What happened
On 6 September 2006, a Piper PA-18-150 Super Cub, registration G-BAKV, was involved in an accident at Thruxton Airfield, Hampshire. The pilot, who was conducting solo flight training to convert a National Private Pilot’s Licence, was performing a series of takeoffs and landings. Following a successful initial landing on Runway 25, the pilot began a second approach. Believing the aircraft was slightly low, the pilot momentarily opened the throttle to maintain altitude until crossing the threshold.
During the landing flare, the aircraft ballooned, prompting the pilot to initiate a go-around. To do this, the pilot advanced the throttle rapidly. The engine failed to respond immediately, instead spluttering without an increase in power. In an attempt to lower the nose and maintain airspeed, the pilot pushed the control column forward. As the engine eventually began to deliver power, the aircraft accelerated and swerved, departing the left-hand edge of the runway and onto an adjacent grass taxiway. The aircraft' and nose-down attitude caused the propeller to strike the grass, leading the aircraft to somersault and come to rest inverted. There were no injuries to the pilot.
The investigation
The AAIB investigation examined the engine's response to the sudden throttle movement and the pilot's handling of the aircraft. The investigation established that the engine, a Lycoming O-320-A2B, experienced a rich cut. This phenomenon occurs when the fuel-air mixture becomes too rich to support combustion, often triggered by abrupt throttle applications when carburettor heat is active. The pilot did not recall selecting carburettor heat, but the investigation noted the engine's susceptibility to this issue under such conditions. The investigation also reviewed the pilot's control inputs, noting that the focus on immediate surroundings led to a loss of awareness regarding the aircraft's attitude during the critical moments of the go-around.
Findings
- The primary cause of the engine's failure to respond was a rich cut caused by the rapid advancement of the throttle.
- The pilot's decision to push the control column forward to lower the nose, combined with the sudden surge of power, contributed to the loss of directional control.
- The aircraft's momentum and the nose-down attitude during the engine's power delivery led to the propeller striking the ground, resulting in the aircraft somersaulting.