What happened
On 12 December 2000, a Boeing 747-236B, registration G-BDXP, was completing a passenger flight from Boston to London Heathrow. During the approach, the crew noted that forecasted winds might exceed the aircraft's crosswind limitations for the primary landing runway, Runway 27R. While Runway 23 was available, the crew opted to continue for Runway 27R to utilize the precision approach aid, despite being aware of potential delays and turbulence.
The landing occurred under significant wind conditions, with a reported mean wind of 25 knots and gusts reaching 33 knots. Although the touchdown was initially smooth and close to the centerline, the aircraft began to track toward the left side of the runway. The crew attempted to maintain control using full right rudder. After a brief period where the aircraft appeared to regain the centerline, a sudden change in the wind component caused the aircraft to swing rapidly to the left. Despite the application of full right rudder, the aircraft departed the paved surface and came to rest on a grass area between two taxiways. There were no injuries to the 116 passengers or 18 crew members, and no significant damage to the aircraft was reported.
The investigation
The AAIB investigation utilized data from the Flight Data Recorder, Cockpit Voice Recorder, and Quick Access Recorder to reconstruct the landing roll. The analysis revealed that the aircraft encountered a significant wind change during the deceleration phase. Specifically, as the aircraft slowed through 100 knots, it encountered a relative increase in the headwind component, which triggered a sudden yaw.
Investigators also examined the runway and meteorological conditions, noting that the surface was wet. The investigation looked into the aircraft's technical state, noting an unserviceable autothrottle, and reviewed the airport's runway alternation and allocation procedures. The investigation also assessed the effectiveness of the aircraft's steering systems, noting that the rudder fine steering system had been deactivated by the operator for fleet commonality.
Findings
- The aircraft was landing within the manufacturer's crosswind limits, but encountered a sudden gust that reinforced the corrective rudder input, leading to an uncontrollable over-swing.
- A critical wind change occurred while the aircraft was transitioning between speeds where rudder effectiveness is high and speeds where steering tiller or differential braking is required.
- The crew had not practiced the use of differential braking for directional control in such scenarios.
- The use of Runway 27R during strong south-westerly winds exposed the aircraft to building-induced turbulence and windshear.
- The aircraft's rudder fine steering system was deactivated to maintain commonality with other aircraft in the fleet.