© 2008 Eaton Corporation. All rights reserved.

Aircraft Hydraulic System Design

Peter A. Stricker, PEProduct Sales Manager

Eaton Aerospace Hydraulic Systems Division

August 20, 2010

2

Purpose

• Acquaint participants with hydraulic system design principles for civil aircraft

• Review examples of hydraulic system architectures on common aircraft

3

Agenda

• Introduction• Review of Aircraft Motion Controls• Uses for and sources of hydraulic power• Key hydraulic system design drivers• Safety standards for system design• Hydraulic design philosophies for conventional, “more

electric” and “all electric” architectures • Hydraulic System Interfaces • Sample aircraft hydraulic system block diagrams• Conclusions

4

Introduction As airplanes grow in size, so do the forces needed to move the flight controls … thus the need to transmit larger amount of power

Ram Air Turbine Pump

Hydraulic Storage/Conditioning

Engine Pump

Electric Generator

Electric Motorpump

Flight Control Actuators

Air Turbine Pump

Hydraulic system transmits and controls power from engine to flight control actuators

2

Pilot inputs are transmitted to remote actuators and amplified

1

3

Pilot commands move actuators with little effort

4

Hydraulic power is generated mechanically, electrically and pneumatically

5

Pilot Inputs

5

Introduction • Aircraft’s Maximum Take-Off Weight (MTOW) drives

aerodynamic forces that drive control surface size and loading • A380 – 1.25 million lb MTOW – extensive use of hydraulics• Cessna 172 – 2500 lb MTOW – no hydraulics – all manual

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Controlling Aircraft MotionPrimary Flight Controls

Definition of Airplane Axes

1 Ailerons control roll

2 Elevators control pitch

3 Rudder controls yaw

1

3 2

7

Controlling Aircraft MotionSecondary Flight Controls

High Lift Devices:

►

• Flaps (Trailing Edge), slats (LE Flaps) increase area and camber of wing

• permit low speed flight

Flight Spoilers / Speed Brakes: permit steeper descent and augment ailerons at low speed when deployed on only one wing

Ground Spoilers: Enhance deceleration on ground (not deployed in flight)

Trim Controls:

• Stabilizer (pitch), roll and rudder (yaw) trim to balance controls for desired flight condition

8

Example of Flight Controls (A320) REF: A320 FLIGHT CREW OPERATING MANUAL

CHAPTER 1.27 - FLIGHT CONTROLS

PRIMARY

SECONDARY

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Why use Hydraulics?

• Effective and efficient method of power amplification • Small control effort results in a large power output

• Precise control of load rate, position and magnitude• Infinitely variable rotary or linear motion control • Adjustable limits / reversible direction / fast response

• Ability to handle multiple loads simultaneously• Independently in parallel or sequenced in series

• Smooth, vibration free power output• Little impact from load variation

• Hydraulic fluid transmission medium• Removes heat generated by internal losses • Serves as lubricant to increase component life

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HYDR. MOTOR

TORQUE TUBE

GEARBOX

Typical Users of Hydraulic Power• Landing gear

• Extension, retraction, locking, steering, braking• Primary flight controls

• Rudder, elevator, aileron, active (multi-function) spoiler

• Secondary flight controls • high lift (flap / slat), horizontal stabilizer, spoiler, thrust

reverser• Utility systems

• Cargo handling, doors, ramps, emergency electrical power generation

Flap DriveSpoiler Actuator

Landing Gear

Nosewheel Steering

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Sources of Hydraulic Power

Ram Air Turbine

AC Electric MotorpumpMaintenance-free

Accumulator

Engine Driven Pump

• Mechanical • Engine Driven Pump (EDP) - primary hydraulic power source,

mounted directly to engines on special gearbox pads• Power Transfer Unit – mechanically transfers hydraulic

power between systems• Electrical

• Pump attached to electric motors, either AC or DC• Generally used as backup or as auxiliary power• Electric driven powerpack used for powering actuation zones• Used for ground check-out or actuating doors when

engines are not running

Pneumatic• Bleed Air turbine driven pump used for backup power• Ram Air Turbine driven pump deployed when all engines

are inoperative and uses ram air to drive the pump• Accumulator provides high transient power by releasing

stored energy, also used for emergency and parking brake

Power Transfer Unit

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Key Hydraulic System Design Drivers

