What happened
On 29 June 2003, a Ponsford Bensen B8MR (Modified), registration G-BIGU, crashed near Shipdham Airfield in Norfolk during a private flight. The pilot, who had recently completed a gyroplane training course, was performing what appeared to be a series of maneuvers, including flying along the runway in both directions. Witnesses observed the aircraft exhibiting a "porpoising" motion during the flight.
While the aircraft was flying at an estimated altitude of 250 to 300 feet, a loud bang was heard, followed by an immediate change in the aircraft's attitude. The gyroplane pitched nose-down and fell vertically to the ground. Subsequent examination of the wreckage revealed that the rotor blades had struck the rudder, causing a significant portion of the rudder to detach. The impact resulted in 1 fatality.
The investigation
The AAIB investigation focused on the aircraft's stability and the pilot's recent training. Examination of the wreckage showed that the rotor blades had red marks consistent with striking the rudder, and the teeter stop plate was bent, indicating violent vertical motion. The engine and flight controls were found to be in normal working order, and no medical issues or pilot incapacitation were identified.
Computer modeling conducted by the University of Glasgow suggested that the aircraft possessed potentially unstable longitudinal stability characteristics. Specifically, the thrust line was located approximately 4.8 inches above the vertical center of gravity, which exceeds the recommended safety margin. The investigation also noted that the aircraft used a "pump-action" control stick, which can exacerbate pilot-induced oscillations (PIO) during pitch changes.
Findings
- The primary cause of the accident was a pilot-induced oscillation (PIO) in pitch, which led to the rotor blades striking the rudder.
- The aircraft's design features, including the high thrust line, the lack of a horizontal tail, and the use of a pump-action control stick, contributed to its susceptibility to instability.
- The pilot was relatively inexperienced on this specific aircraft type, having performed much of his recent training on a more stable VPM M-16 model.
- The aircraft's throttle characteristics, specifically the non-linear power response of the Rotax 532 engine, may have aggravated the onset of the oscillation.