What happened
Two separate incidents occurred involving an Evolve Dynamics Sky Mantlis unmanned aerial system (UAS) in Skegness, Lincolnshire, during commercial operations.
On 14 January 2021, during a familiarisation flight, the aircraft was descending to land at approximately 10 m altitude when an unusual noise was heard and a component detached. The UAS yawed left and struck the ground, resulting in moderate damage to the fuselage, landing gear, and one motor. An inspection revealed that a screw securing a propeller blade to the motor hub adaptor had fractured.
On 17 February 2021, during a training flight, the same aircraft was hovering at 100 m altitude when the pilot observed a change in motor sound and video instability. The aircraft descended uncontrollably and struck the ground approximately 170 m from the launch point. This second impact also caused moderate damage to the landing gear and propellers, with one propeller blade missing.
The investigation
Investigators examined the failed screws using optical and scanning electron microscopy. The first failure involved a manufacturer-designed hub adaptor using M3 zinc-plated screws. The second failure involved a third-party adaptor using M3 screws with a black oxide finish.
Metallurgical testing revealed that the hardness of the screws in both incidents exceeded the required specification. Specifically, the hardness values were significantly above the 345 HV threshold, which makes high-strength steel susceptible to both stress corrosion cracking (SCC) and hydrogen embrittability (HE).
Findings
- The first incident was caused by stress corrosion cracking, potentially exacerbated by hydrogen embrittlement.
- The second incident was primarily a fatigue failure, which may have been initiated and accelerated by hydrogen embrittlement.
- A primary causal factor in both failures was that the hardness of the screws exceeded the design specification, increasing their vulnerability to environmental and manufacturing-related degradation.
Safety action
Following these events, the manufacturer implemented several design modifications to the motor hub adaptor, including:
- Increasing the screw diameter from M3 to M4
- Implementing a stress test to check for hydrogen embrittlement
- Utilizing single-use lock nuts and serrated washers
- Removing the use of thread-locking compound
- Adding a brass spacer bush to the propeller blade root