David Learmount/LONDON

A faulty attitude director indicator (ADI) on the captain's side appears to have been a major factor in the Korean Air (KAL) Boeing 747-200 freighter crash on 22 December near London Stansted Airport, UK, according to details in a UK Air Accident Investigation Branch (AAIB) interim bulletin. The two pilots, flight engineer and ground engineer were killed.

The aircraft (HL-7451) was airborne for just over 55s after take-off, bound for Milan, Italy. It hit the ground in an attitude 40° nose down with 90° left bank 21s after reaching a maximum height of 2,150ft (655m), with all engines at high power, flaps and leading edge slats in the take-off position and the gear retracted, according to the AAIB's investigation so far.

Speed at impact was between 250kt (460km/h) and 300kt, says the report. There is no indication of pre-impact structural failure. The cockpit voice recorder (CVR) and flight data recorder have been recovered, both serviceable.

The technical log records that the captain of the inbound flight had reported his ADI (effectively an artificial horizon) "unreliable in roll". During the turnaround, the AAIB reports, work on the ADI was carried out by the engineer killed in the crash, but details are unknown because the technical log was also lost in the accident. The captain was flying the aircraft as it took off from runway 23 in rain with a surface wind of 190°/18kt and a 400/500ft cloudbase.

As the aircraft climbed through 900ft the ADI comparator audible alert sounded three times, indicating disparity between the two ADIs. It sounded nine more times as the aircraft climbed to its maximum height. The aircraft then began a left turn and the pitch angle began to decrease.

During the remaining 21s that the CVR was operating, the engineer was calling warnings to the captain about bank angle. The co-pilot, meanwhile, was acknowledging air traffic control's instruction to change radio frequency to London control, and the captain was expressing concern with distance measuring equipment (DME) readings. The DME had given unexpected ranges before take-off.

The acknowledgement of the frequency change instruction was the last communication with the aircraft. "A short while later an explosion and fireball were seen about 1.5 miles [2.5km] south of the airfield," says the AAIB release. Small pieces of wreckage found the next day on the airfield are believed to have been blown into the sky by the explosion and carried by the wind from the impact site.

Source: Flight International