History of the Flight
The pilot and a passenger departed Dunkeswell Airfield for a short pleasure flight to the Sidmouth area. The flight duration was planned for 30 minutes, but the helicopter carried fuel for three hours, resulting in a relatively high operating weight. During pre-flight magneto checks, engine speed drops of 150 rpm (left) and 175 rpm (right) exceeded the maximum 125 rpm limit, and the engine ran roughly on the right magneto alone. The pilot attributed this to oil fouling from prolonged idling and cleared the plugs by running at 3,000 rpm for one minute, after which a subsequent magneto check showed 125 rpm drops on each magneto.
The flight to Sidmouth was uneventful. Approximately six nautical miles from Dunkeswell at 650 ft, the pilot realized the helicopter would not climb in response to collective inputs. He confirmed the collective friction lock was free and slowed to 60 kt while attempting to climb. Some height was gained with cyclic inputs, but the collective failed to produce a positive climb rate. Height decreased to 500 ft, and due to an approaching ridge, the pilot elected to land in a large field ahead. Clearing the ridge at 400 ft, he prepared to land into wind, but telegraph wires forced a westerly approach, resulting in a tailwind component (120°/15 kt). To avoid the far boundary hedge, the pilot attempted a slow landing with available power. The descent initially appeared normal with rotor speed in the green, but the helicopter began to twitch and the descent rate increased rapidly. Raising the collective lever was ineffective, and the helicopter landed heavily, rolling onto its left side. The engine continued running until the main rotor blades struck the ground. The pilot exited unaided and assisted the injured passenger from the wreckage.
Aircraft and Pilot Information
The Schweizer 269C (G-OGOB), manufactured in 1987, was powered by a Lycoming HIO-360-D1A piston engine. It was configured with twin fuel tanks (18.8 US gallons each) and a modified fuel vent system (Schweizer kit SA 269K-101-1). The pilot held a Private Pilot’s Licence, was aged 39, and had 273 hours total flying experience (120 on type), with 10 hours in the last 90 days and 4 hours in the last 28 days.
Examination
Post-accident examination by the maintenance engineer revealed no significant engine failure. Spark plugs were free of oil fouling, in good condition, and of similar colour. The fuel filter was uncontaminated. Further AAIB examination confirmed no catastrophic engine failure, and no failures or disconnections in the flight controls or transmission system. The main rotor gearbox was disassembled and found in good condition with no adverse wear or cracking. The fuel tank vent system was removed for detailed testing.
Fuel Tank Vent System Tests
Testing simulated rollover conditions. With free fluid flow from an unsealed header tank, a few drops were released before the vent valve closed at approximately 91° roll angle. When the outlet pipe was closed to simulate overfilling of the lower tank, fluid flowed at 0.5 litres per minute at roll angles above 45°, ceasing at 95° when the valve closed. Sealing the header tank reduced flow to 0.3 litres per minute, with valve closure at 91° on most occasions; in three tests, the valve shuttled and released a rapid drip for about 45 seconds until the roll angle reached 99°. At smaller roll angles preventing valve closure, fluid flowed at 0.3 litres per minute initially, decreasing to a slow drip after 2 minutes, releasing a total of 0.53 litres.