What happened
On 3 February 2021, a UAVE Prion Mk 3 unmanned aircraft was conducting a training flight within a protected area of MoD Area 7, Salisbury Plain, Chitterne, Wiltshire. The flight was being operated by the aircraft manufacturer. After the aircraft had climbed to a pre-programmed altitude of 400 ft agl, the engine experienced a loss of power, causing the aircraft to begin a descent.
In an attempt to address the power issue, the remote pilot station operator issued a command to limit the throttle to 80%. However, this did not rectify the engine performance, and the aircraft continued to lose altitude and airspeed. At the time, the autopilot was operating in its default longitudinal mode, which uses throttle to manage airspeed and elevator to manage altitude. As the aircraft descended, the autopilot responded by pitching the nose up to regain the programmed height. This maneuver caused the aircraft to decelerates until it reached a stall, entered a spin, and impacted the ground at approximately 100 ft agl. The impact resulted in substantial damage to the forward fuselage, wings, propeller, rear boom, and front skids.
The investigation
The investigation examined the engine's performance and the autopilot's logic. It was established that the engine's power loss was caused by a loose spark plug cap. Furthermore, the investigation looked into the autopilot's longitudinal modes. While the system is designed to automatically switch modes to prevent stalls under certain conditions, the decision to limit the throttle to 80% prevented the autopilot from transitioning from Mode 0 to Mode 2, which would have otherwise increased throttle to 100% to maintain speed.
Findings
- The engine lost power because of a loose spark plug cap.
- The attempt to mitigate the power loss by restricting the throttle prevented the autopilot from automatically switching to a mode that would have prevented the stall.
- The autopilot's longitudinal mode cannot be changed once the flight is underway.
- The crew had insufficient time to intervene and take manual control once the airspeed began to decay.
Safety action
- The manufacturer has implemented a fleet-wide engine modification to include a safety feature that ensures spark plug caps are securely fitted.
- The operator has updated training and operational procedures to prevent the risk of limiting throttle in a manner that interferes with autopilot logic.