What happened
During a departure from Singapore Changi Airport, the flight crew of an aircraft experienced a flap drive fault while transitioning the flap selector from a setting of 5 to 1. This mechanical issue prevented the left inboard and outboard flaps from moving, creating an asymmetric flap condition. The flight control electronics responded to this misalignment by shutting down the flap drive system and issuing a fault message.
Because the aircraft was unable to complete the flap retraction, the crew performed the necessary checklists and decided to return to the departure airport. The return flight required a landing at a higher-than-normal speed because the aircraft was operating at an overweight condition. Despite these challenges, the aircraft's protective systems functioned as intended, and the crew managed a safe landing.
The investigation
Investigators identified that the mechanical failure was triggered by the delamination of the number 6 wheel tyre tread during the takeoff roll. Debris from the failing tyre was thrown with enough energy to penetrate the left underwing panel, subsequently breaking the flap torque tube. The airport operator identified two separate debris fields on runway 20C; the first was likely created during the high-speed takeoff, while the second was likely caused by further tyre degradation during the landing touchdown.
Analysis of the tyre indicated that it had been operating on a grooved runway surface, which caused chevron cutting damage. While the tyres had been certified as inspected according to the manufacturer's requirements, the investigation noted that the pre-departure service check certification might have been an acknowledgement that a check was not required rather than a confirmation that one had been performed. It was also noted that the specific site of the tread damage might have been obscured from view during the arrival inspection.
Findings
- The number 6 wheel tyre experienced shoulder step-wear, leading to cracking and undercurrent of the tread.
- The delamination of the tyre tread caused debris to strike the wing panel and damage the flap torque tube.
- The tyre failed in fewer life cycles than the typical average.
- The aircraft's protective systems and the crew's adherence to checklists enabled a safe return despite the asymmetric flap condition and the overweight landing.