What happened
On 27 June 2023, a Malloy Aeronautics T150 unmanned aircraft was being operated for a skills currency flight in South Scarle, Lincoln. The flight was being conducted in manual flight mode by a remote pilot, supported by a ground control station operator.
As the aircraft approached the upper limit of its established geofence, the ground station operator issued a warning, though the terminology used was not immediately understood by the pilot. The aircraft subsequently breached the vertical limit of the geofence, which triggered an automated Return to Launch (RTL) mode. To prevent the aircraft from climbing further, the pilot reduced the throttle to approximately 2.6%.
In an attempt to regain manual control, the pilot cycled the flight mode selector switch. However, because the throttle was at a very low setting when the aircraft reverted to manual mode, the aircraft could not maintain level flight. The aircraft entered an erratic flight path with significant pitch and roll oscillations. An attempt to recover by applying full throttle resulted in an overcorrection. The aircraft eventually descended rapidly and struck the ground, resulting in the destruction of the aircraft.
The investigation
The AAIB investigation examined the aircraft's flight data, the operator's safety documentation, and the pilot's training procedures. The investigation established that the ground station's audible alerts had been disabled for the flight. Furthermore, the investigation found that the geofence dimensions—40 m high by 300 m radius—had not been specifically evaluated for appropriateness prior to the flight.
Review of the Operating Safety Case (OSC) revealed that while the geofence was intended as a risk mitigation tool, the documentation lacked specific procedures for handling a geofence breach. Additionally, the investigation noted that the pilot's training relied on self-identified weaknesses rather than a structured programme of non-normal procedure practice.
Findings
- The height of the geofence was a causal factor, as it was not assessed for suitability before the flight.
- The pilot's decision to cancel the automated RTL mode was a result of normalised behaviour to save time.
- Loss of control occurred because the flight mode was changed while the throttle was at a low setting, leaving the aircraft with insufficient power to maintain stable flight.
- The lack of standardised phraseology between the ground operator and the pilot hindered effective communication during the critical moments of the breach.
- The operator's safety documentation lacked necessary definitions and procedures for geofence breaches and contingency actions.
Safety action
- The operator has ceased using the manual flight mode and has implemented standardised phraseology between the ground station operator and the remote pilot.
- The operator has committed to using appropriately sized geofences for each flight and ensuring audible alerts are enabled.
- Safety Recommendation 2024-106: It is recommended that the UK Civil Aviation Authority, when granting Operational Authorisations for Unmanned Aircraft Systems in the specific category, ensure that any safety feature used to mitigate risks is adequately defined in the Operating Safety Case and includes the necessary operational procedures.