• High Level certification requirement per aviation regulations:

Maintain control of the aircraft under all normal and anticipated failure conditions

• Many system architectures* and design approaches exist to meet this high level requirement – aircraft designer has to certify to airworthiness regulators by analysis and test that his solution meets requirements

* Hydraulic System Architecture: Arrangement and interconnection of hydraulic power sources and consumers in a manner that meets requirements for controllability of aircraft

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Considerations for Hydraulic System Designto meet System Safety Requirements

• Redundancy in case of failures must be designed into system

• Any and every component will fail during life of aircraft

• Manual control system requires less redundancyFly-by-wire (FBW) requires more redundancy

• Level of redundancy necessary evaluated per methodology described in ARP4761

• Safety Assessment Tools• Failure Modes, Effects and Criticality Analysis –

computes failure rates and failure criticalities of individual components and systems by considering all failure modes

• Fault Tree Analysis – computes failure rates and probabilities of various combinations of failure modes

• Markov Analysis – computes failure rates and criticality of various chains of events

• Common Cause Analysis – evaluates failures that can impact multiple components and systems

• Principal failure modes considered• Single system or component failure• Multiple system or component failures occurring

simultaneously• Dormant failures of components or subsystems

that only operate in emergencies• Common mode failures – single failures that

can impact multiple systems• Examples of failure cases to be considered

• One engine shuts down during take-off – need to retract landing gear rapidly

• Engine rotor bursts – damage to and loss of multiple hydraulic systems

• Rejected take-off – deploy thrust reversers, spoilers and brakes rapidly

• All engines fail in flight – need to land safely without main hydraulic and electric power sources

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Civil Aircraft System Safety Standards(Applies to all aircraft systems)

Failure Criticality Failure Characteristics

Probability of Occurrence

Design Standard

Minor Normal, nuisance and/or possibly requiring emergency procedures

Reasonably probable

NA

Major Reduction in safety margin, increased crew workload, may result in some injuries

Remote P ≤ 10-5

Hazardous Extreme reduction in safety margin, extended crew workload, major damage to aircraft and possible injury and deaths

Extremely remote P ≤ 10-7

Catastrophic Loss of aircraft with multiple deaths Extremely improbable

P ≤ 10-9

Examples

Minor: Single hydraulic system fails

Major: Two (out of 3) hydraulic systems fail

Hazardous: All hydraulic sources fail, except RAT or APU

(US1549 Hudson River A320 – 2009)

Catastrophic: All hydraulic systems fail

(UA232 DC-10 Sioux City – 1989)

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System Design PhilosophyConventional Central System Architecture

• Multiple independent centralized power systems

• Each engine drives dedicated pump(s), augmented by independently powered pumps – electric, pneumatic

• No fluid transfer between systems to maintain integrity

• System segregation• Route lines and locate components far

apart to prevent single rotor or tire burst from impacting multiple systems

• Multiple control channels for critical functions

• Each flight control needs multiple independent actuators or control surfaces

• Fail-safe failure modes – e.g., landing gear can extend by gravity and be locked down mechanically

LEFT ENG.

SYSTEM 1

SYSTEM 3 RIGHT ENG.

SYSTEM 2

EDP EDP

ROLL 1

PITCH 1

YAW 1

OTHERS

EMP

EMP RAT

PTU

ROLL 2

PITCH 2

YAW 2

OTHERS

EMP

ROLL 3

PITCH 3

YAW 3

LNDG GR

EMRG BRKNORM BRK

NSWL STRG

ADP

EDP Engine Driven Pump

EMP Electric Motor Pump

ADP Air Driven Pump

PTU Power Transfer Unit

RAT Ram Air Turbine

Engine Bleed Air

OTHERS

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System Design PhilosophyMore Electric Architecture

• Two independent centralized power systems + Zonal & Dedicated Actuators

• Each engine drives dedicated pump(s), augmented by independently powered pumps – electric, pneumatic

• No fluid transfer between systems to maintain integrity

• System segregation• Route lines and locate components far

apart to prevent single rotor or tire burst to impact multiple systems

• Third System replaced by one or more local and dedicated electric systems

• Tail zonal system for pitch, yaw• Aileron actuators for roll• Electric driven hydraulic powerpack for

emergency landing gear and brake• Examples: Airbus A380, Boeing 787

LEFT ENG.

