aircraft instruments— types and cockpit layout

Aircraft Instruments— Types and Cockpit

Layout 22

2.1 Introduction

An aircraft is a very complex machine, which has to be monitored and controlled, either manually by the pilot(s) or by the dedicated fl ight control computers. In the latter case, however, pilot(s) will have the fi nal authority to fl y the aircraft. The modern aircraft has a large number of trans-ducers which convert physical parameters such as airspeed, altitude, attitude, temperatures, engine para meters, etc., and present them to the pilot in the most convenient way for him to see, compre-hend and act to fl y the aircraft in a safe and purposeful manner. In the early days, a scarf around the pilot’s neck provided vital data on the attitude (pitch, roll and yaw) of the aircraft, angle of attack, side slip, etc.

Arrangement of instruments in the cockpit is such that pilot very naturally sees effortlessly most vital and fl ight-critical data, namely, airspeed, altitude, attitude and vertical speed. Such data are presented to the pilot just below the wind shield in the main instrument panel which is discussed in more detail later in this chapter. It is also very important, that he comprehends the data he sees with minimum mental effort. Therefore, it is necessary to arrange the instruments, taking into account the importance of the data as well as their location, and the format of the displays. This becomes the more important in military fi ghter aircraft capable of fl ying at low altitudes and high speeds.

This deals with broad overview of aircraft instruments—their type and location and cockpit lay-outs of modern aircraft.

AIRCRAFT INSTRUMENTS—TYPES AND COCKPIT LAYOUT 11

panel”—meaning pilots do not rely on outside cues which could be misleading; instead, they are trained to rely always on aircraft instruments, keeping blind eye to the view outside.

In basic-six instruments, Gyro Horizon, which shows the aircraft’s attitude (displaying pitch, roll and yaw motions of aircraft) occupies the central top position. By far this is the most important aircraft instrument which is relied upon by the pilots. The attitude of the aircraft, in turn depends on the air speed and aircraft’s vertical speed, hence they are positioned on the two sides of the Gyro Horizon as shown in Figure 2.2.

Another important instrument is the Direction Indicator (DI) which gives to the pilot, in which direction, the aircraft is heading. Directional changes are achieved by rolling (banking) and yaw-ing (turning) the aircraft. Hence very naturally, a Turn and Bank Indicator is positioned at the right side of DI. To the left of DI is situated the Altitude Indicator (ALTI) giving information of fl ight level, usually in fl ight level units of 100 feet; for example a fl ight level of 300 represents an altitude of 30,000 feet.

Another grouping of instruments is known as “ basic T”, which is of more recent origin than the “basic six”, and is shown in Figure 2.3. This is the present standard even in modern aircraft of recent origin. In larger civil transport aircraft both pilot and co-pilot have independently, such identical display elements.

After reviewing modern instruments, requirements of newer generation of aircraft, and pilot feedbacks, designers arrived at this “basic T” grouping of most important fl ight instruments required for safely fl ying the aircraft, without much effort and eye scan by the pilots. Most important indicators are: Air Speed Indicator (ASI), Attitude Director Indicator (ADI) and Alti-tude Indicator—ALTI, which from the horizontal bar of T. Horizontal Situation Indicator (HSI) is at the centre and makes up the vertical bar of T, as shown in Figure 2.3. HSI gives direc-tional information to the pilots. On the right side of HSI is located a Vertical Speed Indicator (VSI), which gives rate of climb or descent of the aircraft—not really so fl ight critical. A Radio

ASI

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AltitudeIndicator

DirectionIndicator

Turnand BankIndicator

GH VSI

Air SpeedIndicator

VerticalSpeed

IndicatorGyro Horizon

Fig. 2.2 Basic six grouping of aircraft instruments in earlier aircraft–now outdated.

