What happened
On 1 August 1 and 1998, a Rotorway Executive 90, registration G-ROTO, was conducting a scheduled flight test to renew its Permit to Fly. The flight departed from Cambridge Airport with the pilot, an observer, and the aircraft owner on board. The weather conditions were favorable, with good visibility and light winds.
During the flight, which involved testing various performance parameters including climb rates and autorotations, the aircraft was traveling at approximately 500 feet altitude. Witnesses observed the helicopter traveling in a straight line before it suddenly appeared to turn or dive sharply. Observers noted pieces of debris falling from the aircraft and described the main rotors as either stopping or rotating very slowly as the helicopter descended. The aircraft struck a field of wheat, causing a significant ground fire and the destruction of the helicopter. The accident resulted in two fatalities.
The investigation
AAIB investigators examined the wreckage and analyzed radar data and witness accounts. The investigation focused on the structural integrity of the aircraft and the sequence of events leading to the impact. Engineering analysis of the debris indicated that the main rotor blades had struck the tail boom while the aircraft was still in flight. This impact caused the tail rotor section to separate and scattered debris across a wide area.
Investigators also reviewed recent maintenance work. Approximately three weeks prior to the accident, the owner had performed adjustments to the yaw and cyclic controls to address stiffness and movement restrictions. While these adjustments were not performed according to standard CAA procedures and lacked an independent engineering check, investigators found no evidence that these specific modifications contributed to the crash.
Findings
- The primary cause of the accident was contact between the main rotor and the tail boom during flight.
- The reason for this contact could not be definitively established, though it may have resulted from a system malfunction or an inability to apply corrective controls.
- The aircraft was performing a flight test profile, likely involving autorotations, at the time of the incident.
- The pilot was experienced and the aircraft was within weight and balance limitations.
- The destruction of the aircraft by the subsequent ground fire prevented a full assessment of potential engine or system failures that might have preceded the rotor strike.