Investigators have been unable to find what caused flames to burst from an Air New Zealand jumbo jet's engine as it neared San Francisco.
The Transport Accident Investigation Commission released its report today on an Air New Zealand Boeing 747-419 which had flames coming from its number-four engine as it approached San Francisco on September 16, 2011.
The crew, who were unaware of the flames, were alerted by another aircraft. There was no indication on the flight deck of any problem.
TAIC said the engine was shut down and the plane landed without incident.
A local engineer inspected the engine and conducted extra checks, and the plane returned to normal service.
The aircraft flew two more sectors without incident, but on its approach to land at Auckland the number four engine "surged" and was shut down by the crew.
The report said engine compressor stalls and surges "can be dramatic, especially for passengers". A surge is often caused by an air pressure change in the engine and can be accompanied by a loud bang.
TAIC said "a review by Rolls-Royce of reported RB211 engine surge events showed that while [an engine] stall could result in damage or having to shut down the engine, the safe operation of the aeroplane should not be affected".
TAIC found the actions following the San Francisco incident were "appropriate".
"The cause of the surge at Auckland was not identified and it could not be determined whether the San Francisco occurrence contributed to the more severe occurrence at Auckland."
The commission said it became aware of two other engine shutdowns in Air New Zealand's fleet.
"Although these incidents had involved different aeroplane types, each type, like the Boeing 747, had been scheduled for replacement in the short-to-medium future.
The inquiry found no link between the three engine incidents and nothing to suggest that the operator was accepting lower engineering or safety standards as the three aeroplane types neared replacement.