AIRCRAFT INSTRUMENTATION AND SYSTEMS12

Magnetic Indicator (RMI) is located on the left side of HSI, and gives: (i) magnetic heading derived from fl ux gates which detect the direction of the aircraft with reference to earth’s mag-netic fi eld and (ii) bearings to two radio stations, located on ground, hence the joint name Radio and Magnetic Indicator—or Radio Magnetic Indicator. In some designs, RMI position is fi lled up by a TBI—the Turn and Bank Indicator or by another instrument of recent origin called as Turn coordinator (TC).

In aircraft manufactured after 1980, more sophisticated, all Electronic Flight Instruments Systems (EFIS) replace individual ADI and HSI. Present day aircraft (2009) use just one AMLCD colour monitor for each of the pilot and co-pilot, located directly in front of them. A third shared colour monitor displays all Engine Indicators and Crew Alert System (EICAS). Such monitors replace a large number of cluster of instruments, which makes pilot invest considerable effort and eye scan movements to see, understand, analyse and take consequential steps for safe and desired fl ight of the aircraft. All the computer-generated dial instruments follow the “basic T” confi guration. The on-board computers automatically decide and select as to which instruments need to be presented to the pilot on a “need-to-know” basis, depending on the phase of the fl ight. There are various well-identifi ed phases of fl ight such as ground taxiing from departure point, take off, climb, cruise, descent and ground taxiing to arrival terminal.

Pilot(s) should at all times be able to easily read and interpret the data presented to him by aircraft instruments in order to either maintain the aircraft in a steady and stable condition of fl ight or changing conditions of fl ight through manoeuvres. Aircraft instruments and display systems play an extremely important role in assisting the pilot to fl y the aircraft safely and in a desired attitude.

We now consider the aircraft display systems. Figure 2.4 shows the classifi cation of displays along with some examples.

The qualitative and quantitative displays are further described.

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VerticalSpeed

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ADI ALTIAir SpeedIndicator

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Fig. 2.3 Basic T arrangement of aircraft instruments.


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2.3 Aircraft Display Types

Aircraft displays form an important link between the pilots and the aircraft (man–machine loop). Some of the more important requirements of the display system are:

1. They must be easy to interpret. 2. The display should be unambiguous. 3. They must follow natural sense of pilots. 4. Reliability should be very high. 5. Pilot effort should be minimum to read and absorb data content. 6. Accuracy of indication should be high. 7. Adequate sensitivity is required to sense small deviations. 8. Repeatability should be high to reduce repeated calibration efforts.

There are broadly two types of display:

1. Quantitative display, and 2. Qualitative display.

2.3.1 Quantitative Displays

In this type of displays, the data is displayed quantitatively as numbers either using a pointer-scale instrument or using an alphanumeric LED/LCD type numeric displays. Examples of quantitative displays are: air speed indicator, altitude indicator, vertical speed indicator, etc. All of them pro-vide numeric display of concerned parameters. Some examples of quantitative displays are shown in Figure 2.5.

The circular scales are good for ASI, ALTI, engine oil pressure and temperature, etc. Sometimes clustered straight scale as in Figure 2.5(b) is ideal, for example, to indicate exhaust gas tempera-tures (EGT) of a 4-engined large transport aircraft. A quick glance shows how the temperatures vary in comparison with each other and to know if any particular engine is malfunctioning and its EGT is wildly straying off the normal value.

In circular scales the range can be extended by having a dynamic counter as in Figure 2.5(d) for an altimeter. One full rotation of pointer advances the counter main scale by one. For example, the reading shown is 34,400 feet above sea level, and after the main pointer increases to full scale of 10, the main counter reaches 35,000 feet. In addition, there is a static counter at the bottom of the circular scale. The static counter is used to adjust the atmospheric barometer pressure to the appropriate ambient pressure value by using the BARO knob located at the bottom left of the instrument. More on altimeter will be covered in subsequent chapters. Such BARO corrections are required frequently as will be explained later.

