What happened
On 2 December 2020, a DJI Phantom 4 RTK was conducting a commercial aerial survey of railway infrastructure near Newtongrange, Midlothian. The flight was being performed in an automated mode to capture data for Network Rail. The aircraft was operating at a height of approximately 70 m (2/3 of its planned altitude) when it reached a turning point in its programmed route.
During this maneuver, the pilot noted a short beep from the controller, though no error messages were immediately visible. Shortly thereafter, the aircraft began a rapid vertical descent. The observer noted that the aircraft appeared to be in free fall, inverted, with the motors appearing to stop. The drone eventually struck the ground in the rear garden of a house within a densely populated estate. While no persons were injured, the aircraft was located near a member of the public who was present in the area during the flight. The impact caused significant damage to the motors, propellers, arms, landing gear, and fuselage.
The investigation
An investigation by the AAIB examined the flight logs and the physical wreckage. The data revealed that as the aircraft attempted its turn, the left rear motor suddenly accelerated to maximum speed, causing the aircraft to spin and tumble. The logs also showed that the descent rate actually exceeded free fall at one point because the remaining three propellers were still providing thrust while the aircraft was upside down. The kinetic energy at impact was calculated at approximately 900 Joules.
The manufacturer analyzed the flight logs and determined that the left rear propeller had detached in flight. The investigation also reviewed the regulatory framework for UAS operations in the UK at the time, specifically regarding the risks of overflying uninvolved persons and the adequacy of existing risk assessments.
Findings
- The primary cause of the loss of control was the in-flight detachment of the left rear propeller.
- The aircraft's descent was characterized by a rapid vertical drop and tumbling motion following the loss of propulsion symmetry.
- The impact energy of 900 Joules significantly exceeded the 80 Joule threshold often cited as a limit for safe overflight of people.
- Existing mitigations, such as maintaining a 50m horizontal distance from people, were noted as difficult to implement in congested urban environments.
Safety action
- The CAA is recommended to collect data on UAS failure rates per flying hour to improve risk assessments.
- It is recommended that the CAA consider prohibiting the overflight of uninvolved persons with aircraft in the Specific category that rely solely on propulsion for lift and could impact the ground with energy exceeding 80 Joules.
- Following the incident, the operator ceased using DJI Phantom 4 models for their survey activities.