AAIB investigation to Evolve Dynamics Sky Mantis, (UAS registration N/A)

Loss of propeller blade after failure of attachment screw, Skegness Lincolnshire, 14 January 2021 and 17 February 2021 (same UAS for both events)

• From: Air Accidents Investigation Branch
• Published: 13 January 2022

• Category: Unmanned Aircraft Systems (UAS)
• Report type: Bulletin - Correspondence investigation
• Date of occurrence: 14 January 2021
• Aircraft type: Evolve Dynamics Sky Mantis
• Location: Skegness, Lincolnshire
• Registration: N/A

Summary:

A screw which attached the propeller blade of a UA to the motor hub adaptor failed during a training flight. A second screw failure was experienced by the same operator after the UA had been repaired and had been fitted with a different design of hub adaptor and screws.

The first failure was caused by stress corrosion cracking possibly with the presence of hydrogen embrittlement. The second was a fatigue failure which may have been initiated and accelerated by hydrogen embrittlement. The hardness of both screws exceeded the specification which increased the susceptibility of the screws to hydrogen embrittlement.

The UAS manufacturer has introduced several design changes to prevent reoccurrence.

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Evolve Dynamics Sky Mantis, UAS registration N/A 01-22

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Glossary of abbreviations

Published 13 January 2022