SYSTEM 1

RIGHT ENG.

SYSTEM 2

EDP EDP

ROLL 1

PITCH 1

YAW 1

OTHERS

EMP

GEN1 RAT

ROLL 2

PITCH 2

YAW 2

OTHERS

EMP

ROLL 3

ZONAL PITCH 3 YAW

3

NORM BRK

EMRG BRKLNDG GR

NW STRG

GEN2

EDP Engine Driven Pump

EMP Electric Motor Pump

GEN Electric Generator

RAT Ram Air Turbine Generator

Electric Channel

OTHERS

ELECTRICAL ACTUATORS

LG / BRK EMERG POWER

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System Design PhilosophyAll Electric Architecture

“Holy Grail” of aircraft power distribution ….• Relies on future engine-core mounted electric generators

capable of high power / high power density generation, running at engine speed – typically 40,000 rpm

• Electric power will replace all hydraulic and pneumatic power for all flight controls, environmental controls, de-icing, etc.

• Flight control actuators will like remain hydraulic, using Electro-Hydrostatic Actuators (EHA) or local hydraulic systems, consisting of

• Miniature, electrically driven, integrated hydraulic power generation system

• Hydraulic actuator controlled by electrical input

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Fly-by-Wire (FBW) SystemsFly-by-Wire• Pilot input read by computers• Computer provides input to electrohydraulic flight

control actuator • Control laws include

• Enhanced logic to automate many functions• Artificial damping and stability• Flight Envelope Protection to prevent airframe from

exceeding structural limits

• Multiple computers and actuators provide sufficient redundancy – no manual reversion

Conventional Mechanical• Pilot input mechanically connected to flight control

hydraulic servo-actuator by cables, linkages, bellcranks, etc.

• Servo-actuator follows pilot command with high force output

• Autopilot input mechanically summed• Manual reversion in case of loss of hydraulics or

autopilot malfunction

BOEING 757 AILERON SYSTEM

PILOT INPUTS

AUTOPILOT INPUTS

LEFT WING

RIGHT WING

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Principal System InterfacesDesign Considerations

Hydraulic System

Hydraulic power from EDP

Nacelle / Engine

Pad speed as a function of flight regime – idle to take-off

Landing Gear

Power on Demand

Flow under normal and all emergency conditions – retract / extend / steer

Electric motors, Solenoids

Electrical System

Electrical power variations under normal and all emergency conditions (MIL-STD-704)

Flight Controls

Power on Demand

Flow under normal and all emergency conditions – priority flow when LG, flaps are also demanding flow

Avionics

Signals from pressure, temperature, fluid quantity sensors

Signal to solenoids, electric motors

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1,000

10,000

100,000

1,000,000

10,000,000

Cessna

172

Pheno

m 100

KingAir 2

00

Learj

et 45

BAe Jets

tream

41

Learj

et 85

Hawke

r 400

0

Challen

ger 6

05

Falcon

F7X

Global X

RS

Gulfstre

am G65

0

Embraer

ERJ-195

Boeing

737-7

00

Airbus

A321

Boeing

757-3

00

Airbus

A330-3

00

Boeing

777-3

00ER

Boeing

747-4

00ER

Airbus

A380

MT

OW

- lb

LARGE BIZ / REGIONAL J ETS

SINGLE-A ISLE

WIDEBODY

MID / SUPER MID-SIZE B IZ J ETS /

COMMUTER TURBO-PROPS

VERY LIGHT / LIGHT J ETS / TURBO-PROPS

GENERAL AVIATION

Aircraft Hydraulic Architectures Comparative Aircraft Weights

Increasing Hydraulic System Complexity

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Aircraft Hydraulic ArchitecturesExample Block Diagrams – Learjet 40/45

MAIN SYSTEM EMERGENCY SYSTEMMTOW: 21,750 lb

Flight Controls: Manual

Key Features

• One main system fed by 2 EDP’s

• Emergency system fed by DC electric pump

• Common partitioned reservoir (air/oil)

• Selector valve allows flaps, landing gear, nosewheel steering to operate from main or emergency system

• All primary flight controls are manual

Safety / Redundancy

• Engine-out take-off: One EDP has sufficient power to retract gear

• All Power-out: Manual flight controls; LG extends by gravity with electric pump assist; emergency flap extends by electric pump; Emergency brake energy stored in accumulator for safe stopping

