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CUSTOMER SERVICES DIRECTORATE 2 ROND POINT EMILE DEWOITINE 31700 BLAGNAC FRANCE TELEPHONE + 33 (0)5 61 93 33 33
ALERT OPERATORS TRANSMISSION - AOT AOT ref: A73P002-21 Rev 00 Page 1 of 4 Date: 23-FEB-2022 © AIRBUS S.A.S. 2022. ALL RIGHTS RESERVED. CONFIDENTIAL AND PROPRIETARY DOCUMENT SUBJECT: ATA 73 – A350 Trent XWB-97 Engine - Introduction of Hydro-Mechanical Unit (HMU) life limit AIRCRAFT TYPE: A350-1000 OUR REF.: A73P002-21 Rev 00 dated 23-FEB-2022 EFFECTIVITY DATE: 28-FEB-2022
1. AIRCRAFT AFFECTED A350-1000 fitted with Rolls-Royce TRENT XWB-97 engines.
2. REFERENCED DOCUMENTATION
2.1 APPENDIX N/A
2.2 REFERENCES Ref.1 - Rolls-Royce NMSB 73-AK747 - FUEL AND CONTROL – HYDROMECHANICAL UNIT – COMBINING SPILL VALVE ASSEMBLY - WEAR MANAGEMENT – NON-MODIFICATION SERVICE
BULLETIN
3. REASON
3.1 FACTS In 2021, two A350-1000 aircraft experienced transient engine N1 shaft speed exceedance during takeoff. The “ENG x EGT OVER LIMIT” ECAM warning was triggered in one case, leading to a rejected takeoff. Additional cases of temporary N1 shaft speed exceedance were identified by the Rolls-Royce Engine Health Monitoring (EHM) system with no cockpit effect.
For all events, the corrective action was to replace the Hydro-Mechanical Unit (HMU) and the engine fuel pump according to Rolls-Royce EHM recommendations.
Investigations showed that an internal component of the HMU, called the Combining Spill Valve (CSV), was slow to close due to piston wear, affecting the ability of this piston to move fully and freely over its operating range.
3.2 CONSEQUENCES The primary function of the HMU is to supply the required fuel flow to the combustor. This is set by the Engine Electronic Controller (EEC) in response to an N1 target from the aircraft thanks to a Fuel Metering Valve (FMV) within the HMU. The CSV role is to control the fuel spill back to the high pressure fuel pump inlet to maintain the required metering valve pressure drop.
A CSV that is slow to respond (slow to close) can cause a delay in providing the requested high fuel flow demand to fuel nozzles, which leads to N1 shaft transient underspeed. In response to the detected N1 underspeed, the EEC then commands the FMV to fully open to compensate. When the CSV piston finally moves to the fully closed position, an excess of fuel is sent to the fuel nozzles which eventually results in an N1 transient shaft overspeed.
ALERT OPERATORS TRANSMISSION - AOT AOT ref: A73P002-21 Rev 00 Page 2 of 4 Date: 23-FEB-2022 © AIRBUS S.A.S. 2022. ALL RIGHTS RESERVED. CONFIDENTIAL AND PROPRIETARY DOCUMENT This issue only affects the engine high thrust regimes (Max Take Off and Max Continuous) on TRENT XWB-97 engine type.
At aircraft level, an engine fitted with a HMU CSV piston fully and definitively stuck, in combination with an In Flight Shut Down (IFSD) on the opposite engine (due to an independent cause) could lead at worst to the definitive loss of more than 50% of aircraft thrust. Note that such scenario was never experienced in service so far.
3.3 AIM The aim of this AOT is to set a life limit to all HMU P/N G5020HMU02 since new, since last CSV replacement, or since last overhaul (under specific conditions listed in NMSB ref.1) in accordance with Rolls-Royce instructions in NMSB ref.1. This is to ensure the CSV assembly wear remains within acceptable levels.
Refer to chapter 4.1.1 for the life limits definition.
4. SHORT TERM ACTION NOTE: This AOT is classified mandatory or expected to be classified mandatory by an Airworthiness Directive (AD). The accomplishment instructions marked as Required for Compliance (RC) must be done to comply with the AD. “RC” is indicating these paragraphs accordingly.
