History of the Flight
The aircraft, a DHC-1 Chipmunk 22 (registration G-BDCC), was performing an aerotow of a glider. The takeoff and initial climb were normal, with all engine parameters satisfactory. At approximately 500 feet above ground level, the pilot retracted the flaps and switched the fuel boost pump to OFF. As the climb continued through 800 feet, engine speed oscillated between 2,200 and 2,400 RPM. The pilot signaled the glider pilot to release by rocking the wings. He then reselected the boost pump to ON and performed engine failure checks, noting nothing unusual. During a turn back toward the airfield, the engine lost power. Varying the throttle provided short bursts of power but insufficient to maintain altitude. The pilot positioned the aircraft for a northerly approach downwind and downslope. He delayed selecting full flap until confident of landing, but due to tailwind and downslope, he could not land before the northern boundary fence. He attempted to pull up over the fence and land in a field beyond, but the right main landing gear struck something solid. The aircraft came to rest in the field after passing over a cutting containing a quarry conveyor belt system. The pilot sustained serious injuries. He switched off the master and boost pump, pulled the mixture to idle cut-off, and attempted to release his harness due to fire concerns. No fire occurred, and airfield personnel with a fire cover vehicle rescued him and transported him to hospital.
Inspection and Tests
Initial inspection at the accident site revealed no apparent faults with the engine controls or fuel system. All fuel lines were clear, the fuel filter clean, and an adequate quantity of the correct fuel type was present. The engine air intake was unobstructed. The aircraft was dismantled and recovered to a hangar. The carburettor was removed, tested, and strip-inspected at an approved overhaul organisation; no faults were found. It was reassembled, refitted to the engine, and the engine was ground run while the fuselage was secured on trestles. With a serviceable propeller, the engine operated satisfactorily up to 1,600 RPM, with normal gauge readings and acceptable magneto drops of 50 to 75 RPM.
Discussion
Prior to the accident, the aircraft had flown multiple times since its last refuelling without reported engine issues. On 3 June 1999, the aircraft had been involved in a previous accident (AAIB Bulletin 8/99) during which it failed to become airborne, skidded on wet grass, and collided with a fence. Damage was limited to the airframe and propeller leading edge. Following propeller repair and a crankshaft runout check, the aircraft was declared airworthy four weeks later and flew about 50 hours satisfactorily. In light of this history and the satisfactory post-accident engine operation, it was considered that a temporary restriction of fuel flow to the engine likely occurred during the climb, which reselection of the boost pump could not overcome.