What happened
On 6 August 2016, a Rotorway Executive 90, registration G-BVTV, was involved in an accident during a private flight from Landmead Farm Airfield in Oxfordshire. After being parked in the sun for several hours, the pilot attempted a return flight. During the initial hover, the pilot noted that the engine required full throttle to maintain rotor RPM and struggled to maintain a steady hover height.
Attempting to utilize a 'crashed creep' takeoff technique to leverage ground effect, the pilot progressed along the grass strip. However, as the aircraft reached approximately 75 feet above ground level and an airspeed of 50 mph, the pilot identified a continuing power deficiency. Fearing a potential engine failure and unable to safely execute an autorotation from the current altitude and speed, the pilot decided to return to the airfield.
Initially intending a downwind running landing, the pilot instead turned back into the wind to avoid a tailwind component that might have compromised cyclic authority. During this maneuver, the aircraft began to sink with low rotor RPM, resulting in a heavy touchdown and the helicopter rolling onto its starboard side. The pilot escaped through a broken windscreen and sustained no injuries.
The investigation
The investigation examined the engine performance and the environmental conditions at the time of the flight. The aircraft had been exposed to direct sunlight for several hours prior to the departure. The investigation looked into the impact of the 24°C ambient temperature and the use of carburettor heat on engine output. Additionally, the potential for fuel vapour-lock involving Mogas was assessed, noting that its low boiling point makes it susceptible to vaporization in warm conditions.
Findings
- The primary cause of the heavy landing was the inability to maintain sufficient climb performance, which led to the pilot's decision to abort the takeoff.
- High ambient temperatures and the use of carburettor heat may have reduced available engine power.
- Potential fuel vapour-lock due to the vaporization of Mogas in warm conditions may have contributed to the power loss.
- The pilot's operational technique, including a lack of finesse in managing rotor RPM and flying at a speed below the best rate-of-climb, likely contributed to the perception of an engine failure.