Korean Air Cargo Flight 8509 was a Boeing 747-2B5F, registered HL7451 and bound for Milano-Malpensa Airport, that crashed due to instrument malfunction and pilot error on 22 December 1999 shortly after take-off from London Stansted Airport where the final leg of its route from South Korea to Italy had begun.
The aircraft crashed into Hatfield Forest near the village of Great Hallingbury, close to but clear of some houses. All four crew on board perished.
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.
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.
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, but the comparator alarm sounded repeatedly.
The first officer, whose own ADI would have shown the true angle of bank, failed to participate in full crew resource management techniques, saying nothing to challenge his captain’s actions nor made any attempt to take over the flight with his own controls.
The older and more experienced flight engineer did call out “bank” four times in 19 seconds, but the captain ignored his warnings, continued to ignore the chiming alarm, made no verbal response, and actually continued to increase the left bank angle.
At 18:38, 55 seconds after take-off, Flight 8509’s left wing dragged along the ground, then the aircraft plunged into the ground at a speed of between 250 and 300 knots, in a 40° pitch down and 90° left bank attitude. The aircraft exploded on impact.
CAUSAL FACTORS: The following causal factors were identified:
The pilots did not respond appropriately to the comparator warnings during the climb after takeoff from Stansted despite prompts from the flight engineer.
The commander, as the handling pilot, maintained a left roll control input, rolling the aircraft to approximately 90° of left bank and there was no control input to correct the pitch attitude throughout the turn.
The first officer either did not monitor the aircraft attitude during the climbing turn or, having done so, did not alert the commander to the extreme unsafe attitude that developed.
The maintenance activity at Stansted was misdirected, despite the fault having been correctly reported using the Fault Reporting Manual. Consequently the aircraft was presented for service with the same fault experienced on the previous sector; the No 1 INU roll signal driving the captain’s ADI was erroneous.
The agreement for local engineering support of the Operator’s engineering personnel, was unclear on the division of responsibility, resulting in erroneous defect identification, and misdirected maintenance action.
After the investigation, the United Kingdom’s Air Accidents Investigation Branch (AAIB) issued recommendations to Korean Air to revise its training program and company culture, to promote a more free atmosphere between the captain and the first officer.