A Korean Air Cargo plane crashed in 1999........
It was dark when the plane took off from London Stansted Airport, with the captain flying. When the captain tried to bank the plane to turn left, his ADI showed it not banking and the comparator alarm sounded repeatedly. The first officer, whose instrument would have shown the true angle of bank, said nothing, although the flight engineer called out "bank".[1] The captain made no response and continued banking farther and farther left. At 18:38, 55 seconds after take-off, Flight 8509's wing dragged along the ground, then the aircraft plunged into the ground at a speed between 250 and 300 knots, in a 40° pitch down and 90° left bank attitude.[1] The aircraft exploded on impact.
Following the plane's departure from Tashkent on the previous flight segment, one of its inertial navigation units (INUs) had partially failed, providing erroneous roll data to the captain's attitude director indicator (ADI or artificial horizon). The first officer's ADI and a backup ADI were correct, a comparator alarm called attention to the discrepancy, and in daylight the erroneous indication was easily identified. The ADI's input selector was switched to the other INU and the correct indications returned.[1]
At Stansted, the engineers who attempted to repair the ADI did not have the correct Fault Isolation Manual available and did not think of replacing the INU. One of them identified and repaired a damaged connecting plug on the ADI. When the ADI responded correctly to its "Test" button, they believed the fault had been corrected, although this button only tested the ADI and not the INU. The ADI's input selector was left in the normal position.[1]
