What happened
On the afternoon of the accident, a Boeing 747, registration HL-7451, arrived at London Stansted from Tashkent. Upon arrival, the flight engineer noted in the technical log that the captain's Attitude Director Indicator (ADI) was unreliable during roll. This defect was also communicated verbally to ground engineers. While maintenance work was being performed and cargo was being exchanged for a subsequent flight to Milan-Malpensa, the aircraft prepared for departure.
The flight, designated KAL 8509, departed from Stand Alpha 6 at 18:36 following an ATC-related delay. During the initial climb phase, while passing through 900 feet, the crew heard the ADI comparator buzzer sound three times. As the aircraft climbed further, the pilot reported issues with DME indications, and the warning buzzer sounded twice more. While executing a left turn as part of the Dover 6R Standard Instrument Departure, the comparator warning sounded approximately nine additional times. The aircraft reached a maximum altitude of 2,532 feet amsl before entering a steep left bank of roughly 90 degrees and a nose-down pitch of 40 degrees. The aircraft descended at high speeds, between 250 and 300 kt, before impacting the ground.
Findings
Investigations into the crash identified several critical contributing factors:
- The crew failed to react correctly to the repeated comparator warnings during the climb.
- The pilot flying maintained a left roll input that led to the extreme bank angle and failed to provide necessary pitch corrections.
- There was a lack of effective monitoring by the first officer regarding the aircraft's developing unsafe attitude.
- Maintenance actions were misdirected, as the reported ADI fault was not properly resolved, leaving an erroneous roll signal from the No 1 INU active.
- Confusion regarding the division of responsibility between the operator's engineers and local support led to incorrect defect identification.