History of the Flight
On 20 July 2006, at 1815 UTC, a private Enstrom 280FX (registration G-VVWW) departed from an unspecified location. After climbing to 1,300 ft amsl, the pilot leaned the fuel mixture until the exhaust gas temperature (EGT) gauge indicated 1,600ºF. The manifold pressure was 29 inches, engine rpm was in the green, and fuel flow decreased to 80 lb/hr. A few minutes later, the pilot felt a significant engine vibration, followed by the low rotor rpm warning horn and warning light. The pilot lowered the collective lever and increased engine power, which briefly restored rotor rpm. As engine rpm continued to decrease, the pilot transmitted a Mayday call and commenced an autorotation. He selected a landing site and reduced airspeed while keeping rotor speed in the green. During the landing flare, the pilot used the collective to cushion touchdown but did not level the helicopter in time, causing the tail rotor to strike the ground. The pilot could not recall if he moved the mixture control toward rich after the initial power problem or during the autorotation. Weather at the time was wind 240º at 8 kt, scattered cloud at 3,000 ft, temperature 29ºC, QNH 1026 mb.
Injuries and Damage
The pilot was uninjured. The tail rotor blades were bent and the tail rotor transmission system was shock loaded.
Engineering Examination
A licensed aircraft engineer who maintains the helicopter, together with the AAIB, examined the helicopter. Testing and partial strip of the fuel injector unit revealed no fault or failure of the engine or engine systems that would explain the circumstances. Spark plugs, engine exhaust, and surrounding cowling showed evidence of a very lean fuel mixture for a period of time.
After repairs to the tail rotor, engine runs and test flights were conducted. During the first two test flights, full engine power was achieved, but an unusual engine-generated vibration was noted, and the No 4 cylinder temperatures were marginally lower than the other three cylinders. The No 4 cylinder fuel injector nozzle was removed and found partially restricted by a light sandy-coloured hard deposit, similar to that on the spark plugs and engine exhaust. This restriction would have reduced maximum fuel flow and disrupted the fuel spray pattern. The nozzle was cleaned and refitted. A subsequent test flight showed satisfactory engine performance without unusual vibrations.