What happened
On 18 November 2004, a Hiller 12 OH-23B helicopter, registration N33514, was performing a series of private flight maneuvers at Northampton (Sywell) Aerodrome. After completing two initial sorties, the pilot commenced a third flight with a new passenger. During the approach to land, while at approximately 60 feet altitude and a speed of 40 knots, the engine suffered an instantaneous loss of power.
As the pilot attempted to transition into a hover, the engine failure left insufficient height and speed to execute a full autorotation. The pilot managed to maintain a level attitude and cushion the impact, but the subsequent touchdown was firm, causing the aircraft to bounce. During this sequence, the pilot lost tail rotor authority, and the rotor disc toppled, causing the main rotor blades to strike and sever the tail boom. Both the pilot and the passenger escaped the aircraft without injury.
The investigation
The AAIB investigation focused on the cause of the engine power loss and the accuracy of the aircraft's fuel monitoring systems. Investigators found that the aircraft was equipped with a 'Datcon' fuel gauge that was not approved for use on this aircraft type. Post-accident testing revealed that the gauge significantly overestimated the remaining fuel.
When the helicopter was tested in a nose-high attitude—simulating the transition to a hover—the fuel flow ceased due to starvation, even though the gauge indicated a higher quantity. Furthermore, the investigation noted that the aircraft had been 're-manufactured' by a US corporation whose president had previously been sanctioned by the FAA for using unapproved parts. The investigation also highlighted that the owner did not maintain a technical log, which prevented an accurate tracking of fuel consumption and uplift.
Findings
- The engine failure was caused by fuel starvation occurring during the transition to a hover.
- The fuel gauge installed in the N33514 was inaccurate and not approved for use on the aircraft.
- The pilot relied on the fuel gauge readings, which showed a much higher fuel level than was actually present.
- The lack of contemporaneous flight and fuel records meant the discrepancy between the gauge and actual fuel levels could not be identified through consumption tracking.
- The sudden loss of power occurred at a critical point in the flight profile, where the height and velocity made a safe autorotative landing extremely difficult.