History of the Flight
The pilot and two passengers departed from Redditch for Bedstone on a private flight. Approximately 15 minutes into the flight, while at 1,000 feet above ground level, the pilot felt a couple of yaw kicks, which he attributed to turbulence from a ridge he had just flown over. He initiated a gentle right turn to move away from the ridge and lowered the collective lever. Around the same time, the low rotor rpm horn sounded, the low rpm warning light illuminated, and the pilot noticed that engine noise had ceased. He entered an autorotation and selected a landing site on a ridge with woods and isolated trees. During the approach, the main rotor blades clipped several trees. The helicopter made a fast run-on landing before colliding with a fence and a metal farm gate. The pilot and passengers were uninjured. The helicopter sustained minor damage to the rotor blades, landing skids, canopy, and cockpit area.
Meteorological Conditions
At 1200 hours on the day of the accident, an area of high pressure covered the British Isles, having been present for at least five days, resulting in generally dry weather. At the accident location, the wind at 1,000 feet was from 350 degrees at 5 to 10 knots, with a temperature of 4.5°C, a dew point of 0.6°C, and relative humidity of approximately 80%. Nighttime temperatures during the preceding period dropped to around -1°C.
Engineering and Fuel System Aspects
The helicopter was a Robinson R44 Astro, G-HEPY, with a Lycoming O-540-F1B5 piston engine. The fuel system comprised a main tank and an auxiliary tank with a combined capacity of 190 litres. The auxiliary tank fed directly into the main tank. Each tank had a water drain point and a refuelling orifice. Fuel flowed from the main tank to a gascolator and then to the carburettor fuel bowl. The gascolator also had a water drain.
An engineering inspection revealed no obvious engine problems. However, the gascolator and carburettor fuel bowl were found to be full of water. Approximately one litre of water was drained from the main fuel tank and half a litre from the auxiliary tank. The owner, who was the sole user, normally refuelled at Wellsbourne Mountford Airfield and parked the helicopter at his house. On this occasion, the helicopter had been parked for two days with a fuel load of about 115 litres. The owner stated he had performed fuel/water sediment checks the night before the flight rather than during pre-flight checks. After the accident, the fluid sample from the gascolator was clear, but the owner did not realize it was water because Avgas is blue. He expected to see globules or a meniscus.
Fuel Contamination
The source of the water was likely condensation that accumulated in the fuel tanks due to temperature variations over the previous days. With the meteorological conditions present, condensation could have produced up to a teaspoon of water in each tank since the previous flight. There was no recent rain, and the fuel caps were in good condition. No water contamination was found in the fuel supply at the local airfield. The owner did not check the colour of the fuel sample and may not have detected water because the sample consisted solely of water.
Comments from the Investigation
The engine stoppage was a result of water in the fuel system. The yaw kick felt by the pilot just before the failure was likely an early indication of water entering the engine. Because the pilot had lowered the collective lever, unloading the main rotor, the low rotor rpm warning and loss of engine noise alerted him to enter autorotation. The investigation noted that helicopter pilots may not be aware of carburettor icing due to governing systems, but in this case, water contamination was identified as the cause of the engine failure.