The high range can also be accomplished by having another smaller circular scale and pointer as shown in Figure 2.6. The full range, here is split into two concentric scales; the inner scale is an extension of outer scale. A common aircraft instrument using this type of instrument is the engine speed indicator—the large outer scale has a multiplication factor of ×100 rpm, while the

AIRCRAFT INSTRUMENTATION AND SYSTEMS


inner scale has a ×1000 rpm. These instruments are however replaced by modern digital, unam-biguous indicators, where RPM value is digitally displayed with no room for confusion.

Linear and Non-Linear Scales

Some physical parameters are non-linear and therefore direct linkage results in a non-linear scale. For example, air speed measures a differential pressure q = ( pt − ps), where pt is the total pilot pressure and ps is the static pressure and q = ½ ρv 2, where ρ is atmospheric density and v is air speed which is being measured. The capsule responds to the pressure only. Figure 2.7 shows capsule defl ection vs. air speed, which of course follows square law. Capsule defl ection is propor-tional to the pressure and pressure in turn depends on square of air speed. Observe that for same speed change Δv, defl ection δ2 is much larger, at high end of scale.

If the capsule defl ection is directly magnifi ed and linked to pointer, the display will be non-linear as indicated in Figure 2.7(b). It will be crowded at the low end of scale and gets expanded at higher speeds. Sometimes this non-linear displays are useful—because the readings are expanded

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Fig. 2.7 Non-linear scale displays.


in the range of interest (i.e. between 200–300 knots) to the pilot. However, the non-linearity can be compensated by using non-linear magnifi cation to compensate for parameter non-linearity. If this is done, the display will be linear as shown in Figure 2.7(c).

Sometimes logarithmic scale is preferred to open up at low end and to get crowded at near full scale, as in vertical speed indicator. (More information will be presented in later chapters.) A typi-cal VSI instrument is shown in Figure 2.8. Scale is opened up at low end, which is often of importance. Such representation provides improved readability near level fl ight, i.e. near zero rate of climb.

Note: In an aircraft the units are funny and use of knots for speed, feet for altitude and feet per minute for rate of climb, because the pilots are used to them.

2.3.2 Display Colour and Markings

Pilots should be able to easily interpret, comprehend and be alerted about certain parameters exceeding maximum limits. In order to achieve this, there are coloured arcs, radial lines and sec-tors, in order to highlight the limits of operation. A pilot can also set “bugs” which may be manu-ally moved around to fi x desired limits of performance, he has chosen by experience or aircraft manufacturer.

The markings use the following standard colour conventions:

Markings Purpose

Red markings on scale Maximum and minimum limitsYellow arc Take-off/precautionary sectorsGreen arc Safe and normal operational zoneRed arc Prohibited zone

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Fig. 2.8 Vertical speed indicator—logarithmic scale.


be evident in subsequent chapters. Both qualitative and quantitative indications are made available on a single multi-coloured electronic display.

Most aircraft are equipped with a standard set of instruments, which informs the pilot about the attitude (pitch, roll and yaw) air speed and altitude, of the aircraft. Many aircraft will have these basic fl ight instruments:

1. Altimeter: giving the aircraft’s height above some reference level by measuring the local air pressure. The altimeter has a provision to adjust to local barometric pressure which must be correctly set to obtain accurate altitude data, (BARO adjustment). This adjustment is mandatory after a certain prescribed altitude by all aircraft so that they are all properly separated vertically during cruise.

2. Attitude Indicator: shows the pitch, and roll and angles relative to the horizon. This is also known as the artifi cial horizon. By reading this instrument, pilot will be able to know whether the wings are level and if the aircraft nose is pointing above or below the horizon. This is a primary instrument for instrument fl ight and is very useful in poor visibility condi-tions. In modern aircraft EADI replaces this altitude indicator.

3. Air Speed Indicator (ASI): This instrument displays the aircraft’s speed in knots: (1 knot = 1.18 mph = 1.85 kmph) relative to the surrounding air. The indicated speed should be corrected for air density (which varies with altitude, temperature, and humidity) in order to get the True Air Speed—TAS, and further corrected for wind conditions to obtain the ground speed.

