History of the Flight
The unmanned aircraft (UA), an Avy Aera 1.5, was performing a demonstration flight and hovering at an altitude between 50 m and 65 m above ground level. After approximately three minutes in the hover, a rhythmic sound lower than normal was heard coming from the UA. Moments later, the UA dipped to the left and began a slow spiral descent. The pilot attempted to regain control and fly the UA back to the launch site, but the UA was unresponsive and eventually struck the ground, sustaining severe damage to the lift motor boom, fuselage, and landing gear.
Aircraft Description
The Avy Aera 1.5 is a fixed-wing, payload-carrying UA that takes off and lands vertically. It is equipped with four electric lift motors mounted on twin booms, each driving a propeller for vertical flight, and a single electric pusher propeller for horizontal thrust. During forward flight, the lift propellers stop rotating and remain stationary until a transition to hover is commanded. The lift propellers are two-bladed, with blades held between upper and bottom hub plates by a single M3 bolt and stiff nut, allowing them to pivot. A stop pin on the blade boss engages a groove in the bottom plate to prevent overshooting.
Examination Findings
Post-accident examination revealed that one blade of a lift propeller had detached from its hub in flight. The bolt and nut that secured the blade failed near the interface between the plain shank and the threaded portion of the bolt. The threaded portion remained inside the nut. The failure allowed the hub plates to separate, releasing the blade.
Manufacturer's Investigation
The manufacturer conducted a metallurgical examination of the failed M3 bolt and concluded that it failed due to fatigue within the threaded portion under the nut. Further investigation, including test running of example propellers, identified that the probable causes of the fatigue failure were related to the design of the propeller and the loads encountered in flight. Design features of the hub plates and blades could lead to backlash between the blade boss and the bolt, resulting in vibrations when the propeller rotates. These vibrations are believed to cause cyclical shear and bending loads on the bolt. Additionally, the stop pin groove on the failed propeller hub assembly was damaged, with evidence of fretting on the underside of the blade boss, which may have increased backlash and exacerbated the cyclical loads. The manufacturer also noted that there is no specified torque setting for the nut, meaning any inconsistency in tightening during assembly or maintenance could contribute to fatigue failure.
Safety Actions
In response to the accident, the manufacturer introduced a 10-hour replacement schedule for the propellers and requires immediate replacement if a Quadchute event occurs. The manufacturer is also reviewing propeller designs for UAs under development.