History of the Incidents
On 14 January 2021 at 1100 hrs, an Evolve Dynamics Sky Mantis unmanned aircraft (UA) was on a familiarisation flight near Skegness, Lincolnshire, with an instructor and observer present. During descent to landing at about 10 m agl, an unusual noise was heard and a part was seen to depart. The UA yawed left and fell to the ground. Inspection revealed that the screw attaching one propeller blade to the motor hub adaptor had fractured. The threaded part of the screw remained in the adaptor plate, but the rest was not recovered. The blade was found about 20 m away. The motor hub adaptor had accumulated approximately 1.16 million motor revolutions.
After the first incident, the manufacturer repaired the UA and replaced all motor hub adaptors with a different design. However, on 17 February 2021 at 0930 hrs, a different pilot was conducting a training flight under supervision with the same UA. While in loiter mode at about 100 m agl and 120 m from the pilot, a change in motor sound and video image wobble were noticed. The UA wobbled, dropped, and then wobbled again. The pilot initiated return to launch site and ascend control, but the UA did not respond and continued descending until it struck the ground. It was recovered about 170 m from the launch point with one propeller blade from the rear left motor missing, along with part of the retaining screw. The threaded part of the screw and a jam nut remained attached to the motor hub adaptor. Approximately 3.6 million motor revolutions had been completed since the repair.
Aircraft Information
The Evolve Dynamics Sky Mantis is a remotely operated UAS for the emergency services sector. It has four electric motors driving two-bladed propellers. Two motor hub adaptor designs were used: type 1 (UAS manufacturer design) and type 2 (third-party design). Type 1 used a single M3 zinc-plated class 12.9 steel socket head shouldered cap screw per blade, secured with a split pin. Type 2 used a similar M3 screw with black oxide finish, a jam nut, lock washers, and thread lock compound.
Examination of Failed Screws
The failed screws were examined by a specialist metallurgist. The type 1 screw failed at the second thread from the plain diameter. The fracture surface was intergranular, consistent with stress corrosion cracking (SCC). Radial marks indicated crack propagation direction. Light oxidation was present. Hydrogen embrittlement (HE) could not be excluded as a contributing factor.
The type 2 screw failed primarily through fatigue, initiating from the thread root and propagating across 90% of the cross-section before final separation. About 20% of the stable crack growth region showed intergranular fracture consistent with HE; the remainder was transgranular fatigue. The level of embrittlement was insufficient to cause failure alone but may have reduced the crack initiation threshold and accelerated crack growth.
Hardness Testing
Ten screws of each type were randomly selected and tested for Vickers hardness along with the failed screws. The material specification (BS EN ISO 898-1:2013 12.9) requires hardness of 385 HV to 435 HV. Susceptibility to HE and SCC increases above 345 HV. All type 1 sample screws and the failed screw exceeded the specification, with similar hardness values. All but one type 2 sample screw also exceeded specification; the failed screw averaged almost 10 HV higher than the sample average.
Design Changes
In response, the UA is now supplied with type 2 adaptors using M4 screws, single-use locking nuts, and shakeproof washers. Assembly is more detailed and controlled. A brass spacer bush is fitted in the composite propeller blade root. Random sampling for hydrogen embrittlement has been incorporated. Further changes under consideration include switching screw material to a corrosion-resistant steel with lower yield strength.