What happened
On 15 February 2005, a HS 748 Series 2A, registration G-BVOU, was performing a cargo operation at Belfast (Aldergrove) Airport when the aircraft experienced a significant incident during landing. The crew reported that the left engine had stopped during the landing sequence. Following the touchdown, the crew noted a fire indication and attempted to taxi the aircraft, but found steering to be extremely difficult.
Upon exiting the runway, it was discovered that both tyres on the left main landing gear had deflated. The damage to the tyres included local flat spots where the tread and carcass structures had worn through. Despite the engine failure and the tyre issues, there were no fatalities or injuries to the two crew members on board.
The investigation
The AAIB investigation examined the aircraft's technical logs, the flight data recorder, and the cockpit voice recorder. While the flight data recorder provided limited useful parameters, the cockpit voice recorder confirmed that the crew followed standard procedures, including the withdrawal of the propeller pitch stop and the engagement of the gust-lock lever.
Investigators also performed a technical analysis of the propeller control unit (PCU) and the engine nacelle environment. They examined the possibility of grease-related combustion in the nacelle, which could explain the fire warning. Additionally, the investigation looked into the braking system and the potential for the left wheels to have locked during the landing roll.
Findings
- The investigation identified an intermittent defect in a micro-switch within the control console as a primary factor. This defect could trigger the auto-coarsening function of the left propeller.
- It is possible that the left propeller pitch increased significantly while the engine RPM decayed, leading to a left engine flame-out.
- This asymmetric propeller performance likely created a lift imbalance, potentially causing the left wheels to remain on the ground longer or experience different braking forces than the right wheels.
- The engine fire warning was likely caused by a temporary rise in temperature in the nacelle, where accumulated grease ignited due to reduced ventilation after the engine stopped.
- The deflation of the left tyres was attributed to the possibility that brake pressure was applied to the left units while the wheels were unexpectedly still in contact with the runway, causing them to lock and develop flat spots.