What happened
On 19 February 2021, a Parrot Anafi was conducting a commercial UAS operation near Fareham, Hampshire. The aircraft was flying at an altitude of 58 metres to maintain a safe distance from nearby structures. During the flight, the pilot briefly diverted his attention to a tablet to check battery levels and camera imagery. Upon looking back, the pilot observed the drone spinning and descending. Although the pilot managed to stabilise the aircraft using full thrust, the instability recurred shortly after he attempted to turn the drone back toward the launch site.
During this second period of violent spinning, the aircraft descended rapidly, flying over the pilot and observer before colliding with trees in a nearby woodland. The impact caused substantial damage to the aircraft, including a detached battery and a damaged front left propeller arm.
The investigation
The AAIB examined the wreckage and identified that the bonding between the upper and lower halves of the front left propeller arm had failed. This separation allowed the two components to move independently. Further investigation revealed that the operator had discovered similar bonding issues on other aircraft in their fleet, both before and after the accident.
Analysis suggested that the separation of the arm halves likely caused the motor mount to lose rigidity. The combination of motor vibration and gyroscopic forces from the propeller would have caused the arm to bend and twist unpredictably. This movement likely altered the direction of thrust, creating flight dynamics that the onboard control system could not compensate for, leading to the observed loss of control.
Findings
- The failure of the bonding between the upper and lower halves of the front left propeller arm was the primary cause of the loss of control.
- The structural instability caused by the arm separation led to unpredictable thrust vectors.
- The aircraft was part of a specific batch of 269 units identified by the manufacturer as potentially having similar bonding vulnerabilities.