On 20 October 1998, in the North Sea, an Eurocopter AS332L Super Puma operated by Norsk HeliKopter AS, experienced engine failure with autorotation and subsequent lost of height. The crew misidentified the malfunctioning engine and reduced the power of the remaining serviceable engine. However, the error was detected just in time for the crew to recover control of the helicopter.
The following is an extract from the official Investigation Report published by the Aircraft Accident Investigation Board (AAIB), Norway:
“[…]The flight proceeded normally at an altitude of 2 000 ft until the crew at 12:23:04 hours discovered that the torque on the right engine (Tq 2) started to rise and became unstable. The RPM of the gas turbines (Ng 1 and Ng 2) increased correspondingly. No other indications were observed at that time. The crew then noticed an unusual engine noise. The torque indicator was set to indicate separate values for Tq 1 and Tq 2 (crosshatched position). This showed that the torque value on the left engine was correspondingly low. The crew analysed the situation and got the impression that the problem was related to the right engine.”
The crew decreased the power of the right engine to approximately 75-80%.
“The noise from the engine area then increased substantially, and the warning indicator for high Nr [rotor RPM] came on for a short period. Later analysis of the helicopter’s Flight Data Recorder (FDR) showed that the rotor RPM reached 109% for one second before it was stabilised by the Co-pilot. The Ng 1 reached in this period 101.3% and this may explain the high Nr. The collective pitch was then reduced to 7 degrees and the helicopter entered autorotation at a speed of 80 kt.
[…]the Pilot-in-Command sent a distress signal, 3 times MAYDAY. At that time, the crew was confronted with a large number of alarms and warnings in the form of lights and sound signals. At the same time, the intense noise from the engine area began to diminish. The crew then observed that Ng 1 had dropped to below 20%, but an attempt to start this engine only resulted in excessive turbine temperature. The Co-pilot informed the passengers that they should prepare for an emergency landing at sea, and asked them to assume the ‘brace position’. […]
[…]the crew ascertained that the right engine apparently was operating normally with Ng 2 of approx. 80%, and the right FFCL was pushed in to the ‘Flight detent’ position. The engine reacted normally and the descent was halted at an altitude of 500 ft. The crew then set course towards land. At that time, the left engine was shut down and the cockpit was ‘cleaned up’. The helicopter gradually climbed to an altitude of 1 000 ft. […]”
Further the report presents the following comments from AAIB Norway:
“The problem with the left engine became a serious incident because the crew reduced the power on the wrong engine in response to the observations they had made. As a result the helicopter was almost without engine power for a period of time. The AAIB/N is of the opinion that the possibility of engine failure cannot be avoided. Safety must therefore depend on making sure that flying can proceed in a satisfactory manner with the one engine that is still functioning properly.
This incident highlights a problem that is commonly present when two engines are connected to a common gearbox. The challenge lies in ascertaining whether the one engine is failing in such a way as to place a greater load on the other engine, or whether one engine is taking on too great a load because a fault in that engine makes the other engine ‘redundant’. “
The AAIB Norway, produced the following recommendations based on the serious incident:
- “The AAIB/N recommends the company to set up a training programme to improve the crew’s skill in dealing with engine problems on the Eurocopter AS 332 Super Puma […].
- The AAIB/N recommends the Civil Aviation Authority of Norway (CAA/N) in co-operation with the aviation authorities in France to assess whether it should be mandatory to install magnetic plugs with warning lights in all civil Eurocopter AS 332 Super Puma aircraft […].“
For further information see the full incident report published by AAIB/N.