What happened
On 4 February 2006, a Robinson R44 Astro, registration G-HEPY, was conducting a private flight from Redditch to Bedstone. While cruising at 1,000 ft, the pilot experienced a yawing sensation, which was initially attributed to turbulence. Shortly thereafter, the engine ceased operation. The pilot noted the activation of the low rotor RPM warning horn and the loss of engine noise, prompting an immediate transition into an autorotation.
Seeking a suitable landing area amidst a landscape of trees and woods, the pilot targeted a ridge. During the landing sequence, the rotor blades struck several trees, and the helicopter subsequently collided with a farm gate and a fence. The zero fatalities and zero injuries were reported, though the aircraft sustained minor damage to the cockpit, landing skids, tail pylon, and rotor blades.
The investigation
Investigators examined the fuel system and the meteorological conditions preceding the event. The aircraft had been parked for two days following a period of dry weather, during which night-time temperatures had dropped to approximately -1°C. An engineering inspection of the fuel system revealed that the gascolator and the carburettor fuel bowl contained significant amounts of water. Specifically, approximately 1 litre of water was drained from the main fuel tank and 0.5 litres from the auxiliary tank.
While the airfield manager confirmed that recent fuel sediment checks were clear, the owner noted that the pre-flight fuel/water check had been performed the previous night rather than immediately before flight. It was also noted that the owner did not typically use the colour of the fluid to distinguish between Avgas and water, instead looking for visible globules or a meniscus.
Findings
- The engine failure was caused by water in the fuel system.
- Water likely accumulated in the unusable portion of the fuel tanks due to condensation caused by temperature fluctuations between day and night.
- The pilot may not have detected the contamination during pre-flight checks because the sample taken contained only water, lacking the characteristic blue colour of Avgas or a visible fuel-water interface.
- The pilot's decision to lower the collective lever just prior to the failure contributed to the engine's inability to maintain power during the onset of the issue.