On 20 June 2012, an Airbus A320 (ZK-OJQ) being operated by Air New Zealand on a domestic passenger flight from Wellington to Auckland experienced sudden surging of the right hand V2500 engine late on final approach at destination in day Visual Meteorological Conditions (VMC). The engine involved was set to idle and the intended landing was completed without further event with taxi-in accomplished using the remaining serviceable engine. On the previous flight, the aircraft had ingested a bird into the same engine when landing and was operating in accordance with an approved deferral of a required borescope check of the engine for one further flight.
After receiving preliminary information about the occurrence, in which it was clear that IAE 2500 engine involved had sustained substantial core damage, the New Zealand Transport Accident Investigation Commission (TAIC) decided to open an Investigation.
It was found that although the flight crew were initially unaware that the bird had been ingested into the IAE V2527-A5 engine and used reverse thrust on both engines to decelerate as usual, soon after this, the strong smell characteristic of bird ingestion into a turbine engine core became evident. On arrival at the parking gate, ATC and line maintenance were advised. An initial inspection of the engine by a maintenance engineer confirmed that a bird had entered the core of the right engine and the aircraft was withdrawn from service for the inspection prescribed in the Aircraft Maintenance Manual. Following this inspection, in which no evidence of damage was found, the aircraft was released to service and departed for Auckland in revenue service. Bird remains were sent for analysis and the bird was later subsequently identified by DNA analysis as a male Southern Black-Backed Gull (an adult male typically weighs over 1kg).
It was noted that that the AMM procedure after bird ingestion required an internal examination of the engine using a borescope to conclusively verify that no damage had occurred. However, with no obvious signs of damage found on the initial inspection and only one engine involved, the procedure allowed the borescope inspection to be deferred for up to 10 hours’ flying or one further flight whichever came first and this conditional release to service was made. The aircraft subsequently left Wellington in revenue service on the incident flight to Auckland with a different flight crew to the one which had experienced the earlier bird strike.
The 35 minute flight was uneventful until the aircraft was between 1500 feet and 1000 feet on approach at Auckland with the runway in sight when loud banging noises were heard from the right engine indicating compressor stall. A smell of burnt bird became increasingly evident both on the flight deck and in the passenger cabin. In response, the right engine thrust lever was set to idle and the banging noises stopped. The Captain "elected not to spend time trying to find a thrust lever position where the stall ceased, as the runway was clear but a heavy rain shower was approaching the far end" and the left-engine thrust lever was advanced to maintain the required flight path. A 'PAN' call was made to ATC requesting the Rescue and Fire Fighting Services to attend the aircraft after landing and in a brief conversation between the First Officer and the Inflight Service Manager to confirm that the cabin was secure for landing, it was established that flames had been seen coming from the right engine tail pipe when the banging noise had occurred.
After touchdown, both thrust levers were selected to reverse with a normal response. Once at taxi speed, the aircraft cleared the runway and was stopped on a taxiway where the right engine was shut down. Once the the RFFS had confirmed that there was no fire or obvious danger, the aircraft was taxied to the gate using the left engine.
A line maintenance engineer made an initial inspection and reported that there was a very strong smell of burnt bird at the rear of the right engine but no obvious signs of an ingestion except two small pieces of bird feather on one of the fan exit guide vanes inside the engine fan case. These were subsequently confirmed to be from the same bird species as the other remains removed earlier from the same engine and had "identical" DNA sequences. Once the aircraft had been removed to a hangar, the deferred borescope inspection was carried out and revealed that one of the third-stage high-pressure-compressor blades was missing and that this missing blade appeared to have caused substantial damage to the engine core as it had exited through the core. The engine was removed from the wing and sent to a specialist repair facility for further assessment.
A full examination of the engine at this facility confirmed extensive engine damage and that it was attributable to bird ingestion which had initially led to cracking of the missing compressor blade. This crack had then grown under the stress of continued engine operation in a damaged state and eventually led to it fracturing completely and causing significant damage to other engine components as it passed through all the other compressor stages. Compressor blades adjacent to the failed one were found to have "soft body impact damage typical of a bird strike".
It was noted that the Operator's Maintenance Operations Centre (MOC) had routinely received ACARS data from the aircraft during the flight from Wellington to Auckland in the form of a 'Cruise Report'. This had contained three alerts which indicated changes in EGT, fuel flow and N2 vibrations on the right engine. However, "an incorrect data character in the report resulted in it being sent to a telex-error-holding folder, so the alerts did not appear in front of the MOC duty manager during the flight". The message was not seen until the evening of the same day after a systems engineer had fixed the problem by correcting the data character which allowed the report to be processed correctly. FDR data for the flight showed similar differences between the two engines throughout the flight.
The Investigation formally documented the following five Findings:
- It is highly likely that this contained engine failure was the result of a single bird strike event on the previous flight when the aeroplane was landing at Wellington Aerodrome, when a black-backed gull was ingested into the engine core.
- The maintenance actions taken by the operator following the bird strike exceeded the engine manufacturer’s requirements.
- Releasing the aeroplane to service under the “fly-on allowance” would have been highly unlikely to result in an unacceptable risk to flight safety.
- Indications that the right-hand engine was not performing well were not detected by the Maintenance Operation Control due to programming logic errors in the automated engine condition report system. However, even if they had been, it is unlikely that any subsequent action would have prevented the engine compressor stall event on landing at Auckland.
- Wellington International Airport is providing an effective bird management programme that is keeping the risk of bird strikes as low as reasonably practicable.
No Safety Recommendations were made but the Commission formally noted the following two Key Lessons:
- Although the safety of the aeroplane and the persons on board was not unduly compromised by releasing the aeroplane to service knowing that a bird had been ingested into the core of one engine, operators will need to balance the cost of having inspection services available at key aerodromes into which they fly with the cost of an engine failure of this scale.
- Even if the minimum mandatory checks are made to an engine that has suffered a bird strike down the core, if the aeroplane is released to service before the required full inspection has been undertaken, the pilots and ground engineering services should maintain increased vigilance of engine performance until the appropriate full maintenance checks can be completed.
The Final Report was approved for publication on 28 May 2015.