4. Magnetic Compass: The magnetic compass is used to indicate the aircraft’s heading rela-tive to the earth’s magnetic north, to know which direction the aircraft is fl ying with respect to the magnetic north. While the compass shows reliable readings in steady and level fl ight, it gives faulty indications when turning, climbing, descending or accelerating. This faulty indication can be compensated by using the gyro- stabilised heading indicator. For naviga-tional purposes, it is necessary to correct the magnetic direction to obtain direction with respect to true geographic north (which points to the earth’s axis of rotation). Note that magnetic north is wandering and slightly to the left of geographic North. Magnetic compass acts as a standby unit when other direction indicators malfunction.

5. Heading Indicator: (also known as Directional Gyro-DG). It is based on the gyro stabil-ity and precession, and is therefore subject to drift errors, which must be periodically cor-rected by calibrating it with respect to the magnetic compass. In modern aircraft, the DG is replaced by a Horizontal Situation Indicator (HSI), which provides the same heading information, but also helps in navigation.

6. Turn and Bank Indicator (TBI): The TBI is a gyroscopic instrument, displaying the direc-tion and the rate of turn, (in degrees per minute). Internally mounted inclinometer shows the turn quality, i.e. whether turn is properly coordinated (i.e. no slip out or skid in) as opposed to an uncoordinated turn. This instrument has become a turn coordinator in newer aircraft, which are manufactured after 1970.

7. Turn Coordinator: typically displays the rate and direction of roll while the aircraft is rolling; displays rate and direction of turn, while the aircraft is not rolling. Internally housed incli-nometer also displays the turn quality. The turn coordinator has replaced the good old TBI in modern aircraft, and shows the rate of turn, but it does not indicate pitch information.


8. Vertical Speed Indicator (VSI): Displays the rate of climb or descent, usually in feet per minute. Vertical speed is indicated by sensing the changing air pressure during ascent or descent.

2.4 Instrument Grouping— Basic T Grouping

As previously mentioned modern aircraft have four of the fl ight instruments arranged as a T—called the basic T as shown in Figure 2.11. Instruments are located as described below. The ADI is in the top centre, ASI is to the left, ALTI is at the right, HSI is situated right below ADI in the bottom row. The turn coordinator or RMI is positioned to the left of HSI. VSI is located to

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PFDATTITUDEDIRECTORINDICATOR

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ALTIMETER(ALTI)

TURNCOORDINATOR

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Navigation display (ND)

T grouping of 4 primaryflight instruments

Magnetic compass (below wind shield)

Fig. 2.11 Basic T confi guration of Main Instrument Panel (MIP).

the right of HSI. The magnetic compass will be located above the instrument panel, often on the windscreen centre post.

In some cases, the position taken by the Turn Coordinator is replaced by the Radio Magnetic Indicator—RMI which incorporates magnetic compass plus the bearing of the aircraft with respect to ground stations of radio navigation systems such as: VOR (Very high frequency Omni Range) and Automatic Direction Finder (ADF). In the newer aircraft with glass cockpit instruments also, the basic T grouping is generally followed. Brief details of glass cockpit is given below.


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Early glass cockpits, as in Macdonald Douglas’ MD80/90, Boeing’s 737, 757 and 767, Airbus’s A-300 and A-310, used EFIS for displaying attitude and navigational parameters only, and they continued to use mechanical gauges for air speed, altitude and vertical speed. Modern glass cockpits have now replaced totally all the mechanical gauges and warning systems, which were present in previous generation aircraft. Modern glass cockpit aircraft are: Boeing 737NG, 747-600, 767-400, 777 and 787; Airbus A-320, A-330 and the very new largest transport aircraft—A-380.

