What happened
On 11 May 2009, a Boeing 747-400, registration G-BYGA, was performing a scheduled international flight from OR Tambo Airport, South Africa, to London Heathrow. During the takeoff roll, the crew observed amber EICAS messages indicating thrust reverser in-transit status for the No. 3 and No. 2 engines.
As the aircraft reached rotation speed, the Group ‘A’ leading edge flaps automatically retracted. This sudden change in configuration caused the aircraft to experience significant buffeting and intermittent stick shaker activation, signaling a near-stall condition. The pilot flying managed to maintain control and prevent a full stall. Once the landing gear was retracted, the leading edge flaps automatically re-extended, restoring normal aerodynamic performance. The crew subsequently declared an emergency, returned to the airport, and landed safely after dumping fuel.
The investigation
SACAA AIID investigators examined the aircraft's flight data and conducted ground testing on the engine systems. The investigation focused on why the leading edge flaps retracted while the aircraft was still in ground mode.
Testing revealed that the thrust reverser locking gearboxes on both the No. 2 and No. 3 engines had partially unlocked, meaning the reversers were not fully stowed. This mechanical discrepancy triggered the automatic retraction logic. The investigation also noted that the crew had no clear visual indication of the flap retraction on the flight deck, as the EICAS color change was difficult to perceive.