What happened
On 25 September 2021, a Grumman AA-5, registration G-BBSA, departed Teessable International Airport for a local flight with two passengers. Shortly after takeoff from Runway 23, the aircraft experienced a significant reduction in engine power. At an altitude of approximately 120 ft, the pilot reported the power loss to ATC and requested to land on Runway 05.
Believing the aircraft had already passed the airport boundary, the pilot attempted a turnback to reach the airfield. During this maneuver, the engine speed dropped further, and the aircraft's airspeed decreased. As the pilot executed a left turn, the aircraft's bank angle increased, leading to an aerodynamic stall. The aircraft struck the ground west of the runway near the Runway 05 threshold, resulting in three serious injuries and the destruction of the aircraft.
The investigation
The AAIB investigation examined the engine, the carburetor, and flight data from a cockpit tablet and passenger mobile phone video. Investigators discovered that a section of brass tubing, specifically the accelerator pump discharge tube from the carburetor, had become detached and entered the No 4 cylinder. This foreign object prevented the exhaust valve from sealing correctly, causing the engine to operate on only three cylinders.
Analysis of the flight path via simulation revealed that while the pilot attempted a turnback, a landing straight ahead on the remaining runway would have been possible. The investigation also noted that the pilot's workload was high due to performing engine failure checklists while managing a low-altitude maneuver. The investigation confirmed that the aircraft's bank angle during the turn increased the stall speed above the actual airspeed, triggering the stall.
Findings
- The engine suffered a partial power loss because a fragment of the carburetor accelerator pump discharge tube had broken loose and entered the No 4 cylinder.
- The presence of this foreign object prevented the exhaust valve from closing properly, causing significant power loss and vibration.
- The pilot's decision to attempt a turnback at a low altitude, combined with an increased bank angle, led to an accelerated stall.
- The pilot's view of the remaining runway was likely obscured by the aircraft's nose during the initial stages of the power loss.