Photo: Crash site of G-OBME, from Accident Report, figure 1.

Eddie Sez:

The British accident report does a good job of dissecting the events that led to this crash, but they miss the actual cause. The aircraft experienced a compressor stall that did not immediately cause the left engine to fail, but did cause severe vibration and smoke through the air conditioning system. The captain's experience in previous aircraft told him the smoke had to be coming from the right engine and he very quickly disconnected the autothrottles and brought the right engine back to idle. The compressor stalls eased, leading both pilots to believe they got the correct engine. Neither discerned from the engine instruments the real problem and because the right engine was at idle, they were denied a chance to compare the two engines. The captain had several opportunities to discern the real situation from the airplane's yaw before and after retarding the right engine to idle. They shut down the wrong engine and when the left engine finally quit, they had no place to go but down.

The accident report gets all that. But why did both pilots react so quickly to a situation that called for a more deliberate approach? I think they panicked. Both pilots wanted to resolve the situation quickly and fooled themselves along the way that the problem was resolved. There are very few situations in a modern airplane where actions need to be immediate. More about this: Pilot Psychology / Panic.

What follows are quotes from the relevant regulatory documents, listed below, as well as my comments in blue.


Accident Report


Narrative

[Accident Report, ¶1.1]


Analysis

[Accident Report, ¶2.1] After an uneventful takeoff and climb the crew suddenly heard an unusual noise, accompanied by vibration, as the aircraft passed through FL283. The noise was heard in the cabin as a series of thuds and the FDR indicated that it was directly associated with the stalling of the fan and/or LP compressor with attendant surging of the No 1 engine. In addition to the noise and vibration, the lateral and longitudinal accelerations recorded on the FDR were consistent with the reported lower frequency shuddering that was sufficiently marked to shake the walls of the forward galley. Very soon after the onset of these symptoms there was a smell of fire and possibly some visible smoke in the cockpit. The combination was interpreted by the pilots as evidence of a serious engine malfunction, with an associated fire, and appears to have driven them to act very quickly to contain this perceived condition.

[Accident Report, ¶3.(a) 19] The No 1 engine suffered fatigue of one of its fan blades which caused detachment of the blade outer panel. This led to a series of compressor stalls, over a period of 22 seconds until the engine autothrottle was disengaged.

[Accident Report, ¶3.(a) 3] The flight deck crew experienced moderate to severe engine induced vibration and shuddering, accompanied by smoke and/or smell of fire, as the aircraft climbed through FL283. This combination of symptoms was outside their training or experience and they responded urgently by disengaging the autothrottles and throttling-back the No 2 engine, which was running satisfactorily.

[Accident Report, ¶2.1] Neither pilot appears to have assimilated from the engine instruments any positive indication of malfunction, but subsequent tests show the engine instrument system to have been serviceable and there was no evidence to indicate that it did not display the large engine parameter variations that occurred when the compressor surged. The FDR showed four distinct excursions in N1 on the No 1 engine, with a 6 second period of relative stability between the second and third.

[Accident Report, ¶3.(a) 4] After the autothrottle was disengaged, and whilst the No 2 engine was running down, the No 1 engine recovered from the compressor stalls and began to settle at a slightly lower fan speed. This reduced fan speed after the autothrottle was disengaged, and whilst the No 2 engine was running down, the No 1 engine recovered from the compressor stalls and began to settle at a slightly lower fan speed. This reduced the shuddering apparent on the flight deck, convincing the commander they had correctly identified the No 2 engine as the source of the problem.

[Accident Report, ¶2.1]

[Accident Report, ¶3.(a) 24] Fifty three seconds before ground impact, the No 1 engine abruptly lost thrust as a result of extensive secondary fan damage.


Probable Cause

[Accident Report, ¶3 (b)] The cause of the accident was that the operating crew shut down the No 2 engine after a fan blade had fractured in the No 1 engine. this engine subsequently suffered a major thrust loss due to secondary fan damage after power had been increase during final approach to land. The following factors contributed to the incorrect response of the flight crew:

  1. The combination of heavy engine vibration, noise, shuddering and an associated smell of fire were outside their training and experience.

  2. They reacted to the initial engine problem prematurely and in a way that was contrary to their training.

  3. They did not assimilate the indications on the engine instrument display before they throttled back the No 2 engine.

  4. As the No 2 engine was throttled back, the noise and shuttering associated with the surging of the No 1 engine ceased, persuading them that they had correctly identified the defective engine.

  5. They were not informed of the flames which had emanated from the No 1 engine and which had been observed by many on board, including 3 cabin attendants in the aft cabin.

See Also

Pilot Psychology / Panic

Procedures & Techniques / Crew Resource Management


References

Aircraft Accident Report 4/90, Department of Transport, Air Accidents Investigation Branch, Royal Aerospace Establishment, Report on the accident to Boeing 737-400 G-OBME near Kegworth, Leicestershire on 8 January 1989