What happened
On 6 May 2022, an Avy Aera 1.5 was conducting a private demonstration flight near Lamlash, Isle of Arran. While the unmanned aircraft was maintaining a hover at an altitude between 50 and 65 metres, the operator heard a rhythmic, unusual sound coming from the craft. Shortly after this sound was noted, the aircraft dipped to the left and entered a slow spiral descent. Despite efforts to regain control and return the aircraft to its launch point, the craft became unresponsive and struck the ground, resulting in severe damage to the landing gear, fuselage, and lift motor boom.
The investigation
Investigators examined the wreckage and determined that a blade from one of the lift propellers had become detached during flight. The investigation focused on the M3 bolt and nut assembly that secures the propeller blades to the hub. It was discovered that the bolt had snapped near the transition between its plain shank and the threaded section, while the threaded portion remained trapped inside the nut. This failure allowed the upper and lower hub plates to separate, releasing the blade.
Following the accident, the manufacturer performed a metallurgical analysis and conducted tests on similar propellers. This process revealed that the design of the hub plates and blades could allow for backlash between the blade boss and the bolt. This movement creates vibrations during rotation, which subject the bolt to cyclical shear and bending loads. Furthermore, evidence of fretting was found on the underside of the blade boss, and the stop pin groove showed signs of damage, which likely increased the risk of backlash.
Findings
- The primary cause of the accident was a fatigue failure of a blade attachment bolt.
- Design features within the propeller hub allowed for backlash between the blade boss and the bolt, generating vibrations.
- These vibrations created cyclical loads that led to the fatigue of the bolt.
- Damage to the stop pin groove and evidence of fretting likely exacerbated the mechanical instability.
- The lack of a specified torque setting for the bolt assembly may have contributed to inconsistent installation.
Safety action
- The manufacturer has implemented a new 10-hour replacement schedule for propellers.
- Immediate propeller replacement is now required following any 'Quadchute' event.
- A design review of propellers for future unmanned aircraft is currently underway.