . T o r h o n e n

GREECE

Air Traffic Cont ro l le rs Assoc ia t ion o f Greece10, Agios Zonis Street, Athens 804, GreeceP r e s i d e n t C . T h e o d o r o p o u l o sV i c e - P r e s i d e n t N . P r o t o p a p a sG e n e r a l S e c r e t a r y E . P e t r o u l i a sT r e a s u r e r S . S o t i r i a d e s

H O N G K O N G

Hongkong Air Traffic Control AssociationHongkong AirportP r e s i d e n t K . M a l c o l m

S e c r e t a r y M . A . W i g h t m a nP. LeungE . C o l l i e rI F A T C A L i a i s o n O f fi c e r

I C E L A N D

A i r T r a f fi c C o n t r o l A s s o c i a t i o n o f I c e l a n d

Reykjavik Airport, IcelandC h a i r m a n G . K r i s t i n s s o n

S e c r e t a r y S . T r a m p eT r e a s u r e r K . S i g u r o s s o n

31

I R A N N O R W A Y

I r a n i a n A i r T r a f fi c C o n t r o l l e r s A s s o c i a t i o n Lufttrafikkledelsens ForeningM e h r o b a d I n t e r n a t i o n a l A i rpor t Box 51, 1330 Oslo Lufthavn, NorwayTeheran, Iran C h a i r m a n G. E. NilsenSecretary General E. A. Rahimpour V i c e - C h a i r m a n K . C h r i s t i a n s e n

Secretary J . K a l v i kT r e a s u r e r E. Feet

I R E L A N D

I r i s h A i r T r a f fi c C o n t r o l O f fi c e r s A s s o c i a t i o n R H O D E S I AA T S S h a n n o n Rhodes ian A i r Tra ffic Cont ro l Assoc ia t ionI r e l a n d Private Bag 2, Salisbury Airport RhodesiaP r e s i d e n t J. E. Murphy P r e s i d e n t C . W . D r a k eGen. Secretary J . K e r i n Secretary C. P. F l ave l lT r e a s u r e r P. J.O'Herlihy T r e a s u r e r W . V a n d e w a a lAsst. Gen. Secretary M . D u r r a c k

S W E D E N

I S R A E LS w e d i s h A i r T r a f fi c C o n t r o l l e r s A s s o c i a t i o nFack 22, 1 90 30 Sigtuna, Sweden

A i r T r a f fi c C o n t r o l l e r s A s s o c i a t i o n o f I s r a e l C h a i r m a n Fl. JelveusP. O. B. 33 Secretary A. KarlahogLod Airport, Israel T r e a s u r e r G . KanhamnC h a i r m a n W . K a t z IFATCA Representative B . F l i n n e r s o n

V i c e - C h a i r m a n E. MedinaT r e a s u r e r D . F u r r e r S W I T Z E R L A N D

Swiss A i r Tra ffic Cont ro l le r s A s s o c i a t i o n

P. O. Box 271I T A L Y CH 1215, Geneva Airport, SwitzerlandA s s o c i a z i o n e N a z i o n a l e Ass is ten t i e Cont ro l lo r i C h a i r m a n J . D . M o n i n

della Civil Navigazione A e r e a I t a l i a IFATCA Secretary T. R o u l i n

V i a C o l a d i R i e n z o 2 8 L ia i son O ffice rRome, Italy for Zurich Airport J . G u b e l m a n n

P r e s i d e n t Dr. G. Bertoldi, M. P.Secretary L . M e r c u r i T U R K E Y

T r e a s u r e r A . G u i d o n i Tu rk i sh A i r Tra ffic Con t ro l A s s o c i a t i o n

Yesilkoy Airport, Istambul, TurkeyPres iden t Alton Koseoglu

L U X E M B O U R G

Luxembourg Guild of Ai r T r a f fi c C o n t r o l l e r s U N I T E D K I N G D O M

Luxembourg Airport Gu i ld o f A i r Tra ffic Con t ro 1 O f fi c e r s

P r e s i d e n t A . K l e i n 14, South Street, Park LaneSecretary H . T r i e r w e i l e r London W 1, EnglandT r e a s u r e r J. Ronk M a s t e r W. E. J. Groves

Executive Secretary W . R i m m e r

T r e a s u r e r E . B r a d s h a wN E T H E R L A N D S

N e t h e r l a n d G u i l d o f A i r• T r a f f i c C o n t r o l l e r s U R U G U A YPostbox 7590 A s o c i a c i o n d e C o n t r o l a d o r e s

