British Midland Airways 092
Accident Case Study
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.
Photo: Crash site of G-OBME, from Accident Report, figure 1.
- Date: 8 January 1989
- Time: 20:25
- Type: Boeing 737-4Y0
- Operator: British Midland Airways
- Registration: G-OBME
- Fatalities: 0 of 8 crew, 47 of 126 passengers
- Aircraft Fate: Destroyed
- Phase: Approach
- Departure Airport: London-Heathrow Airport (LHR/EDLL), United Kingdom
- Destination Airport: Belfast-International Airport (BFS/EGAA), United Kingdom
[Accident Report, ¶1.1]
- At 2005.05 hrs, as the aircraft was climbing through FL283 some 20 nm south-south-east of East Midlands Airport, the crew experienced moderate to severe vibration and a smell of fire.
- Replay of the FDR showed that severe vibration had occurred in the No 1 (left) engine at this time, accompanied by market fluctuations in fan speed (N1), a rise in exhaust gas temperature (EGT) and low, fluctuating, fuel flow.
- The commander took control of the aircraft and disengaged the autopilot. He later stated that he looked at the engine instruments but did not gain from them any clear indication of the source of the problem. He also later stated that he thought that the smoke and fumes were coming forward from the air conditioning system, which, from his appreciation of the aircraft air conditioning system, led him to suspect the No 2 (right) engine. The first officer said that he monitored the engine instruments and, when asked by the commander which engine was causing the trouble, he said 'IT'S THE LE ... IT'S THE RIGHT ONE.', to which the commander responded by saying, 'OKAY, THROTTLE IT BACK'.
- The commander's instruction to throttle back was given some 19 seconds after the onset of the vibration when, according to the FDR, the No 2 engine was operating with steady engine indications. During the 11 seconds that elapsed between the disengagement of the autopilot and the throttling back of the No 2 engine, the aircraft rolled slowly to the left through 16 degrees but the commander made no corrective movement of aileron or rudder.
- Within 1 to 2 seconds of the closure of the No 2 throttle the aircraft rolled level again, the fluctuation in lateral and longitudinal accelerations ceased, the No 1 engine fan speed settled at a level 3% below its previous stable speed, and the EGT stabilised at 50°C above its previous level.
- The commander then ordered the first officer: 'SHUT IT DOWN'. This order was given 43 seconds after the onset of the vibration buts its execution was delayed when the commander said 'SEEMS TO BE RUNNING ALRIGHT NOW. LET JUST SEE IF IT COMES IN'.
- 2 minutes 7 seconds after the start of the vibration and during a short pause in radio communications with BMA Operations, the fuel cock (start lever) of the No 2 engine was closed and the auxiliary power unit was started.
- The three flight attendants in the rear of the cabin saw evidence of fire from the No 1 engine, and two of them briefly saw light coloured smoke in the cabin.
- The commander then broadcast to the passengers on the cabin address system that there was trouble with the right engine which had produced some smoke in the cabin, that the engine was now shut down and that they had could expect to land at East Midlands Airport in about 10 minutes. The flight attendants who saw signs of fire on the left engine later stated that they had not heard the commander's reference to the right engine. However, many of the passengers who saw signs of fire on the left engine heard and were puzzled by the commander's reference to the right engine, but none brought the discrepancy to the attention of the cabin crew.
- During the descent the commander did not re-engage the autopilot but flew the aircraft manually, whilst the first officer dealt with radio communications. Flight deck workload remained high as the first officer obtained details of the actual weather at East Midlands and attempted without success to programme the flight management system to display the landing pattern at East Midlands. This last activity engaged the first officer's attention for 2 minutes.
- When the aircraft was 13 nm from touchdown on this new heading, and descending to 3,000 feet amsl, ATC advised a right turn to bring the aircraft back to the centerline. At 2020.03 hrs, during this turn, power was increased on the No 1 engine to level aircraft momentarily at 3,000 feet and maximum indicated vibration was again recorded on the FDR. The aircraft was then cleared to descend to 2000 feet and the commander began a slow descent, calling successively for 2° and then 5° of flap. After joining the centreline, at 2000 feet above ground level (agl), the commander called for the landing gear to be lowered and, as he paused the outer marker at 4.3 nm from touchdown, called for 15° of flap. One minute later, at 2023.49 hrs, when the aircraft was 2.4 nm from touchdown at a height of 900 feet agl, there was an abrupt decrease in power from the No 1 engine. The commander called immediately for the first officer to relight (ie restart) the other engine and the first officer attempt to comply.
- The commander then raised the nose of the aircraft in an effort to reach the runway. 17 seconds after the power loss the fire warning system operated on the No 1 engine and 7 seconds later the ground proximity warning system (GPWS) glideslope warning sounded and continued with increasing repetitive frequency as the aircraft descended below the glidepath. The commander ordered the first officer to carry out the fire drill. At 2024.33 hrs the commander broadcast a crash warning on the cabin address system using the words 'PREPARE FOR CRASH LANDING' (repeated). 2 seconds later, as the airspeed fell below 1245 kts, the stall warning stick shaker operated, and continued to operate until the aircraft struck the ground at 2024.43 hrs.
The commander was basing his idea that air conditioning smoke comes from the right engine from a previous aircraft he had flown. The correlation does not exist in this aircraft. The speed at which both pilots reacted was contrary to their training and robbed them of time they could have better spent studying the engine instruments.
Had the commander kept the aircraft in coordinated flight prior to throttling back the right engine, he would have another clue that the left engine was producing less thrust than the right, because he would have needed right aileron to level the wings followed by right rudder to eliminate the yaw and return the ailerons to neutral.
Here again was another clue. If the No 2 engine was at idle and there was no adverse yaw, the No 1 engine could not have been producing much thrust.
Even had their diagnosis of which engine was causing the vibration and smoke in the cabin been correct, there was no need to shut the engine down and the smoke could have been addressed by closing bleed switches or even shutting down the air conditioning system completely once they had descended to a lower altitude.
The commander may have preferred to fly the airplane manually because it was a familiar activity and provided a comfort. Using the autopilot, however, would have freed his attention span to take over some of the first officer's duties.
They could have done this a few minutes earlier, but at this point they were outside the engine's relight envelope.
[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]
- It is also likely that, if the No 2 engine had not been shut down, the accident would not have happened, and some explanation must be sought for the commander's decision to shut it down. It is now known that the engine was operating normally but, because the decision to shut it down was made after is throttle had been closed, having failed to recognize its normal operating parameters before closing the throttle, the crew could no longer confirm its normal operation by comparison with the No 1 engine instruments. There is, however, no evidence from the CVR that the crew consulted the engine instruments or attempted any other analysis of their situation before shutting down the No 2 engine. Indeed, it appears that they were so sure that they had contained the situation that the commander engaged in lengthy communications with BMA Operations just after the No 2 throttle had been closed.
- In the aircraft 'non-normal' checklist severe vibration does not necessitate an engine shut-down, provided it is not accompanied by abnormal engine indication, nor does the presence of smoke or fumes in the cockpit.
[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.
[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:
- The combination of heavy engine vibration, noise, shuddering and an associated smell of fire were outside their training and experience.
- They reacted to the initial engine problem prematurely and in a way that was contrary to their training.
- They did not assimilate the indications on the engine instrument display before they throttled back the No 2 engine.
- 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.
- 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.
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