Main Focus

Integrity under extreme conditions

Aircraft and spacecraft both have the highest requirements for the integ-rity and reliability of hydraulic sys-tems, under extreme conditions of use. ISO/TC 20/SC 10, Aerospace fluid sys-tems and components, has determined to fulfil those requirements by creating international rules, specifications and test methods. Its work programme rec-ognizes the importance of including all involved parties such as airframe manu-facturers, airlines, equipment suppliers and airworthiness authorities. Anoth-er vital aspect of aerospace standard-ization is to consider the whole prod-uct life cycle from concept, through design, manufacture, maintenance and recycling.

The development of standards requires close teamwork among specialists in a variety of specialist fields. Membership of SC 10 includes expert representatives from them all ; and SC 10 is in close con-tact with European and American stand-ardization organizations such as ASD 1) (formally AECMA) and SAE 2).

About the author

Martin Hbner has worked in the standardiza-tion department of Airbus Germany, since graduating from the Technical University of Darmstadt, Germany, in

2001. He is responsible for mechanical standard parts. Mr. Hbner has chaired ISO/TC 20/SC 10 since 2003 and can be reached at martin.huebner@airbus.com

Aircraft hydraulic systems

by Martin Hbner, Chair, ISO/TC 20/SC 10, Aerospace fluid systems and components

Almost all modern aircraft use hydraulic systems general-ly, applied to flight controls and landing gear. Such applications, one assumes, were not imagined by the inventor of hydraulics, Ctesibius of Alexandria (285-222 BC) nor by their re-inventor Benedetto Castelli (1578-1643 AD), a student of Gali-leo Galilei.

Standardization and maintenance

International standardization is par-ticularly important in relation to main-tenance. Modern aircraft have a work-ing life of several decades. That entails close attention to testing and replace-ment of components according to spec-ified schedules and avoidance of any unnecessary ground time. An aircraft earns money only when it is flying. Worldwide customer support systems

Aircraft engines produce the neces-sary pressure and flow which are trans-mitted, via thousands of metres of pipes, hoses and fittings, to actuators work-ing on brake flaps or landing gear. For safety reasons two, three or four inde-pendent hydraulic circuits are installed on every aircraft. The architecture of these circuits ensures that a hydrau-lic failure in one of them leads only to the loss of some so-called redundan-cies . All the functions necessary for safe flight and landing remain availa-ble. Even in the event of total engine failure, vital hydraulic functions con-tinue to work, taking their energy from a special propeller (Ram air turbine), in order to control the aircraft.

A harmonized and reduced diversity of

parts, achieved through International Standards, greatly improves the cost

effectiveness of such customer support.

1) ASD is the Aerospace and Defence Industries Association of Europe

2) SAE is the Society of Automotive Engineers

30 ISO Focus October 2005

Aerospace : the new frontier

is an amazing, deplorable and costly anachronism.

Whos who The structure of TC 20/SC 10

is as follows : Chair, Martin Hbner (Germany), Secretary, Jrgen Schwindt (Germany)Convenors :

WG 01, Seals and seal retainers Keith Allen (UK)

WG 03, Tubing Hans van der Velden (USA)

WG 04, Retaining devices for rigid and flexible tubes Antony Olszak (UK)

WG 05, Miscellaneous fluid pow-er and fuel systems Terence Chin (USA)

WG 06, Couplings for rigid pipes Oliver Harten (Germany)

WG 08, Hydraulic fluids and clean-liness Francois Cros (France)

WG 09, Hydraulic power and actu-ation equipment Peter Keenan (UK)

WG 14, Hose assemblies Al Baer (USA).

Member countries of SC 10 are : France, Germany, USA, UK, Japan, China, Republic of Korea, Russia and Ukraine.

and whats whatSince its formation back in 1971,

the main work of SC 10 has been the development of specifications, defini-tions and test methods for aerospace fluid systems. The text of its very first resolution was : The cord section pre-viously stated in ISO/TC 20 Resolu-tion 194 is amended to change the 7,1 mm size to 7 mm. As a result of more than 30 years of work, today there are 68 ISO standards for aerospace fluid systems. Currently, 56 work items are in progress in SC 10. Its 2005 plenary was held in Forth Worth, Texas in con-junction with the SAE 2005 AeroTech Congress & Exhibition.

Some of the standards developed by SC 10 :

ISO 6771:1987, Aerospace Fluid systems and components Pressure and temperature classifications

ISO 11217:1993, Aerospace Hydraulic system fluid contamina-tion Location of sampling points and criteria for sampling

ISO 11218:1993, Aerospace Clean-liness classification for hydraulic fluid

ISO 12333:2000, Aerospace Con-stant displacement hydraulic motors General specification for 35 000 kPa systems

ISO 12334:2000, Aerospace Hydrau-lic, pressure-compensated, variable delivery pumps General require-ment for 35 000 kPa systems

ISO 7169:1998, Aerospace Sepa-rable tube fittings for fluid systems, 24 degrees cone General specifi-cation

to provide spare parts within hours are necessary to satisfy demands, whether they be in Tokyo, Sydney, New York or Dsseldorf at very short notice, and for all parts of every aircraft currently in service. This implies a tremendous logistical effort. A harmonized and reduced diversity of parts, achieved through International Standards, great-ly improves the cost effectiveness of such customer support. Many airlines fly a diversity of aircraft types. But close teamwork between competitors within the aircraft industry is still evi-dent and enhanced by the standardi-zation process. Such harmonization is one goal of SC 10.

Higher pressures will bring higher revenues

Increased hydraulic pressure at 5 000 psi in the latest civil aircraft projects, versus 3 000 psi on todays aircraft allows a significant weight sav-ing, improves system reliability due to the smaller fittings and pipes, and eas-es equipment handling thanks to more compact components. It also saves vol-ume on the aircraft, since less fluid is necessary. All this adds up to extra rev-enue payload.

The interests of the airframe builder and those of the manufacturer of fittings, pipes and other equipment are merged within SC 10, in order to establish common, worldwide, accept-ed standards for this new higher-pres-sure technology.

Military metricsHydraulic systems are a main

topic for SC 10, but not the only one. It also handles fuel and water systems and standards for military applications. One big difference between military and civil aviation is that military aircraft are designed using the metric system where-as civil aircraft use the inch system. This

International standardization is

particularly important in relation to maintenance.

ISO Focus October 2005 31