What happened
On 21 November 2021, a DJI Matrice M3/00 was being prepared for a commercial aerial search operation in Blacon, Chester. Immediately following takeoff, and before the completion of standard airborne safety checks, the pilot heard a distinct cutting noise emanating from the aircraft. The unmanned aircraft system (UAS) began to move toward the left, yawed to the right, and entered a spiralling ascent. This uncontrolled motion was followed by a rapid descent, resulting in the aircraft striking the ground. The impact caused severe damage to the airframe, battery, propellers, and camera, rendering the unit a total loss.
The investigation
Following the accident, the operator conducted an examination of the wreckage. Investigators identified that the propeller blades on the front and rear right sides had intermeshed and collided. This mechanical interference was traced back to the rear right motor arm, which had folded forward during the initial phase of the flight.
Further analysis of the DJI Matrice M300 design revealed that the arm, hinge, and collar assemblies are manufactured to very tight tolerances. While the arms are secured using a 'twist to lock' collar, the high quality of the fit means that simply sliding the collar into position can make the arm feel rigid and secure even if the locking rotation has not been performed. The investigation noted that the vibrations produced by the motors and propellers during flight can cause an unlocked collar to slide outward, releasing the hinge and allowing the arm to fold under the forces of thrust and precession.