A320 - Dual Hydraulic Loss - Airbus

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A320 - Dualhydraulic lossBy: Michel PALOMEQUEFlight Safety AdvisorA318/A319/A320/A321 program1 IntroductionThe A320 may experience a series of dual hydraulicloss when, at low altitude, a leak in the greenhydraulic system causes the loss of the yellowcircuit.To understand how this may happen, this articlewill first describe the aircraft’s hydraulic systemand explain the respective roles of the PowerTransfer Unit (PTU) and ECAM caution in case ofpressure differential between the green and yellowsystems.The second part of the article will describe howthe combination of a major leak in the green circuitand the ECAM inhibition below 1,500 feet maylead to the dual hydraulic loss.The third and last part will develop on the correctiveactions proposed to avoid this type of occurrences.2 General overviewThe A320 hydraulic system is composed of threedifferent and fully independent circuits: Green,Yellow & Blue. The users are shared between thesystems in order to ensure the control of the aircraft,even when one system is inoperative.Speed Motor/ Generator (CSM/G) is used to provideaircraft electrical power in case of emergency.On the green & yellow systems, the normal sourceof pressure is the Engine Driven Pump (EDP) andthe auxiliary source is the Power Transfer Unit(PTU). The PTU is a hydraulic motor pump whichtransfers hydraulic power between the green andyellow systems without transfer of fluid.It operates automatically, whenever the pressuredifferential between the two systems reaches500 PSI.In case of low fluid level in either the green or yellowsystem, an amber caution is triggered on the ECAM,which requests the pilot to switch off the PTU aswell as the EDP. This to avoid having the PTUrunning at maximum speed and causing theoverheating and loss of the properly functioninghydraulic system.According to the Airbus philosophy of notoverloading the flight crew during the critical phasesof flight, the above amber caution is inhibited below1,500 feet.The figure below shows all the systems, which areinterconnected to the hydraulic systems:On the blue hydraulic system, the normal sourceof pressure is the electrical pump, and the auxiliarysource is the Ram Air Turbine (RAT). The ConstantSafety first #04 June 2017 - 1/5

FLIGHT CONTROLSLANDING GEARBRAKESNOSE WHEEL STEERREVERSER ENG 1FLIGHT CONTROLSBRAKESPARKING BRAKEREVERSER ENG 2FLIGHT CONTROLSCONSTANT SPEEDMOTOR/GENERATOR3000 PSI3000 PSIACCUMULATORACCUMULATORPOWER TRANSFERUNIT (PTU)3000 PSIACCUMULATORCARGO DOORENGINE DRIVENPUMP (EDP)1ENGINE DRIVENPUMP (EDP) 2SMENG 1FIRESHUT-OFFVALVEGREENRESERVOIRSRAM AIRTURBINE(RAT)ELECTRICPUMPMENG PYELLOWRESERVOIRSYSTEM OVERVIEW / RATExample of architecture.Aircraft pre modification 26925.PTU is automaticallyoperated when a certainΔP (500PSI) appearsbetween Greenand Yellow circuits.Safety first #04 June 2017 - 2/5

3 Dual hydraulic lossscenarioThe scenario of the dual hydraulicloss is the following:1) During takeoff, a leak in thegear retraction circuit leads tothe loss of the green hydraulicsystem. This loss stops theretraction of the landing gear.2) The 500 PSI pressure differential between the green andyellow hydraulic pressure isreached and the PTU automatically switches ON.3) The loss of the green systemnormally triggers the corresponding ECAM caution, whichrequests the crew to switch offthe PTU.4) However, below 1,500 feet, theECAM caution is inhibited.5) The PTU therefore remainsON, and operates at its maximum speed.6) As a consequence of the nonretraction of the landing gear,the crew may be busy communicating with ATC and managing the situation, leaving theaircraft flying below 1,500 feetlonger than usual, and thuskeeping the ECAM cautioninhibited.7) Within less than two minutes,the PTU overheats the yellowhydraulic system and causesits loss. In case of green low level, if PTU is not switched off this will lead to A Yellow overheat (around 2 minutes later) Then to a double G Y hydraulic failureAs a summary,the following figurecan be used to describeshortly the scenario ofthe dual hydraulic loss:G RSVR LOLVL (inhibitedbelow 1500ft)G Y SYS LO PR(displayed above 1500ft)Y RSVROVHT(inhibited below1500ft)1500ftLanding gear retractiontentativeGreen hydraulicleakageLift offAutomoaticstarsof the PTUSudden increase ofthe hydraulic leakPTU notswitchedOFFPTU &pumps areswitchedOFFPTUdurationSafety first #04 June 2017 - 3/5

