ANZ A320 at Wellington and Auckland on Jun 20th 2012, bird strike in Wellington, engine failure in Auckland

Last Update: June 18, 2015 / 15:15:02 GMT/Zulu time

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Photo of ANZ Air New Zealand ZK-OJQ, Airbus A320 — ANZ Air New Zealand ZK-OJQ, Airbus A320 (Photo credit: Aero Icarus / Flickr / License: CC by-sa)

Incident Facts

Date of incident
Jun 20, 2012

Classification
Report

Cause
Engine failure

Airline
ANZ Air New Zealand

Flight number
NZ-406

Departure
Auckland, New Zealand

Destination
Wellington, New Zealand

Aircraft Registration
ZK-OJQ

Aircraft Type
Airbus A320

ICAO Type Designator
A320

An ANZ Air New Zealand Airbus A320-200, registration ZK-OJQ performing flight NZ-406 from Auckland to Wellington (New Zealand), completed a seemingly uneventful flight with a safe landing on Wellington's runway 16. After the thrust reversers were selected a smell consistent with a bird strike into an engine developed. The crew notified tower and maintenance of a possible bird strike.

Maintenance agreed with the assessment of the odour by the flight crew, that a bird strike into an engine (V2527) had occurred, initial examination identified a bird had been ingested into the core of the right hand engine, and removed the aircraft from service, the scheduled return flight NZ-418 was cancelled.

A full examination of the right hand engine was initiated, bird debris was collected and sent for analysis which later revealed a male black-backed gull had struck the engine. Following the maintenance manuals the maintenance engineers completed all required steps, in addition the first stages of the low pressure compressor blades were inspected with a mirror and torch, no damage was found. Maintenance subsequently undertook a low power ground run verifying the engine did not show any vibration and checking whether the odour from the engine was clearing, after about 8-9 minutes staff people being sent in from the terminal stated that no odour was noticeable anymore. Maintenance released the aircraft to service according to the maintenance manual which permitted continued operation for 10 flying hours or one sector (whichever ended first).

A replacement crew positioned from Auckland to Wellington, was advised of the occurrence and advised, that a smell was still possible during climb on high engine power, that should subside however. The crew accepted the aircraft. Maintenance asked the captain whether it was possible to give the engine a "a good run-up before take-off to check that the engine performance parameters were normal". The captain announced to passengers, that it was possible that a smell would occur during the flight.

The aircraft departed for flight NZ-428 from Wellington to Auckland with 167 passengers and 5 crew. After lining up on the departure runway the crew stood on the brakes while accelerating the engines. After the engines had reached takeoff power, the crew verified that all engine parameters were normal, no smell was noticed in the cabin, then released the brakes.

As expected, during initial climb another strong smell developed in the cabin, which seemed to soon reduce again but did not dissipate.

While in cruise at FL350 for a few minutes the first officer took note of the engine parameters noting that the right hand engine showed 19 degrees C higher EGT than the left hand engine.

On final approach to Auckland's runway 23L, while descending between 1500 and 1000 feet AGL, the right hand engine stalled emitting a series of bangs, a strong odour of burnt bird developed in the cabin and cockpit. The crew reduced the engine to idle, declared PAN, completed the landing checklist and briefly talked to purser who reported streaks of flame had been seen from the tail pipe of the engine, and continued for a safe landing on runway 23L.

New Zealand's TAIC released their final report concluding the probable causes of the incident were:

- 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.

