What happened
On 17 September 2015, a Piper PA-18-150 Super Cub, registration G-BJCI, was being prepared for glider aerotow operations at Milfield Airfield. During the engine start sequence, the engine immediately accelerated to a much higher power level than anticipated. Despite the pilot applying the heel brakes, the aircraft began moving forward toward a vehicle parked approximately 20 feet away. In an attempt to steer the aircraft away from the car, the pilot turned left, but the right elevator struck the rear of the parked vehicle. The incident resulted in no injuries, though the aircraft sustained minor damage to its right elevator and the vehicle suffered slight damage to a rear light lens.
Evidence revealed that the aircraft had experienced similar uncommanded power surges on the preceding day. During a pre-takeoff check, a different pilot observed the engine running up to full power without command. Later that same day, following a period of normal operation, the engine again surged to full power immediately after a glider release. The pilot managed a successful engine-off landing, and the aircraft was returned to the hangar.
The investigation
The investigation established that the aircraft's engine behavior was consistent with a known mechanical issue where a loose clamp on the throttle cable allows the cable outer to shift. This movement triggers a fail-safe spring, which inadvertently drives the throttle to the fully open position. While a technician had inspected the damage and tightened the clamp, the engine was not run to verify if this action had resolved the underlying defect.
Furthermore, the investigation found that the pilot performing the start on the day of the accident was unaware of the engine issues reported the previous day. Communication failures were also noted, as the club's method for relaying technical information between pilots had not been utilized, and no formal technical logs were maintained for the tug aircraft.
Findings
- A loose clamp on the throttle cable caused the uncommanded high power setting.
- The pilot was unaware of the previous day's engine malfunctions.
- There was a lack of effective communication regarding the aircraft's technical status between crew members.
- The club lacked a formal technical logging system for its tug aircraft.