Accident
On 27 June 2023 at 1443 UTC, a Malloy Aeronautics T150 unmanned aircraft was destroyed when it struck the ground near South Scarle, Lincoln. The aircraft was being operated in manual flight mode during a skills currency training flight. The remote pilot (RP) was assisted by a ground control station (GCS) operator, both in two-way radio communication. The geofence was set to 40 m height and 300 m radius.
As the aircraft approached the upper limit of the geofence, the GCS operator informed the RP, but the terminology used was not immediately understood. Shortly after, the aircraft breached the geofence and automatically switched to Return to Launch (RTL) mode. The RP instinctively reduced throttle and then cycled the flight mode selector switch to loiter and back to stabilised (stab) mode. The aircraft diverged from level flight, reaching a maximum pitch of -41° and roll of -60.9°. The RP increased throttle to 100%, causing an overcorrection with pitch up to 85.3° and 60° roll, descending from 37 m. The RP switched to loiter mode but then closed throttle, and the aircraft struck the ground approximately 50 m from the RP, within the horizontal geofence boundary. Twelve seconds elapsed from the breach to impact. Data showed throttle reduced to 2.6% in RTL mode, then mode changes with throttle increase to 100% and large roll inputs.
Aircraft
The Malloy Aeronautics T150 is a military-standard cargo unmanned aircraft with eight electric motors, capable of lifting 68 kg payload, with a maximum takeoff mass of 123 kg and 240 kg thrust. It can operate in multiple flight modes, including manually flown modes and automated modes for pre-planned missions. The RP used a FrSky Taranis X9D plus transmitter with a three-way switch to select flight modes: alt hold, stab, and loiter. RTL mode is fully automated with no effect from RP inputs.
Personnel
The RP held a General Visual line of sight Certificate (GVC) and an A2 Certificate of Competency, with 150 total flight hours (2 on type). The GCS operator also held a GVC and A2 CofC and was experienced on multiple unmanned aircraft types. Four other people were present at the site but not actively engaged.
Operating Safety Case
The aircraft operated under a CAA Operational Authorisation granted on 30 May 2023, referencing an Operating Safety Case (OSC) with volumes on operations, technical ability, and safety risk assessments. The OSC did not contain definitions or procedures for geofence use or actions after a breach. A safety recommendation was made to the CAA because these omissions affect other operations where geofences are used as a mitigation.
After the accident, the operator discontinued use of manual flight mode and promoted standardised phraseology between GCS operator and RP. Further action was taken to apply suitably sized geofences for each flight.