REF.: AIR5005A (SAE)

Mid-Size Jet

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Aircraft Hydraulic ArchitecturesExample Block Diagrams – Hawker 4000

MTOW: 39,500 lbFlight Controls: Hydraulic with manual reversion

exc. Rudder, which is Fly-by-Wire (FBW)Key Features• Two independent systems• Bi-directional PTU to transfer power between

systems without transferring fluid• Electrically powered hydraulic power-pack for

Emergency Rudder System (ERS)

Safety / Redundancy• All primary flight controls 2-channel; rudder has

additional backup powerpack; others manual reversion

• Engine-out take-off: PTU transfers power from system #1 to #2 to retract LG

• Rotorburst: Emergency Rudder System is located outside burst area

• All Power-out: ERS runs off battery; others manual; LG extends by gravity

Super Mid Size

REF.: EATON C5-38A 04/2003

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Aircraft Hydraulic ArchitecturesExample Block Diagrams – Airbus A320/321

MTOW (A321): 206,000 lbFlight Controls: Hydraulic FBWKey Features• 3 independent systems • 2 main systems with EDP

1 main system also includes backup EMP & hand pump for cargo door3rd system has EMP and RAT pump

• Bi-directional PTU to transfer power between primary systems without transferring fluid

Safety / Redundancy• All primary flight controls have 3 independent

channels• Engine-out take-off: PTU transfers power from

Y to G system to retract LG• Rotorburst: Three systems sufficiently

segregated• All Power-out: RAT pump powers Blue; LG

extends by gravity

Single-Aisle

REF.: AIR5005 (SAE)

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Aircraft Hydraulic ArchitecturesExample Block Diagrams – Boeing 777

LEFT SYSTEM

Wide Body

RIGHT SYSTEMCENTER SYSTEMMTOW (B777-300ER): 660,000 lbFlight Controls: Hydraulic FBWKey Features• 3 independent systems • 2 main systems with EDP + EMP each • 3rd system with 2 EMPs, 2 engine bleed air-

driven (engine bleed air) pumps, + RAT pumpSafety / Redundancy• All primary flight controls have 3 independent

channels• Engine-out take-off: One air driven pump and

EMP available in system 3 to retract LG• Rotorburst: Three systems sufficiently

segregated• All Power-out: RAT pump powers center

system; LG extends by gravity

REF.: AIR5005 (SAE)

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Aircraft Hydraulic ArchitecturesExample Block Diagrams – Airbus A380

Wide Body

MTOW: 1,250,000 lbFlight Controls: FBW (2H + 1E channel)Key Features / Redundancies• Two independent hydraulic systems

+ one electric system (backup)• Primary hydraulic power supplied by 4

EDP’s per system• All primary flight controls have 3 channels

– 2 hydraulic + 1 electric• 4 engines provide sufficient redundancy

for engine-out cases

REF.: EATON C5-37A 06/2006

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Conclusions

• Aircraft hydraulic systems are designed for high levels of safety using multiple levels of redundancy

• Fly-by-wire systems require higher levels of redundancy than manual systems to maintain same levels of safety

• System complexity increases with aircraft weight

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Suggested References

Federal Aviation RegulationsFAR Part 25: Airworthiness Standards for

Transport Category Airplanes FAR Part 23: Airworthiness Standards for

Normal, Utility, Acrobatic, and Commuter Category Airplanes

FAR Part 21: Certification Procedures For Products And Parts

AC 25.1309-1A System Design and Analysis Advisory Circular, 1998

Aerospace Recommended Practices (SAE)ARP4761: Guidelines and Methods for

Conducting the Safety Assessment Process on Civil Airborne Systems and Equipment

ARP 4754: Certification Considerations for Highly-Integrated or Complex Aircraft Systems

Aerospace Information Reports (SAE)AIR5005: Aerospace - Commercial Aircraft

Hydraulic Systems

Radio Technical Committee Association (RTCA)

DO-178: Software Considerations in Airborne Systems and Equipment Certification (incl. Errata Issued 3-26-99)

DO-254: Design Assurance Guidance For Airborne Electronic Hardware

TextMoir & Seabridge: Aircraft Systems –

Mechanical, Electrical and Avionics Subsystems Integration 3rd Edition, Wiley 2008