What happened
On 15 May 2000, a Boeing 747-236B, registration G-BDXJ, was conducting a scheduled passenger flight from Miami to London Heathrow. The flight was operating as a public transport service with 336 passengers and 18 crew members on board. During the arrival, the crew was performing an automatic landing on Runway 2-7L for practice purposes.
While the aircraft was on approach, the 'A' channel autopilot disengaged at 1,000 feet radio height; however, the approach continued using the remaining two autopilot channels. The flare manoeuvre was successfully initiated at 50 feet radio height, with the control column moving backwards to raise the nose. However, at approximately 30 feet radio height, the control column unexpectedly moved forward, causing the aircraft to pitch nose-down. Although the pilot immediately disengaged the autopilot and applied significant manual nose-up input, the rate of descent increased sharply. The aircraft subsequently made a very heavy landing. There were no injuries to the passengers or crew.
The investigation
Investigators analysed data from the Quick Access Recorder (QAR), which confirmed a peak vertical acceleration of 2.1g, verifying the heavy nature of the impact. The data showed that the rate of descent had surged from 586 to 1191 feet per minute just two seconds before touchdown. While a brief, momentary change in wind direction was noted, it was deemed insignificant due to the low wind speed.
The investigation also examined the Landing Control Logic Units (LCLUs) and the radio altimeter system. Testing confirmed that the radio altimeters were providing accurate height data and that the LCLUs were functioning correctly. Furthermore, ground-based 'Ramp Down' checks were performed using all three autopilot channels as 'first in command' to simulate an automatic landing; these tests were all successful, and no fault could be identified that would explain the unexpected nose-down movement.
Findings
- The aircraft sustained damage to the No. 3 engine pylon skin and the keel beam behind the wing.
- The cause of the nose-down pitch during the flare could not be determined through the technical examination of the aircraft systems or flight data.
- The radio altimeter system and the Landing Control Logic Units were operating within normal parameters during the approach.