What happened
On 14 July 2022, a GEN 3.8 unmanned aircraft was conducting an urban delivery mission in the Tankardstown area of Balbriggan, Co. Dublin. The operation involved transporting small packages to customers via a pre-programmed flight path. Approximately 52 seconds after departing from its base at a local shopping centre, the aircraft experienced a significant mechanical failure. A propeller blade from the upper rear-right motor detached from the aircraft, causing intense vibrations.
These vibrations led to the subsequent failure of the lower rear-duty electric motor. As the aircraft struggled to maintain stability, its flight control unit switched to backup systems, and the automated Smart Return To Land feature activated. However, the loss of propulsion and increasing instability caused the aircraft's descent rate to accelerate. This triggered the onboard Flight Termination System, which deployed an emergency parachute. The aircraft eventually landed on grassy terrain near a public footpath, sustaining minor damage to its landing gear and outer shell.
During the incident, the detached propeller blade struck an individual on the ground, resulting in minor injuries to a small cut on the head. A second individual walking nearby was also struck by a small fragment of a propeller blade but remained uninjured.
The investigation
The AAIU investigation focused on the mechanical failure of the propulsion system and the sequence of events leading to the parachute deployment. Investigators examined the aircraft's hardware, including the remaining propellers and the motor assemblies. Metallurgical analysis was performed on the hardware to identify the origin of the failure.
Technical examination of the propeller hub revealed that while the alloy steel retaining bolts remained intact, the specific bolt securing the upper rear-right propeller blade had fractured. The investigation also reviewed the operator's maintenance procedures and the suitability of the propeller components for the aircraft's coaxial configuration.
Findings
- The primary cause of the accident was a fatigue fracture of a bolt securing the propeller blade to the hub.
- The fracture was caused by high-cycle fatigue resulting from insufficient assembly torque, which allowed the bolt to be subjected to fluctuating loads during flight.
- The separation of the blade created severe vibrations that caused the failure of the adjacent electric motor.
- The aircraft's emergency parachute system functioned as intended, reducing the descent rate before impact.
- The propeller manufacturer had previously noted that their polymer propellers are not ideally suited for the complex load conditions found in coaxial configurations.
Safety action
Following the incident, the operator implemented several safety improvements. They revised their bolt tightening procedures and removed all existing bolts from service. Additionally, the operator redesigned the propeller hub with a failsafe mechanism to prevent blade separation in the event of a bolt failure and committed to transitioning from a coaxial design to a single-propeller-per-arm configuration.