Synopsis
On 15 May 2000, a Boeing 747-236B (registration G-BDXJ) operated by a public transport airline performed a heavy landing during an automatic approach to Runway 27L at London Heathrow Airport. The aircraft sustained minor damage to the keel beam and No. 3 engine pylon, but no injuries were reported among the 336 passengers and 18 crew members.
History of the Flight
The aircraft was on a scheduled return flight from Miami to London Heathrow. Weather conditions at arrival were dry with light winds and good visibility. The crew conducted an automatic landing for practice with all three autopilots engaged. At 1,000 feet radio height, the 'A' channel autopilot disengaged, but the approach continued using two autopilot channels. The flare was correctly initiated at 50 feet, with the control column moving aft. However, at about 30 feet, the control column unexpectedly moved forward, causing the nose to pitch down. The pilot disengaged the autopilot and applied a sharp aft column input, but the aircraft touched down firmly. The landing was heavy but resulted in no injuries.
Autopilot Operation
The Boeing 747-236B automatic landing system requires selecting 'LAND' on the Autopilot/Flight Director Mode Selector Panel and engaging all three autopilot channels ('A', 'B', 'C') to 'COMMAND'. Confidence checks are performed at selection and at 1,000 feet. The disengagement of the 'A' channel at 1,000 feet was consistent with a failed confidence check. The system is designed to land with two channels. At 1,000 feet, control switches from autopilot computers to Landing Control Logic Units (LCLUs) which manage the flare using radio altitude data.
Quick Access Recorder Data Analysis
Data from the Quick Access Recorder showed a peak vertical acceleration of 2.1g. Both 'B' and 'C' channels indicated flare capture four seconds before touchdown, followed by a brief forward control column movement that initiated a nose-down pitch. The autopilot was disengaged two seconds before touchdown when a manual nose-up input was applied. The rate of descent increased from 586 to 1,191 feet per minute. A wind shift from 318° to 113° at 2 knots for one second was not considered significant. The radio altimeter data were accurate.
Engineering Investigation
Heavy landing inspections revealed damage to the keel beam aft of the wing and the No. 3 engine pylon skin. The pylon was repaired; the keel beam was deemed acceptable for continued service with periodic re-inspection. LCLU tests showed no faults, and their service histories had no related issues. Triple channel ramp-down checks simulating automatic landings with each channel as 'first in command' were satisfactory. No explanation was found for the nose-down pitch during the flare. The aircraft returned to service after a successful automatic landing, and the autopilot system was upgraded to full operational status. No further problems were reported.