Schiphol Airport Centra1, Nether lands Aeropuerto Nacional de G□ r r a s c o

P r e s i d e n t T h . M . v a n G a a l e n Tor re de Con t ro lSecretary F. M . J . M e n t e Montevideo, UruguayT r e a s u r e r P. K a l f f C h a i r m a n U . P a l l a r e s

Member, Publicity A . V i n k Secre ta ry J . B e d e r

Member, IFATCA-affairs B . H . v a n O m m e n T r e a s u r e r M . P u c h k o f f

Y U G O S L AV I AN E W Z E A L A N D Jugoslovensko Udruzenje Kontrolora LetenjaAi r Tra ffic Cont ro l Asso ie l a t i o n Direkcija Za Civiinu Vazdu s n u P l o v i d b u

Dept. of Civil Aviation, i3th Floor, Dept. BIdgs. Novi Beograd, Lenjinov Buievar 2, YugoslaviaStout Street P r e s i d e n t A. StefanovicWellington, New Zealari d Vice -Pres iden t Z . Ve r e s

Pres ident E . M e a c h e n Secretary D. Zivkovic

Secretary C . L a t h a m T r e a s u r e r D . Z i v k o v i c

IFATCA L ia i son O ffice r G. N. McLindon M e m b e r B. Budimirovic

3 2

control in trainingElliott airspace controi -

first name in digital radar simulationOur unrivalled experience in this field enables us to offer fully developed training systemswhich provide a reaiistic environment for Airways, Area, Approach & Terminal Control tasks.

The simulator may be readily integrated with allother sub-systems of a control unit or complex.A l t e rna t i ve l y i t can s imu la te o the r sub -sys temsinterfaces when required.The established Elliott range includes all threeforms of simulation system:-(a) Autonomous installation - Training school(b) Add-on sub system | - Continuation and(c) In-built facility j Conversion trainingFor de ta i l s p lease con tac t :

H e r e a r e s o m e o f t h e f e a t u r e s w h i c h m a k e E l l i o t t

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■ Realistic radar responses ■ Authentic trackbehaviour ■ Track capacity to suit customerenvironment ■ Any form of display presentationmay be employed ■ Simple exercise preparation■ Sound ergonomic interface between man andmachine ■ Individual task supervisory facilities■ Modular construction using standard hardwareunits ■ Full range of proven software packages■ Facilities for operational and statistical analysis■ A tool for system evaluation and developmentB General purpose off-line computing facilitiesB Optimization oftraining time,standardsandcosts

Elliott Airspace Control DivisionMarconi Radar Systems Ltd..Elstree Way, Borehamwood. Herts. U.K.Telephone 01-953 2030 Telex 22777(Member of the G.E.C.-Marconi Electronics Group)

AIR TRAFFIC CONFUSION OR. . .

' ■m f

r k ^ "

i J tT f

I t

The answer to increasing air traffic confusion is an accurate, comprehensive, automatic and reliable Nav/ATCsystem incorporating a Data Link.Decca-Harco is the only system that can meet thenavigation and ATC demands of both sub- and supersonic air traffic. And only Decca-Harco can provide theflexibility and accuracy that permits close lateral separation of aircraft throughout the route structure.At the control centre the Decca Data Link providesthe controller with accurate displays of the identity, altitude and precise position of all co-operating aircraft, ■using the common reference of a high accuracy, areacoverage system The necessity for R/T communicationis reduced by the use of two-way Alpha-Numeric messages and routine reports are eliminated, reducing thework load and increasing the reliability of the ATC system.

On the flight-deck Decca Omnitrac—the world's mostadvanced lightweight digital computer—provides thepilot with undistorted pictorial presentation and automatic chart changing. The 'ghost beacon' facility giveshim bearing and distance to any point. Omnitrac alsoprovides auto-pilot coupling and automatic altitude control which maintain respectively any required flightpath and flight profile. The ETA meter indicates either timet o d e s t i n a t i o n o r E TA .

It is only through an integrated system, operating from acommon reference, such as Decca-Harco, that a greatmany aircraft of different types flying at various speedsand altitudes can be efficiently co-ordinated into a singledisciplined traffic pattern.

ECCA-HARCOThe comprehensive Nav/ATC systemThe Decca Navigator Company Limited ■ London