NOTE: The accomplishment instructions of this AOT include procedures given in other documents or in other sections of the AOT. When the words ’refer to’ are used and the operator has a procedure accepted by the local authority he belongs to, the accepted alternative procedure can be used. When the words ’in accordance with’ are used then the given procedure must be followed.
NOTE: The access and close-up instructions, not comprising return to service tests, in this AOT do not constitute or affect the technical intent of the AOT. Operators can therefore, as deemed necessary, omit or add access and/or close-up steps to add flexibility to their maintenance operations as long as the technical intent of the AOT is met within the set parameters.
4.1 PLANNING AND MANPOWER
4.1.1 RC - ACCOMPLISHMENT TIMESCALE The life limits of the HMUs installed on wing are provided in the below Table 1 and instructed through Rolls-Royce NMSB Ref.1. There are two life limits defined in two different time periods. From 31 March 2022 to 29 June 2023, the HMU life limit is 11,000 engine hours. From 30 June 2023 and on, the HMU life limit becomes 7,500 engine hours. These life limits are summarized in Table 1. HMU installed should not exceed the corresponding life limits defined in Rolls-Royce NMSB Ref.1.
Table 1: HMU Life Limits and Effectivity dates Effectivity date Life limit From 31 March 2022 to 29 June 2023 11,000 engine hours * From 30 June 2023 7,500 engine hours * * since new, since last CSV replacement, or since last overhaul (under specific conditions listed in NMSB ref.1).
ALERT OPERATORS TRANSMISSION - AOT AOT ref: A73P002-21 Rev 00 Page 3 of 4 Date: 23-FEB-2022 © AIRBUS S.A.S. 2022. ALL RIGHTS RESERVED. CONFIDENTIAL AND PROPRIETARY DOCUMENT
4.1.2 MANPOWER The estimated time per engine is 0.5 man-hour (0.5 hour x 1 person) to gain access to the HMU, 2 manhours (1 hour x 2 persons) to replace the HMU and 4 man-hours (1.33 hours x 3 persons) to restore serviceable condition (a Fuel and Oil Low-Power Leak-Test is required).
The total estimated time per engine is 6.5 man-hours.
4.2 DESCRIPTION
4.2.1 ACCESS Refer to Rolls-Royce NMSB Ref.1 for access.
4.2.2 RC - INSPECTION REQUIREMENTS Before reaching the life limits defined in the section 4.1.1 “ACCOMPLISHMENT TIMESCALE”, replace the HMU by another unit. This newly installed HMU must be less than 2,000 engine hours since new, since last CSV replacement, or since last overhaul (under the specific conditions defined in NMSB ref.1).
4.2.3 RC - FINDINGS If an HMU installed is above the limits defined in the section 4.1.1 “ACCOMPLISHMENT TIMESCALE”, remove and replace the affected HMU in accordance with current Maintenance Procedure (MP) tasks within 3 flight cycles.
4.3 SPARES AND TOOLING Refer to Rolls-Royce NMSB Ref.1 for spare parts and tooling.
5. FURTHER INFORMATION
5.1 FOLLOW-UP PLAN The HMU life limits are expected to be mandated by the Airworthiness Authorities.
Further technical follow-up, if needed, will be provided through Airbus Technical Follow Up (TFU) ref.
73.21.00031.
This AOT will be cancelled with the introduction of a new HMU part number.
5.2 IMPACTED DOCUMENTATION None.
6. AOT APPROVAL The technical content of this document is approved under the authority of the DOA ref.EASA.21J.031.
7. REPORTING
ALERT OPERATORS TRANSMISSION - AOT AOT ref: A73P002-21 Rev 00 Page 4 of 4 Date: 23-FEB-2022 © AIRBUS S.A.S. 2022. ALL RIGHTS RESERVED. CONFIDENTIAL AND PROPRIETARY DOCUMENT Questions about this AOT are to be addressed to Airbus Customer Services through TechRequest on Airbus World, selecting Maintenance & Engineering Domain, Engineering Support Section and ATA 73-
00.
This Rolls-Royce NMSB Ref.1 is the subject of an AIRBUS monitored retrofit campaign. For the organization of this retrofit, you are kindly requested to contact AIRBUS retrofit management department: monitored.retrofit@airbus.com.
Best Regards, Ahmed CHABBOUH