On 27 September 2016, the left GE90-115B engine of a Boeing 777-300 (A6-ETL) being operated by Etihad Airways on an international passenger flight from Abu Dhabi to Sydney as EY450 failed during takeoff shortly after rotation in day VMC. After completing the takeoff and declaring a MAYDAY to ATC, tyre debris was discovered on the runway and reported to the augmented flight crew who identified that they had a nose landing gear problem. The subsequent return to land was uneventful although nose wheel steering was unavailable because of damage to the nose wheel steering system and the aircraft was stopped on the runway. After confirmation that there was no fire, occupants were disembarked on the runway using steps positioned at exits on the right hand side.
An Investigation was carried out by the UAE GCAA Air Accident Investigation Sector. The aircraft CVR and DFDR were removed and their data successfully downloaded.
It was noted that the 39 year-old Captain, who was PF for the departure, had accumulated 8,130 total flying hours which included 1,325 hours on type. The 47 year-old First Officer had accumulated 13,000 total flying hours which included 2,309 hours on type. A relief crew consisting of a 50 year-old Captain with 12,300 hours including 392 hours on type and a 41 year-old First Officer with 8,927 hours including 1,420 hours on type were occupying the flight deck supernumerary crew seats.
It was established that takeoff from the 4100 metre-long runway 13R had been conducted at a surface air temperature of 36° C in the late morning. As the aircraft accelerated towards Vr, the crew reported that they had felt vibration in the flight deck and that at rotation, which FDR data showed had commenced after 54 seconds of takeoff roll at 196 KIAS, a loud bang had been heard, a “L ENG FAILURE” EICAS message was displayed. The takeoff was completed with No 1 engine auto shutdown taking place soon after the aircraft became airborne. The AP was engaged at approximately 400 feet agl, the landing gear selected up at a recorded 539 feet agl and at 182 KIAS, the engine failure checklist was completed and a MAYDAY was declared. ATC then alerted the airport RFFS and requested an inspection of the runway during which tyre debris was discovered and the flight crew advised of this finding.
The flight crew identified that the aircraft had a problem with the NLG but could not determine its condition. After just over half an hour airborne, during which preparations were made for an overweight (340 tonne) single engine landing on runway 13L at a calculated Vref of 201 KIAS with autobrake 4 set, this was completed uneventfully. It was noted that touchdown occurred approximately 1,280 metres past the runway threshold and the aircraft stopped after a further 2,590 metres with just 230 metres remaining. An RFFS inspection confirmed to the Captain that there were no signs of fire and all occupants were disembarked on the runway using passenger stairs at the R1 and R2 doors and bussed to the terminal building.
The extent of internal damage to the failed No. 1 engine was considerable and included damage to the fan, compressor, and hot section. The prolonged landing roll created high brake temperatures and all MLG tyres deflated after their thermal fuses melted. Both nose gear tyres maintained their pressure but the left tyre was observed to have shed 90% of its outer tread (see the illustration below) which had disintegrated with several pieces then striking various parts of the aircraft structure and some being ingested into the No. 1 engine. It was also found that there was evidence that the No. 2 engine inboard fan cowling had been struck by tyre debris but had not been damaged by the impact and there was no evidence suggesting that any tyre debris had been ingested. The tyres fitted to the affected NLG were identified as having been manufactured by Bridgestone and it was noted that this manufacturer’s tyres were in exclusive use on all Etihad aircraft with the operator having no previous experience of NLG tread separation. It was noted that the Operator’s policy on re-treading of nose wheel tyres permitted up to three such re-treads and that the tyre involved in the event had been given its third re-tread.
