What happened
On 2 July 2022, a Tekever AR3 was conducting a Beyond Visual Line of Sight (BVLoS) maritime surveillance mission over the English Channel within Temporary Danger Area D098. The flight, operated by a commercial UAS operator, began from a site near Dover under favorable weather conditions. Following standard preflight procedures and successful engine tests, the aircraft proceeded to its mission area.
After approximately 71 minutes of flight, while loitering at 800 ft amsl, the remote pilots observed the engine RPM drop to zero while the throttle command reached its maximum setting of 98%. As the aircraft began to lose altitude, the engine failure became apparent. The aircraft continued along its programmed flight path until it reached 550 ft amsl, at which point the automated emergency parachute system was triggered. The aircraft descended via parachute into the sea. The wreckage was later located and recovered by a fisherman.
The investigation
The investigation focused on the fuel delivery system of the aircraft's single piston engine. The system relied on a high pressure (HP) fuel pump to move fuel from a header tank, while a low pressure (LP) fuel pump transferred fuel from the main tank to that header tank. Both pumps were managed by the Engine Control Unit (ECU).
Examination of the recovered aircraft and telemetry data revealed that the engine RPM decay was consistent with fuel starvation. While the engine and propeller were found to be in good condition, investigators identified internal damage within the LP fuel pump. Analysis of electrical parameters indicated that this damage caused an electrical short circuit. Because the LP and HP fuel pumps shared a single electrical fuse, the short circuit caused the fuse to trip, simultaneously cutting power to both pumps and leaving the engine without fuel.
Findings
- The engine failed due to fuel starvation caused by the simultaneous loss of both the high pressure and low pressure fuel pumps.
- Internal damage within the low pressure fuel pump triggered an electrical short circuit.
- The electrical architecture was vulnerable because the low pressure and high pressure fuel pumps were protected by the same electrical fuse.
Safety action
Following the incident, the operator suspended operations for this aircraft type until design improvements were implemented. The following modifications were made:
- The fuel tank was modified to allow the high pressure pump to draw fuel into the header tank even if the low pressure pump fails.
- The electrical systems for the low pressure and high pressure fuel pumps were separated into individual fuses.
- An upgraded version of the low pressure fuel pump was introduced.