What happened
On 4 September 2020, a Wingcopter 178 Heavylift unmanned aircraft was conducting a training flight near Newbury to verify performance capabilities at its maximum takeoff mass. The flight followed an automated mission profile that had been successfully executed on five previous occasions. After completing necessary pre-flight checks, the remote pilot initiated an automatic flight sequence.
The aircraft successfully lifted off and began a vertical climb in hover mode. After reaching an altitude of approximately 35 m agl, the drone began a programmed translational climb toward a target height of 110 m agl. However, 72 seconds into the flight, as the aircraft approached 100 m agl, it experienced a sudden roll and pitch to the right, becoming temporarily inverted. The aircraft was unable to recover control and entered a rapid, clockwise spinning descent. The pilot attempted to intervene using manual controls, but the aircraft continued to descend, eventually striking a harvested crop field at Mayfield Farm. The impact resulted in the aircraft being damaged beyond economical repair, though there were no injuries.
The investigation
An examination of the aircraft by the operator and the manufacturer determined that one of the rear electronic speed controllers (ESCs) had overheated. This overheating led to the loss of propulsion from the associated motor and propeller, destabilising the aircraft. The investigation established that the manufacturer had previously identified this overheating risk and had implemented a 120-second limit on hover time to mitigate the issue. In this instance, the combination of the aircraft operating near its maximum takeoff mass and the duration of the translational climb caused the component to fail.
Findings
- The primary cause of the loss of control was the overheating of a rear electronic speed controller.
- The failure was driven by the increased electrical load resulting from the aircraft operating near its maximum takeoff mass during a sustained climb.
- The aircraft was performing a programmed mission that exceeded the thermal stability of the ESC under those specific load conditions.