What happened
On 20 July 2006, an Enstrom 280FX, registration G-VVWW, was conducting a private flight near Epsom Racecourse, Surrey. While climbing to 1,300 ft amsl, the pilot adjusted the fuel mixture to lean the engine, resulting in an exhaust gas temperature of 1,600ºF and a fuel flow of 80 lb/hr. Shortly after this adjustment, the pilot experienced significant engine vibration, followed by the activation of the low rotor rpm warning horn and light.
In an attempt to maintain rotor speed, the pilot increased engine power by opening the throttle, which only provided a temporary recovery. As the engine rpm continued to drop, the pilot determined that level flight was no longer sustainable and declared a 'Mayday' via radio. The pilot initiated an autorotation, selecting a suitable landing area and managing the rotor speed. While the touchdown was cushioned, the pilot was unable to level the aircraft in time to prevent the tail rotor from striking the ground.
There were no injuries to the pilot during the incident, though the tail rotor blades were bent and the transmission system sustained shock loading.
The investigation
An engineer familiar with the aircraft type examined the helicopter at a maintenance facility. During the inspection, the spark plugs, engine exhaust, and the interior of the engine cowling all showed evidence that the engine had been operating on an extremely lean fuel mixture for a duration of time.
Following the repair of the tail rotor, subsequent test flights revealed an unusual engine-generated vibration when full power was applied. Further investigation showed that the No 4 cylinder temperatures were slightly lower than the other cylinders. This led to the removal of the No 4 cylinder fuel injector nozzle, which was found to be partially blocked by a hard, sandy-coloured deposit.
Findings
- The engine was running on a very lean mixture for a period of time.
- A partially restricted fuel injector nozzle caused a reduction in maximum fuel flow and a disrupted spray pattern.
- The restriction in the nozzle was caused by a light, sandy-coloured hard deposit.