Future cockpit displays constitute a true departure from all of the above displays. They can cus-tomise the cockpits to various end users, to a greater extent than previous generations, for example they deploy a track ball, thumb pad or joystick as a pilot input device in a computer-like environ-ment. Greater situational awareness, and man–machine interface are now emphasised. Figure 2.13 shows the all- glass cockpit of a modern aircraft. Note that 5 in-line display units are used to present fl ight data to both pilot (left seat) and co-pilot. Also included in line is a Fuel Quantity Indicator (FQI) and control surface position indicators. The fl ight data includes basic T grouping, generally. However, one of the displays shows EADI data and another LRU (Line Replaceable Unit) presents EHSI data. The central display shows EICAS data, viewable by both pilots. Some-times, instead of using two separate displays for EADI and EHSI, a single LRU is used to display both EADI and EHSI data. It is noteworthy that all display units are similar and can be plugged in any of the 5 positions. This greatly simplifi es maintenance and reduces spare part counts.

We now proceed to discuss the broad functionalities of some newer aircraft instruments.

2.5.1 Attitude Director Indicator (ADI)

ADI is a Primary Flight Display (PFD), displaying all information critical to fl ight. Earlier, the Flight Director (FD) or Artifi cial Horizon (AH) or Gyro Horizon was used in an aircraft to indi-cate to the pilot about the orientation of the aircraft relative to earth. It indicates pitch (nose up/down) and roll (wings not level) of the aircraft and constitutes the most important instrument for fl ight in the so called IMC* (Instrument Meteorological Conditions). Attitude indicators also have signifi cant role in VMC,* Visual Meteorological Conditions. Figure 2.14 shows a typical attitude indicator (gyro horizon).

The pilot actions will be made in the natural sense so that there is no confusion or ambiguity to manoeuvre the aircraft. For example, if the pilot desires a level fl ight he rotates the control wheel clockwise as shown in Figure 2.14(b) to make the aircraft fl y straight and level (Figure 2.14(c)).

2.5.2 Electronic Attitude Director Indicator

An improvement was made in EADI (Electronic Attitude Director Indicator), which in addition to displaying the attitude, issues commands using movable command bars. This is shown in the

* IMC and VMC are types of air navigation, that enables pilots to land an aircraft. IMC conditions refer to landing the aircraft under poor visibility. VMC refers to visual conditions permitting a visual landing, i.e. the pilots can visually see the runway.


Note the following:

1. ADI is basically similar to Attitude Director (or gyro horizon). 2. Horizon bar is driven by servo motors driven by remotely located gyroscopes. The sense of

movement is natural to the pilot. 3. Aircraft symbol and the bank pointer are fi xed to the aircraft. (See Figure 2.16(a)) 4. Both horizon bar and roll pointer are gyro stabilised and provide a spatial reference to the

changing aircraft attitude. 5. Command bar displays are preset by pilot in the Mode Select Panel and enables him to fl y

the aircraft in a predetermined fashion, by matching the aircraft symbol coincide with com-mand bars. These bars are particularly useful during take-off and landing phases of fl ight. They ensure fl ight safety.

6. The scale at left of EADI of Figure 2.15 refers to glide slope of ILS (Instrument Landing System) and indicates to the pilot whether he is fl ying above or below “glide slope” beam, which directs him to fl y right into the runway during landing. Similarly, the scale at the bot-tom relates to localiser beam, for lateral alignment of the aircraft, to fl y the aircraft aligned to the centre of the runway. ILS ensures fl ight safety under poor visibility conditions.

2.5.3 Horizontal Situation Indicator (HSI)

The HSI is primarily a Navigation Display (ND), and it is an aircraft instrument located in Main Instrument Panel (MIP), just below the ADI (Attitude Director Indicator). (See Figure 2.11),

(a) A/C flying straight and level

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(c) Nose up command.A/C must fly with nose upto satisfy the command

Fig. 2.16 Different command bar settings to be obeyed.

