What happened
On 13 December 1997, a Piper L21B (Modified) Super Cub, registration G-BAFT, was performing aero-tows at a gliding club. Following the successful release of a glider at 2,500 feet, the pilot began a descent to rejoin the airfield circuit. While flying the base leg, the engine suffered a sudden loss of power. Despite the pilot applying carburettor heat, the engine ceased windmilling and stopped with the propeller in a horizontal position at an altitude of approximately 100 to 120 feet.
Seeking a landing site, the pilot identified a field beyond a set of overhead power cables. During the final approach, the pilot performed emergency procedures, including switching off the fuel and magnetos and unlatching the door. As the aircraft descended to within 10 to 15 feet of the ground, it experienced a sudden deceleration and struck the earth, coming to rest in an inverted position. The pilot was able to exit the aircraft through the hatch without injury.
The investigation
Investigators discovered that the tow rope from the previous glider tow had become entangled around all three overhead power cables. The rope had remained connected to the aircraft during the approach because the pilot had insufficient time to release it; the weak link only failed after the aircraft had come to a halt.
Technical inspections of the aircraft's fuel system showed no signs of contamination or mechanical failure, and the engine performed correctly during subsequent ground testing. Similarly, examinations of the ignition system and magnetos revealed no defects capable of causing the power loss.
Findings
- The engine power loss occurred during a period of high humidity (near 100%) and low temperatures (6°C).
- Carburettor icing is a known risk in such atmospheric conditions when engines are operated at lower power settings.
- The aircraft's descent profile, which involved maintaining a higher power setting to prevent shock-cooling, coincided with conditions conducive to maximum icing effect.