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Aircraft Hydraulic System Design Peter A. Stricker, PE Product Sales Manager Eaton Aerospace Hydraulic Systems Division August 20, 2010 © 2008 Eaton Corporation. All rights reserved. Purpose • • Acquaint participants with hydraulic system design principles for civil aircraft Review examples of hydraulic system architectures on common aircraft 2 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 3 Introduction 1 As airplanes grow in size, so 5 Hydraulic power is generated mechanically, electrically and pneumatically do the forces needed to move the flight controls … thus the need to transmit larger amount of power Air Turbine Pump 2 Hydraulic system transmits and controls power from engine to flight control actuators Electric Motorpump Hydraulic Storage/Conditioning Ram Air Turbine Pump Pilot Inputs 3 Pilot inputs are transmitted to remote actuators and amplified Electric Generator Engine Pump Flight Control Actuators 4 Pilot commands move actuators with little effort 4 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 5 Controlling Aircraft Motion Primary Flight Controls Definition of Airplane Axes 1 3 1 Ailerons control roll 2 Elevators control pitch 3 Rudder controls yaw 2 6 Controlling Aircraft Motion Secondary 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 7 Example of Flight Controls (A320) REF: A320 FLIGHT CREW OPERATING MANUAL CHAPTER 1.27 - FLIGHT CONTROLS PRIMARY SECONDARY 8 Why use Hydraulics? • Effective and efficient method of power amplification • • Precise control of load rate, position and magnitude • • • Independently in parallel or sequenced in series Smooth, vibration free power output • • Infinitely variable rotary or linear motion control Adjustable limits / reversible direction / fast response Ability to handle multiple loads simultaneously • • Small control effort results in a large power output Little impact from load variation Hydraulic fluid transmission medium • • Removes heat generated by internal losses Serves as lubricant to increase component life 9 Typical Users of Hydraulic Power • Landing gear • • Primary flight controls • • Rudder, elevator, aileron, active (multi-function) spoiler Secondary flight controls • • Extension, retraction, locking, steering, braking high lift (flap / slat), horizontal stabilizer, spoiler, thrust reverser Landing Gear Utility systems • Cargo handling, doors, ramps, emergency electrical power generation Spoiler Actuator HYDR. MOTOR Flap Drive GEARBOX TORQUE TUBE Nosewheel Steering 10 Sources of Hydraulic Power • Mechanical • • • Electrical • • • • Engine Driven Pump 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 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 Ram Air Turbine 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 Maintenance-free Accumulator AC Electric Motorpump Power Transfer Unit 11 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 12 Considerations for Hydraulic System Design to 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 redundancy Fly-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 13 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 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 NA 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) 14 System Design Philosophy Conventional Central System Architecture LEFT ENG. • Multiple independent centralized power systems • • • System segregation • • Each engine drives dedicated pump(s), augmented by independently powered pumps – electric, pneumatic No fluid transfer between systems to maintain integrity 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 SYSTEM 3 SYSTEM 1 EDP SYSTEM 2 ADP ROLL 1 PITCH 1 YAW 1 OTHERS NORM BRK EMP RAT ROLL 3 PITCH 3 YAW 3 LNDG GR EDP ROLL 2 PITCH 2 YAW 2 OTHERS EMRG BRK NSWL STRG EMP EDP Engine Driven Pump PTU Power Transfer Unit EMP Electric Motor Pump RAT Ram Air Turbine ADP Air Driven Pump RIGHT ENG. Engine Bleed Air OTHERS EMP PTU 15 System Design Philosophy More Electric Architecture LEFT ENG. • Two independent centralized power systems + Zonal & Dedicated Actuators • • • 