What happened
On 16 September 2011, an Air New Zealand Boeing 747-419, registered ZK-NBT, was approaching San Francisco when the flight crew received an external alert from another aircraft regarding flames visible from the number 4 engine. At the time, the cockpit instruments provided no indication of any malfunction. Following a routine landing and engine shutdown, a local engineer performed inspections and checks according to established procedures, eventually returning the aircraft to service. The aircraft completed two subsequent flight sectors without further issues.
However, during the following flight while approaching Auckland, the number 4 engine experienced a surge. The crew responded by shutting down the engine, and the aircraft landed safely using the remaining three engines.
The investigation
The investigation examined the circumstances of both events and looked for potential links between the two incidents. Investigators also reviewed recent engine failure or shutdown occurrences within the Air New Zealand fleet involving different aircraft types. A review of Rolls-Royce data regarding RB211 engine surge events was conducted, which indicated that while such stalls can cause damage or necessitate engine shutdowns, they do not compromise the overall safe operation of the aircraft.
Findings
- The specific cause of the engine surge during the Auckland approach could not be determined.
- It was not possible to establish if the initial event in San Francisco contributed to the subsequent surge in Auckland.
- The maintenance actions taken by the engineer in San Francisco, including following prescribed procedures and performing extra checks, were deemed appropriate.
- The flight crews acted correctly in managing the engine surge.
- There was no evidence of a link between the three separate engine occurrences across the fleet, nor was there any indication that the operator was reducing engineering or safety standards as certain aircraft types approached scheduled replacement.