Etihad B773 near Abu Dhabi on Jul 11th 2021, engine shut down in flight
Last Update: August 7, 2023 / 16:39:03 GMT/Zulu time
United Arab Emirates' GCAA released their final report concluding the probable causes of the incident were:
The Air Accident Investigation Sector determines that the cause of the in-flight engine shutdown was the damage of the accessory gearbox (AGB) hydro-mechanical unit (HMU) ball bearing. Consequently, the HMU quill shaft sheared by accumulated hard particles contamination, handling damage, and assembly damage as the inner ring found migrated and misaligned.
The GCAA reported that during the flight to Dhaka, about 72 minutes into the flight, the crew received a Engine Indicating and Crew Alerting System (EICAS) message "High-Pressure Turbine Active Clearance Control (HPTACC)" status message, which did not require flight crew action. After landing in Dhaka maintenance action took place, the GCAA summarized:
After inspection, the maintenance engineer advised the flight crew that an engine dry run would be required to clear the status message from the previous sector.
After the dry engine run procedure had been completed and the status message cleared, the maintenance engineer signed the technical log and the Commander accepted the Aircraft for the return flight to OMAA.
Shortly thereafter, the MCC called the crew requesting a second dry run in order to ensure that the status message would not re-appear during pushback. The requested run was carried out and no HPTACC status message appeared.
At 1145, the flight crew received a message via the Aircraft Communications, Addressing and Reporting System (ACARS), informing them about an ‘oil consumption’ message that needed action. After inspection, the maintenance engineer informed the Commander that extra time was needed to inspect the engine and he anticipated that this might take few hours.
Based on the Commander’s statement, the MCC told the engineer that there is a list of checks that need to be done on the engine. This was to eliminate any defects that might affect the engine and to ensure that the engine is in a good condition.
The engineer advised the Commander that it would take around 45 minutes to one hour for clearing the associated message.
At 1253, the flight crew sent a message to NOC stating that they have agreed to extend their FDP and asking for the latest estimated time of departure (ETD) with discretion.
At 1308, the NOC sent a message advising the crew that the latest ETD with discretion is 1500. At 1325, the NOC sent the flight crew a new OFP with the new ETD from VGHS.
The Commander stated that at about 1410, the maintenance engineer requested the electronic aircraft technical log (eATL) from the flight crew for filling the inspection and maintenance information.
At 1422, the NOC sent another load sheet with a change in take-off fuel, and it was annotated as final.
At 1454, the flight crew advised NOC that they are still waiting for maintenance paper work.
At about 1500, the maintenance engineer completed the eATL. As per the Commander’s statement, there was no residual MEL, configuration deviation list (CDL), or any operational comment that prohibited the Aircraft dispatch, and therefore he accepted the flight.
As per the ACARS message record, the Commander notified the NOC about reaching the maximum allowed FDP, and the NOC replied that the flight crew are allowed to extend their duty for 15 minutes.
At 1520, the Aircraft was pushed back from the parking stand. At 1543, the Aircraft took off with the Commander as the pilot flying (PF).
The GCAA then summarized the occurrence flight:
At about 1633, the flight crew observed an EICAS status message of HPTACC Left Valve and Engine Overspeed Left Governor which did not require flight crew action. The flight crew communicated the message to the NOC. The Aircraft was over Kolkata FIR by that time.
At 1844, the Aircraft entered Muscat FIR at flight level 380, and the flight crew requested the weather information for OMAA in preparation for the descent and approach.
The Commander stated that at 1928 he heard two bangs from the engine with moderate airframe vibration. The Commander stated, “I thought after the initial bang that it was due to turbulence.” He added that after the second or third bang, there was an EICAS message of Engine Thrust Left with N1 reducing to 40% and displaying in amber. The Commander later requested for the engine severe damage checklist due to having abnormal engine indications and airframe vibration. The flight crew then initiated the engine out drift down procedure.
The crew made an assessment of the most suitable airport for landing based on the Operations Manual – Part C (OM-C). The Commander chose OMAA as the destination based on the time required for Aircraft preparation for landing.
At 1959, the Aircraft landed at OMAA, runway 31 left.
At 2026, the chocks were positioned when the Aircraft parked at the designated stand.
The GCAA analysed:
Root Cause of the Engine Failure
The analysis of the engine at the manufacturer premises indicated that both the stall/in-flight shutdown and fuel-in-oil root causes are traced to the ball bearing failure in the HMU line of the AGB.
The engine manufacturer analysis of the engine revealed that the root cause of the engine failure is related to:
− Hard particles contamination, which were found in silver plating of the bearing cage
− Signs of handling damage which was evident from the deformation of outer ring raceway
− Assembly damage as the inner ring found migrated and misaligned.
The engine manufacturer had issued a safety action as described in the Factual Information part of this Report.
This is due to the design of the HMU which includes a speed related restriction on the use of the HPTACC. Servo valve which controls the position of the HPTACC valve. Below N2 speed of 45%, control of HPTACC servo valve is not possible. Control of the HPTACC servo valve is only possible at N2 speeds greater than 45%.
The previously FIM defined Fuel Driven Actuator test is preformed while monitoring the engine using the starter. A typical starter monitoring speed is 30% N2. This speed will not allow HPTACC servo valve to function. The Fuel Driven Actuator test will indicate PASS no matter what the condition of the HMU drive shaft is.
The revised FIM defined Air Driven Actuator test is performed while the engine is operating at idle. A typical idle speed is 67% N2. This speed will allow the HPTACC servo valve to function. The Air Driven Actuator test will PASS our FAIL based on how the HPTACC system operates.
The Commander’s Decision to Accept the Flight
The Investigation reviewed all the circumstances surrounding the return flight from VGHS to OMAA in the pre-flight stage.
The Commander stated during the interview that challenges were experienced with in the quality of information exchange with the local engineering staff and MCC regarding the maintenance work undertaken on the Aircraft. This information was considered relevant by the Commander in order to execute a more informed decision about accepting the Aircraft and conducting the return flight.
Despite holding some residual reservations about the Aircraft status and detail on the maintenance actions undertaken at VGHS, the Commander accepted the flight based on the standard practice of reviewing the eATL entries, which declared the Aircraft serviceable.
The maintenance action undertaken at VGHS was in accordance with FIM, and is not considered the root cause of the failure encountered subsequently in flight.
The Commander’s Decision to Continue to OMAA
The Investigation found that the Commander elected to land at OMAA based on the time required for preparation for landing and was consistent with the direction provided in OM-A − Handling of In-flight Abnormalities/Emergencies, and operations in areas in which escape routes and associated diversion airports have been established.
This article is published under license from Avherald.com. © of text by Avherald.com.
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