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 • • • • Each engine drives dedicated pump(s), augmented by independently powered pumps – electric, pneumatic No fluid transfer between systems to maintain integrity Tail zonal system for pitch, yaw Aileron actuators for roll Electric driven hydraulic powerpack for emergency landing gear and brake ELECTRICAL ACTUATORS SYSTEM 1 EDP RIGHT ENG. SYSTEM 2 EDP GEN2 GEN1 RAT EMP EMP ROLL 1 ROLL 3 ROLL 2 PITCH 1 ZONAL PITCH 3 YAW 3 PITCH 2 YAW 1 OTHERS OTHERS LNDG GR NORM BRK YAW 2 OTHERS EMRG BRK LG / BRK EMERG POWER NW STRG Examples: Airbus A380, Boeing 787 EDP Engine Driven Pump EMP Electric Motor Pump GEN Electric Generator RAT Ram Air Turbine Generator Electric Channel 16 System Design Philosophy All 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 17 Fly-by-Wire (FBW) Systems Conventional Mechanical • • • • Fly-by-Wire 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 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 PILOT INPUTS RIGHT WING AUTOPILOT INPUTS LEFT WING BOEING 757 AILERON SYSTEM 18 Principal System Interfaces Design Considerations Electrical System Flight Controls Flow under normal and all emergency conditions – priority flow when LG, flaps are also demanding flow Electric motors, Solenoids Power on Demand Electrical power variations under normal and all emergency conditions (MIL-STD-704) Hydraulic System Power on Demand Hydraulic power from EDP Nacelle / Engine Pad speed as a function of flight regime – idle to take-off Avionics Signals from pressure, temperature, fluid quantity sensors Signal to solenoids, electric motors Landing Gear Flow under normal and all emergency conditions – retract / extend / steer 19 Aircraft Hydraulic Architectures Comparative Aircraft Weights 10,000,000 WIDEB ODY MTOW - lb 1,000,000 SINGLE-A ISLE 100,000 LA R GE B IZ / R EGIONA L J ET S MID / SUP ER MID-SIZE B IZ J E T S / C OMMUT ER T UR B O-P R OP S 10,000 V E R Y LIGHT / LIGHT J ET S / T UR B O-P R OP S GE NE R A L A V IA T ION 1,000 C sn es 72 1 a en h P om 10 0 ng Ki r2 Ai 00 t rj e 45 a re Le ets t eJ A B am 0 00 605 F7X XRS 65 195 700 321 300 300 0E R 0E R 380 0 G 4 A A 57 330 7- 30 7-40 us a k er nger lcon bal am ERJ 737 us 7 e o b b L aw e l A 77 r g Ai r e 74 Ai r Fa G l fstr ing us all ae ein e g g H r h b o o n n u r C B B ei o ei G Emb Ai Bo B 41 t rj e 85 Increasing Hydraulic System Complexity 20 Aircraft Hydraulic Architectures Mid-Size Jet Example Block Diagrams – Learjet 40/45 MTOW: 21,750 lb Flight Controls: Manual MAIN SYSTEM EMERGENCY SYSTEM 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) 21 Aircraft Hydraulic Architectures Super Mid Size Example Block Diagrams – Hawker 4000 MTOW: 39,500 lb Flight 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) • • • REF.: EATON C5-38A 04/2003 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 • • • • 22 Aircraft Hydraulic Architectures Single-Aisle Example Block Diagrams – Airbus A320/321 MTOW (A321): 206,000 lb Flight Controls: Hydraulic FBW Key Features 3 independent systems 2 main systems with EDP 1 main system also includes backup EMP & hand pump for cargo door 3rd 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 • • • • • • • REF.: AIR5005 (SAE) 23 Wide Body Aircraft Hydraulic Architectures Example Block Diagrams – Boeing 777 MTOW (B777-300ER): 660,000 lb Flight Controls: Hydraulic FBW Key Features 3 independent systems 2 main systems with EDP + EMP each 3rd system with 2 EMPs, 2 engine bleed airdriven (engine bleed air) pumps, + RAT pump Safety / 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 LEFT SYSTEM CENTER SYSTEM RIGHT SYSTEM • • • • • • • REF.: AIR5005 (SAE) 24 Wide Body Aircraft Hydraulic Architectures Example Block Diagrams – Airbus A380 MTOW: 1,250,000 lb Flight 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 • 25 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 26 Suggested References Federal Aviation Regulations FAR 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 Text Moir & Seabridge: Aircraft Systems – Mechanical, Electrical and Avionics Subsystems Integration 3rd Edition, Wiley 2008 27