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 TAIC reported that following landing in Auckland four acoustic panels past the engine fan blades, two beyond repair, were found damaged. One third stage compressor blade was found cracked, the TAIC stated: "The fractured blade displayed high-cycle-fatigue crack growth (propagation) followed by aerofoil liberation. High-cycle-fatigue crack growth is caused by stresses placed on the blade during in-service vibrations. The crack originated from the mid chord on the suction side (convex) of the blade about 43 millimetres above the platform and 23 millimetres from the blade leading edge." Nine low compressor blades showed tip curl, which most likely was caused by contact of the blades with the front case either during the bird strike or the engine stall. Bird debris was found on the high pressure compressor variable inlet guide vanes, the TAIC wrote: "All the variable inlet guide vanes, variable stator vanes and subsequent fixed stator vane stages were damaged beyond repair and had to be replaced. The stage 8 rotor path case was replaced due to the amount of foreign object damage." The diffuser section showed damaged caused by foreign objects passing through, the TAIC completed the description of engine damage: "The exit stator case was scrapped and 11 fuel nozzles out of the 20 were scrapped due to impact damage to the inner heat shields. All the liner segments were scrapped in the combustion section due to the large amount of impact damage and metal deposits found on the surface of the liners. The number 4 bearing compartment and stage 1 nozzle guide vanes had heavy maintenance action performed due to the amount of metal debris and metal deposits found in these areas. A significant amount of metal deposits and debris found in the high-pressure turbine module. Eleven number 1 turbine wheel blades were scrapped due to the metal deposits on the external surfaces of the blades. The entire stage 2 nozzle guide vanes and stage 2 blades were repaired due to the metal deposits found on the surface of the parts. Metal deposits were found throughout the engine, with the exception of the exhaust, with associated damage rearward of the third-stage high-pressure turbine."

The TAIC reported, that the aircraft was equipped with automatic reporting of engine parameters via ACARS. Due to the low power ground run in Wellington the automatic reports were inhibited for one takeoff, the ACARS reporting came alive again in cruise where the ACARS messages indicated changes in the EGT, fuel flow and vibrations of the right hand engines, 3 alerts were generated. However, one of the message had contained an invalid character which caused all these messages to be redirected into a holding folder, the airline's operations center therefore did not get to see the messages until a system administrator had reviewed the messages and fixed the illegal character.

The TAIC reported with reference to the flight data recorder that right from the beginning of the engine acceleration for takeoff the engine parameters showed the right hand engine was running about 30 degrees C hotter than the left hand engine, the fuel flow was 3.7% and the vibration level was 0.6 units higher than the left hand engine. Trend data over the 18 months preceding the occurrence did not show any such split between the engines.

The TAIC analysed: "The engineer who carried out the inspection at Wellington followed the prescribed procedure for a bird strike. He also performed several additional actions to identify any damage. With no damage found, and after consulting the operator’s MOC, he released the aeroplane back into revenue service in accordance with the fly-on allowance. The engine subsequently failed early in the 10-hour allowance."

The TAIC ruled out a second bird strike: "The engine failure on the approach to land at Auckland was the result of the continued operation of the engine damaged in the earlier bird strike as the aeroplane was landing at Wellington. The possibility of a second bird strike on the same engine was considered highly unlikely for several reasons. Firstly, the engine failure occurred at a height where, according to bird strike data, a strike from a black-backed gull would not normally be expected. Secondly, the DNA collected at Wellington and Auckland corresponded to the same bird species and sex and, as far as testing allowed, the same bird. Finally, and perhaps most importantly, there was clear evidence of a change in engine performance when the aeroplane departed Wellington."

The TAIC, following extensive analysis with respect to risk assessment and damage estimation, analaysed that "there was theoretically a 41% probability of some damage being present." and continued. "The manufacturer advised that this occurrence was the first recorded event of a blade release within the aircraft maintenance manual fly-on allowance. The manufacturer also reported that until the incident on 20 June 2012 there had been no in-flight shutdowns on any of the 55 preceding flights operating under the fly-on allowance. While this may give confidence in the robustness of the engine to withstand the core ingestion of a medium-sized bird, the engine surge on approach to Auckland occurred within 45 minutes of the aeroplane departing Wellington – well inside the 10-hour limit. The engine run at Wellington was a low-power run of short duration only and was not considered part of the fly-on allowance."

The TAIC concluded analysis: "This analysis of the risk following a single-engine bird strike event involving the IAE engine included a review of that risk assessment by the various regulators and aeroplane and engine manufacturers. The argument supported the hypothesis that a single-engine bird strike on this type of aeroplane fitted with this type of engine was highly unlikely to result in an unacceptable risk to flight safety. Accordingly the Commission has no recommendation to make on that matter."