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July 8, 2014

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3. Kegworth air crash, 1989

Cause of accident: vibration meters

This occurred on 8 January 1989 when a Boeing 737-400 did not quite make the runway at East Midlands Airport, to where it had diverted. The flight from Heathrow to Belfast suffered a blade detachment in the left engine, resulting in noise, vibration and smoke in the cabin.  The pilots shut down what they thought to be the failed engine but in fact they had chosen the wrong one.

The first factor was that of the location of the air conditioning packs, which on previous models of the 737 had been different.  The presence of smoke in the cabin led the pilots to assume that the right engine had failed because that was the one that provided air to the cabin in earlier models; but not on the 737-400. And when the smoke stopped after shutting down the starboard engine, they assumed that they had picked the correct engine, but it was due to them also reverting to manual throttle.

The ergonomic factor was that of the vibration meters. Correct reference to them would have told the pilots which engine had failed. On earlier aircraft, vibration meters had a reputation of being unreliable and were often ignored. On the 737-400, they were in fact very reliable, but the pilots did not know this.

On previous aircraft, the needle was on the inside as is common on most types of meter.  On the 737-400, the meter comprised a series of LEDs which went round the outside of the dial.  And the meters were very small. So, we have meters which were small and worked in a way opposite to that which pilots were used and were required to be read in a situation where there is a lot of vibration. They got it wrong.

To summarise, instrumentation which would have enabled the pilots to identify the correct engine to shut down was small, worked in a different way to what they were used to and was habitually ignored because of previous unreliability.  And there were contrary cues due to the fact that cabin smoke stopped when they took (the incorrect) action.

And for a smaller scale, but more modern example

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