What happened
On 6 March 2023, a UAS Prion Mk3 was conducting a flight test at West Wales Airfield to validate its autopilot systems. During the second flight of the day, the remote pilot observed the engine stop while the aircraft was at 1,000 ft agl, positioned over the center of the airfield.
Following the engine failure, the aircraft lost all electrical power. The remote pilot attempted to take manual control, but the aircraft remained unresponsive to commands from both the remote pilot and the station operator. The aircraft continued to fly for approximately 30 seconds along the edge of Runway 25 before turning and eventually disappearing behind a hedge. The aircraft subsequently landed beyond the southwestern edge of the airfield, sustaining minor damage to the nose gear, fuselage, and right wingtip. There were no injuries or damage to property.
The investigation
The investigation focused on the cause of the engine shutdown and the loss of command and control. It was established that the aircraft's alternator generator was failing to deliver power or charge the emergency lithium polymer battery. This failure was traced to an incorrect wiring connection that had been made during the assembly of the aircraft.
Because the wiring for each airframe was unique, this error went undetected. The investigation also found that while the aircraft's telemetry system had the capability to display the emergency battery's voltage, the field crew had not enabled this specific display option. Furthermore, the aircraft's powerplant had been replaced the previous day, but no validation of the alternator generator's performance was performed following this change.
While the fault was present during the first flight of the day, the emergency battery had not yet been fully depleted. The error remained hidden because the battery was charged via ground power after the first flight, which masked the fact that the alternator had failed to maintain the charge during flight.
Findings
- The primary cause of the incident was the total loss of electrical power resulting from an incorrect wiring connection in the alternator generator system.
- The alternator generator failed to charge the emergency battery or provide sufficient power to the aircraft systems.
- The field crew was unable to monitor the battery's voltage because the telemetry display option had not been activated.
- Pre-flight checks were ineffective at identifying the fault because the battery was replenished by ground power after the initial flight of the day.
- The operator's safety case did not explicitly account for a total loss of electrical power.
Safety action
- The operator has standardised all wiring and schematics across its fleet to prevent assembly errors.
- The after-flight checklist has been amended to require a check of the emergency battery status before connecting ground power.
- The engine monitoring interface has been updated to include a visible voltage indication for the emergency battery.