What happened
On 23 June 2001, a Mistral microlight, registration G-MYST, was performing a private pleasure flight near Tenbury Wells, Shropshire. The aircraft, operated by its owner, was flying at low altitude with the cockpit canopy removed. Witnesses observed the aircraft flying at a low speed and appeared to be in an unbalanced state. Without warning, the aircraft entered a rapid yaw and roll, descending into a steep spin or spiral dive. The aircraft struck the ground in a near-vertical, nose-down position, causing the destruction of the aircraft and two fatalities.
The investigation
The investigation focused on the engine's performance and the aircraft's handling characteristics. Examination of the Rotax 532 engine revealed that a helical gear had failed, which subsequently caused the engine to stop abruptly. This failure was triggered by the ingestion of a foreign object—likely a small steel item—into the rotary valve. This object had likely been lodged within the air filter housing since a previous maintenance period.
Data from a GPS receiver indicated that the aircraft's airspeed had been steadily decaying in the minute prior to the accident, reaching levels very close to the aircraft's stalling speed. Investigators also examined the aircraft's stall warning system and found the sensor housing was insecure. Furthermore, because the canopy was removed, the investigation considered the possibility that a limb from the crew could have extended into the airflow, disrupting the aerodynamics.
Findings
- The primary cause of the accident was an abrupt engine failure caused by foreign object damage to the engine's rotary valve.
- The aircraft was flying at a low airspeed, very close to its stall limit, at the time of the failure.
- The removal of the cockpit canopy may have contributed to the loss of control, as limbs could potentially interfere with the airflow.
- The stall warning system was found to be unreliable due to an insecure sensor mounting and poor audibility of the buzzer while wearing headsets.
Safety action
Following the investigation, the BMAA implemented several mandatory requirements for the Mistral type, including:
- The mandatory fitting of a slip ball to assist with lateral stability.
- A requirement that the canopy must be fitted for all flights.
- The mandatory installation of an airspeed indicator (ASI) correction placard.
- The development of a more reliable stall warning device.