4 Corrective actions5 ConclusionThis scenario already occurred in-service, leadingto the following design change:Below 1,500 feet the PTU is automatically switchedto OFF.The scenario of dual hydraulic loss occurred inservice.l The green hydraulic system was lost during gearretraction.l The PTU automatically switched ON.l The ECAM warning requesting to switch OFFthe PTU remained inhibited below 1,500 feet.l The aircraft remained below that altitude for morethan two minutes after the start of the operationof the PTU, which led the yellow system tooverheat and caused its loss.This solution was preferred to the followingalternatives:l Upgrading the ECAM caution to a red warning.l Cancelling the inhibition of the ECAM warningbelow 1,500 feet.Indeed, red warnings require immediate action,which is not justified in this case, and cancellingthe inhibition would only overload the crew duringa busy flight phase.The operational consequences of such changeare described below:In case of hyd leak in flight: No longer risk of dual hydraulic loss if no rapidcrew action. Follow “G(Y) SYS LO PR” ECAM caution whentriggered.Note: the ECAM procedure requests a manualconfirmation to switch off the PTU.In case of hyd leak on ground: No PTU logic change:Follow “G(Y) SYS LO PR” ECAM cautionIn case of single engine taxi,or hydraulic pump failureor engine failure :A design change has been developed, whichconsists in the automatic switching of the PTU toOFF below 1,500 feet.This change is coveredby the following modifications.MOD 34236 35879 / SB 29-1115:Install provisions for new PTU inhibition logicMOD 35938 / SB 29-1126:Activate new PTU inhibition logicModifications 34236 & 35879are now standard on production linesfrom MSN2740.Modification 35938 is optional,and must be requested.We therefore encourage airlinesto retrofit these modifications. No change : PTU runs to recover the normalpressure in the affected system.No ECAM change with new PTUlogic: ECAM still requests to switch OFF the PTU– Only Pilot confirmation (in flight)with new logic– Common ECAM definition with old logicSafety first #04 June 2017 - 4/5

Safety FirstThe Airbus Safety MagazineFor the enhancement of safe flight throughincreased knowledge and communications.Safety First is published by the Flight Safety Departmentof Airbus. It is a source of specialist safety informationfor the restricted use of flight and ground crew memberswho fly and maintain Airbus aircraft. It is also distributedto other selected organisations.requirements or technical orders. The contents do notsupersede any requirements mandated by the State ofRegistry of the Operator’s aircraft or supersede or amendany Airbus type-specific AFM, AMM, FCOM, MELdocumentation or any other approved documentation.Material for publication is obtained from multiple sourcesand includes selected information from the Airbus FlightSafety Confidential Reporting System, incident andaccident investigation reports, system tests and flighttests. Material is also obtained from sources within theairline industry, studies and reports from governmentagencies and other aviation sources.Articles may be reprinted without permission, except wherecopyright source is indicated, but with acknowledgementto Airbus. Where Airbus is not the author, the contents ofthe article do not necessarily reflect the views of Airbus,neither do they indicate Company policy.All articles in Safety First are presented for informationonly and are not intended to replace ICAO guidelines,standards or recommended practices, operator-mandatedContributions, comment and feedback are welcome. Fortechnical reasons the editors may be required to make editorialchanges to manuscripts, however every effort will be madeto preserve the intended meaning of the original. Enquiriesrelated to this publication should be addressed to:AirbusProduct Safety department (GS)1, rond point Maurice Bellonte31707 Blagnac Cedex - FranceFax: 33(0)5 61 93 44 29safetycommunication@airbus.comSafety First# 04 June 2007Safety First is publishedby Airbus S.A.S1, rond point Maurice Bellonte31707 Blagnac Cedex / FranceEditors:Yannick Malinge,Vice President Flight SafetyChristopher Courtenay,Director of Flight Safety Airbus S.A.S. 2007 – All rights reserved. Confidential and proprietary documents.By taking delivery of this Brochure (hereafter “Brochure”), you accept on behalf of yourcompany to comply with the following guidelines:Ð No other intellectual property rights are granted by the delivery of this Brochure than theright to read it, for the sole purpose of information.Concept Design byHCSGM 20070592Production by Quat’coulÐ This Brochure and its content shall not be modified and its illustrations and photos shallnot be reproduced without prior written consent of Airbus.Copyright: GSEÐ This Brochure and the materials it contains shall not, in whole or in part, be sold,rented, or licensed to any third party subject to payment.Photos copyright AirbusPhotos by ExM:Hervé BerengerPhilippe MascletHervé GousséThis Brochure contains sensitive information that is correct at the time of going to press.This information involves a number of factors that could change over time, effecting thetrue public representation. Airbus assumes no obligation to update any information containedin this document or with respect to the information described herein.Printed in FranceAirbus SAS shall assume no liability for any damage in connection with the use of thisBrochure and of the materials it contains, even if Airbus SAS has been advised of thelikelihood of such damages.Safety first #04 June 2017 - 5/5

or hydraulic pump failure or engine failure : No change : PTU runs to recover the normal pressure in the affected system. No ECAM change with new PTU logic: ECAM still requests to switch OFF the PTU – Only Pilot confirmation (in flight) with new logic – Common ECAM definition with old logic This change is covered by the following modifications. MOD 34236 35879 / SB 29-1115 .