What happened
On 17 December 1999, a Douglas C47 Dakota 4, registration G-AMPZ, was conducting an IFR night cargo flight from Belfast Aldergrove Airport to Coventry. Roughly eight minutes after climbing to a flight level of 060, the aircraft's left engine experienced a loss of power while positioned approximately 7 nm south-east of the RINGA reporting point.
The loss of power was accompanied by audible noise and vibrations suggesting a loss of combustion in a single cylinder. In response, the pilot adjusted the fuel mixture to 'AUTO-RICH' and reduced throttle to mitigate engine stress. After communicating the engine issue to Air Traffic Control, the pilot requested an immediate return to the departure airfield. During the return leg, the No 1 engine suffered further power degradation, leading the crew to shut the engine down entirely and declare a Full Emergency.
The aircraft landed on Runway 25 at Aldergrove at 0138 hrs. While the landing was uneventful, the crew encountered difficulties during ground manoeuvres due to a strong 50-knot wind from the left, which was challenging as power was only available from the right engine.
The investigation
Following the incident, the operator's engineers inspected the left engine. The examination revealed that a spark plug had unscrewed from the No 6 cylinder. Despite the plug being missing, the cylinder itself and the helicoil thread insert remained undamaged. The spark plug was found to be in good condition and was reinstalled.
Records indicated that all spark plugs had been replaced during a maintenance check approximately three flying hours before the flight. Subsequent torque checks of the remaining plugs showed they were correctly secured. Following successful ground runs, the aircraft was returned to service.
Findings
- The engine power loss was caused by a spark plug becoming detached from the No 6 cylinder.
- The loss of the plug was likely due to the component not being sufficiently torque-tightened during the recent maintenance session.
- The crew successfully managed the engine failure and executed an emergency landing without injury to the 3 crew members on board.