The aircraft

The aircraft involved was a Boeing 747-2B5F freighter registered HL7451. Built on 4 April 1980, the aircraft had completed 15,451 flights with a total flight time of 83,011 hours before its fatal flight on 22 December 1999.

INU failure and failed repair

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.

Flight crew

The flight crew consisted of 57-year-old Captain Park Duk-kyu (Hangul: 박득규, Hanja: 朴得圭, RR: Bak Deuk-gyu, M-R: Pak Tŭkkyu), 33-year-old First Officer Yoon Ki-sik (Hangul: 윤기식, Hanja: 尹基植, RR: Yun Gi-sik, M-R: Yun Kishik), 38-year-old Flight Engineer Park Hoon-kyu (Hangul: 박훈규, Hanja: 朴薰圭, RR: Bak Hun-gyu, M-R: Pak Hun'gyu), and 45-year-old maintenance mechanic Kim Il-suk (Hangul: 김일석, Hanja: 金日奭, RR: Gim Il-seok, M-R: Kim Ilsŏk). The captain was a former air force pilot and a highly experienced airman, with a total of 13,490 flying hours – 8,495 of which were accumulated flying Boeing 747s. The first officer, in contrast, was relatively inexperienced with just 195 hours of flying experience on the 747 and a total of 1,406 flight hours. The flight engineer, like the captain, had a lot of experience flying 747s – 4,511 out of his 8,301 total flight hours were accrued in them.

Flight

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". 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. The aircraft exploded on impact.

Aftermath

After the investigation, Air Accidents Investigation Branch of the United Kingdom 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. Korean Air has not had a single crash since this accident in 1999.