What happened
On 18 November 2019, a UAVE Prion Mk 3 was conducting aerial work at West Wales Airfield, Aberporth. During an autonomous landing sequence at the secure facility, the unmanned aerial system (UAS) was executing a left-hand turn to align itself with the final approach path.
As the aircraft began to accelerate and descend below its intended flight profile, the autopilot system attempted to correct the deviation. The system was programmed to manage airspeed via power adjustments and glideslope via pitch control. Consequently, the engine power was reduced to slow the descent, and the elevator was used to pitch the aircraft upward. However, the elevator authority commanded by the autopilot reached its maximum limit and could not provide enough pitch to recover the lost altitude. This resulted in the aircraft striking the ground just before reaching the runway.
There were no injuries or fatalities as the aircraft was unmanned. The impact caused extensive damage to the propeller, tail boom, and both the front and rear landing gear, rendering the aircraft beyond repair.
The investigation
The investigation was based on the aircraft accident report submitted by the pilot. The inquiry focused on the flight control logic of the autopilot and its ability to maintain the descent profile during the turning maneuver. The investigation examined how the interaction between power reduction and pitch command affected the aircraft's flight path.
Findings
- The primary cause of the accident was the insufficient elevator authority available to the autopilot to recover the aircraft's altitude once it had descended below the programmed profile.
- The autopilot's attempt to manage airspeed by reducing power, combined with the physical limits of the elevator, prevented the UAS from maintaining the required glideslope during the turn.
Safety action
Following an investigation conducted by the manufacturer and operator, the autopilot's elevator authority has been increased. Additionally, the pre-programmed turn onto the final approach has been widened and raised to minimize the amount of elevator deflection required during the landing phase.