Drone Manufacturer: Evolve Dynamics
Drone Model: Sky Mantis
Country: United Kingdom of Great Britain and Northern Ireland
Type: Accident
Date: 2021-02-17
Applies: Daytime
Pilot Qualifications: Unknown Status
Pilot Flight Experience: 23 Hours
Link to External Information About This submission: https://assets.publishing.service.gov.uk/media/61bb27788fa8f5037d67b962/Evolve_Dynamics_Sky_Mantis_UAS_registration_N_A_01-22.pdf
File Uploaded: None
Description
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.
14 January 2021 Accident
The pilot was operating the UAS on a familiarisation flight with an instructor and observer present. During a decent to landing, an unusual noise was heard, and a part was seen to depart from the UA when it was at approximately 10 m agl. The UA yawed to the left and fell to the ground. An initial inspection of the UA by the operating team revealed that the screw attaching one of the blades to a motor hub adaptor had fractured. A piece of the threaded part of the screw remained in the motor hub adaptor plate, but the remainder of the screw was not recovered. The blade was recovered approximately 20 m from the UA. Following the accident, the manufacturer repaired the UA and changed all the motor hub adaptors to a different design. The motor hub adaptor had been subjected to approximately 1.16 million motor revolutions when the failure of the screw had occurred.
17 February 2021 Accident
A different pilot was performing a training flight under supervision with the same UAS. The UA was held in ‘loiter mode’ at about 100 m agl and about 120 m from the pilot. Whilst observing through the onboard camera, he noticed a change in motor sound and saw the video image wobble. He looked up to see the UA wobble, drop a distance and then wobble again. The pilot operated the return to launch site function and then the ascend control, but there was no response from the UA, which continued to descend until it struck the ground. The UA was recovered approximately 170 m from the launch point with one of the propeller blades from the rear left motor missing, along with part of the retaining screw. Neither item was recovered. The threaded part of the screw and a jam nut were still attached to the motor hub adaptor. The manufacturer estimated that 3.6 million motor revolutions had been completed since the repair.
Reported Cause
Type 1 Accident
The primary failure mechanism for the type 1 screw was considered to be stress corrosion cracking but a contribution by hydrogen embrittlement could not be discounted. The UAS was being operated near the coast and hence the saline content in the air may have provided an environment conducive for SCC. Radial markings indicated that a crack had initiated on the surface, in the root of a thread and propagated across the diameter and the entire fracture surface area was intergranular in nature. Hydrogen may have been present as a result from the zinc plating process due to ineffective de-embrittlement. The complete coating of the screws with zinc would have trapped the hydrogen in the part and therefore not allowed a path for it to escape. The screw was susceptible to both HE and SCC as it was significantly above the threshold of 345 HV and it also exceeded the hardness specification value.
The stress on the screw would have been a combination of flight loads on the blade and the axial preload as the screw was tightened. Testing done by the manufacturer has demonstrated that the screws can withstand all the operational loads in combination with the tightening torque.
Type 2 Accident
The failure of the type 2 screw was predominately fatigue crack growth but with areas of the fracture surface exhibiting hydrogen embrittlement. No initiating flaw could be identified but it is possible that stress concentration at the thread root in combination with HE was sufficient to initiate a crack which then grew during the 3.6 million motor revolutions before failure. It is possible that the application process for the surface finish was the source of hydrogen in the screw. The high hardness of the material would have reduced the fracture toughness and in combination with the HE further reduced the time to failure.
Conclusion
The type 1 screw failure was probably driven by the operational environment and the presence of hydrogen within the very hard steel. This combination resulted in stress corrosion cracking possibly exacerbated by hydrogen embrittlement. The type 2 screw failure by fatigue crack growth was probably initiated and then accelerated by hydrogen embrittlement.
The use of screws in both accidents with the hardness exceeding the specification value, was determined to be a causal factor in their failures.