The NLG showing the left tyre and associated steering cable damage. [Reproduced from the Official Report]
An extremely detailed examination of the failed NLG tyre and associated debris was carried out by its manufacturer. There was no evidence of any manufacturing defect or retreading issue which could have led to the tread separation. However, it was found that prior to the shedding of the tread, there was evidence that it had been damaged by prior contact with a sharp foreign object and that the tread detachment had subsequently occurred almost simultaneously in three areas as the aircraft rotated during takeoff. The Investigation was unable to determine when, where or how the initial damage had occurred but given the nature of it, it was concluded that either FOD or possibly the sharp edges of embedded guidance lighting on and in the vicinity of parking stands was the most likely cause. A potential connection between the sharp edges to embedded guidance lights and the use of towbarless tugs was found when it was established that at Abu Dhabi, towbarless tug disconnection was known to have resulted in lighting damage. It was, however, noted that the pushback prior to the investigated takeoff had been carried out using a conventional tug and towbar combination, but the initial tyre damage could have occurred during a towbarless tug operation when departing on a recent previous flight from an airport where one was used.
The tyre manufacturer reported that the event under investigation was their first involving NLG tyre failure debris being ingested into and damaging a Boeing 777 engine. They also noted that their record of tyre tread detachment due to FOD between 2012 and 2016 had been 165 failures per million flights.
In order to find out if any embedded manoeuvring area ground lighting might have been a source of tyre tread damage, the lights used which might have been encountered by the aircraft as it taxied from its gate into position on the runway were inspected, and it was found that some of those installed on parking gate centrelines had “sharp edges” which it was considered had potential to cause tyre damage. The reason for this was found to be impact damage caused by towbarless tugs when disengaging from aircraft after pushback and appropriate tug driver retraining was carried out. However, it was not possible to demonstrate that this had happened to the aircraft involved and it was also noted that no FOD item had been found during an inspection of the aircraft’s taxi-out route immediately after the event.
On the wider question of FOD detection and removal at the airport, it was noted that there was no automated runway FOD detection system and detection relied on manual inspections. It was also noted that airport CCTV surveillance did not cover all aircraft parking areas and that FOD sweepers were not equipped with magnets and were therefore “not as effective as they should have been in removing potentially sharp metallic debris”.
On the basis of the evidence gathered, the Investigation concluded that:
- There had been a significant possibility that the No.2 engine might have also ingested some pieces of tyre debris which could have resulted in a more hazardous situation than actually occurred and noted that Boeing had not performed a risk analysis of tyre debris ingestion causing the failure of either one or both engines.
- There was considerable room for improvement in the effectiveness of FOD detection and removal on the manoeuvring areas of Abu Dhabi airport.
- The number of tyre retreads was not a factor in tread separation and neither were the tyre manufacturing or retreading processes.
The Findings of the Investigation included the following:
- Abu Dhabi International Airport was not equipped with an automated FOD detection system to cover the runways, taxiways and manoeuvring areas.
- Boeing had not performed a risk analysis of the possibility of nose wheel tire debris being ingested into one or both engines potentially leading to engine damage or failure.
The Cause of the event was formally documented as: “the shedding of the No.1 nose wheel tyre tread occurred as a result of the tyre contacting foreign object debris (and) the damaged tyre debris was ingested by the No.1 engine causing engine failure”.
A total of six Safety Recommendations were made as a result of the Investigation as follows:
- that the General Civil Aviation Authority (GCAA) of the United Arab Emirates should conduct regular inspections and audits on the airport ground lights fittings to ensure that they remain free of sharp edges or loose bolts that could have the potential to cause damage to aircraft tyres. [SR18/2018]
- that the General Civil Aviation Authority (GCAA) of the United Arab Emirates should implement mitigations to prevent damage to airport ground lights caused by towbarless tugs. [SR19/2018]
- that the General Civil Aviation Authority (GCAA) of the United Arab Emirates should carry out risk assessment to determine whether an automated FOD detection system should be installed or not. [SR20/2018]
- that the General Civil Aviation Authority (GCAA) of the United Arab Emirates should supervise a study on the need of a requirement for airports to be equipped with runway and taxiway sweepers fitted with magnets to attract ferrous foreign objects. [SR21/2018]
- that Abu Dhabi International Airport should install an automatic foreign object detection system. [SR22/2018]
- that the Federal Aviation Administration (FAA) of the United States should evaluate a requirement for The Boeing Company to carry out a risk assessment of nose landing gear tyre debris being ingested into both aircraft engines following tyre failure or tread shedding. [SR23/2018]
The Final Report was issued on 9 October 2018.