(d) Fly left command. Fly the a/c to left to satisfy command bar

(e) Fly right command.Fly the aircraft to rightto satisfy command bar

(f) Commands have beensatisfied


replacing the conventional DG (directional gyro). In the electronic fl ight instrumentation system, it is known as EHSI (Electronic Horizontal Situation Indicator). HSI provides the plan view (map view) of aircraft motion which is important to navigate to the desired airport. Thus, HSI provides a basic horizontal view of the aircraft’s navigation around the earth. It provides an excellent pic-ture for precise navigation. EHSI displays: (i) magnetic heading, (ii) bearing and distance to navi-gation aid (VOR or ADF) and (iii) CDI (course deviation indicator).

An HSI is a combination of two familiar cockpit instruments: the directional gyro (DG) with a heading memory bug and (ii) a VOR (VHF Omni directional Receiver) / ILS (Instrument Landing System) indicator. EHSI reduces pilot’s effort—otherwise the pilot has to eye scan many instru-ments individually.

Figure 2.17 shows the EHSI, which incorporates the following:

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Fig. 2.17 Electronic horizontal situation indicator.

1. Glide slope needle for guiding the aircraft into the airport runway, in a vertical direction. 2. Aircraft Symbol . 3. Course Deviation Indicator (CDI) is shown relative to airplane symbol, in azimuth (plan)

direction. 4. Interlinked to autopilot to hold altitude and while approaching follow glide slope all the

way to decision height (DH) and beyond. 5. Course deviation indicator (CDI) and scale. 6. Compass card operated by DG (directional gyro) and a lubber reference line. 7. Heading select memory bug to aid the pilot to navigate to destination.

Combining the DG and NAV indicator into a single instrument, reduces the pilot workload by pro-viding the following vital information:

• heading—which direction the aircraft is going, • course reference, which direction the aircraft has to go, • course deviation, • glide slope information; while landing.


resolved. Modern aircraft use Twisted Active Matrix Liquid Crystal Displays (TAMLCD), which replaces earlier CRT displays.

2.6.2 Multi Function Display (MFD)

MFD is primarily a Navigation Display (ND) unit. It combines, however, weather data super-imposed on the map.

2.6.3 Engine Indications and Crew Alert Systems (EICAS)

EICAS shows information regarding the aircraft’s systems such as fuel, electrical, engines, etc. EICAS also alerts pilots of unusual or hazardous situations like low engine lubricating oil pres-sure, engine overheat, autopilot malfunction, loss of emergency/utility power, etc.

2.6.4 Mode Control Panels

Pilots can select display range and mode as well he can enter data, using the control panel. For example, command bars mentioned earlier are set using knobs in Mode Control Panels.

2.6.5 Display Data Processors

The visual display of an EFIS is made possible through the SGU (Symbol Generator Unit), which gets data from the pilot, sensors and format selected. SGU is also called display processor, or dis-play electronics unit.

Advantages of EFIS are:

1. The same display can be made to function as PFD, ND, offering versatility. 2. Software upgradable to latest versions without changing hardware. 3. Should one of the display fail, the other can take over and thus act as a redundant system. 4. Any LRU can be plugged in any one of the fi ve slots (see Figures 2.13a and b).

EFIS has become a standard equipment for all modern aircraft like aircraft from Boeing (B-767, B-777 and future 787) and Airbus (A-320, A-330, and the more recent A-380).

With the advent of low-cost computers, liquid crystal colour displays and inexpensive NAV-sensor’s—Fibre optic gyros, GPS and AHRS; EFIS can be adapted to even low-cost general avia-tion aircraft, which was unthinkable a few years ago.

Figure 2.18 is the cockpit of A-330, a large civil transport aircraft and Figure 2.19 is the cockpit of the A-330, which is the largest commercial transport plane today.

Once again it should be reiterated, all of the above information in chapter 2 is to provide a broad overview and requires considerable knowledge to fully understand. Revisit this chapter after cov-ering the full textbook.

aircraft instruments— types and cockpit layout

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