What happened
On 25 July 2022, an Evolve Dynamics Sky Mantis was being operated in St Albans, Hertfordshire, to assist emergency services with a building fire. The unmanned aircraft (UAS) had already completed two successful flights that day. Approximately 13 minutes into its third flight, the aircraft suddenly dropped from a height of roughly 20 m and was destroyed upon impact. The operation was being conducted over wasteland with a 200 m cordon in place, and there were no injuries to any persons.
The investigation
Investigators examined the aircraft and its five batteries, discovering significant thermal damage to the electrical interface. The aircraft's battery connector had been severely affected by heat, resulting in the collapse of 12 internal pins. Examination of the batteries used during the preceding flights revealed varying degrees of damage: the battery from the second flight showed scorched marks and receding pads, while the battery used during the accident flight exhibited much more extensive thermal destruction.
Flight logs confirmed that the aircraft experienced rapid fluctuations in voltage and current in the minute leading up to the crash, followed by a total loss of power at 13 minutes and 2t seconds into the flight. While the pilot performed checks between flights, the progressive degradation of the electrical connections went unnoticed. The manufacturer noted that while the connector design had been previously updated to prevent latching issues, the current failure was driven by a reduction in the number of active pins, which increased electrical resistance and subsequent heat.
Findings
- The aircraft fell due to a complete loss of electrical power caused by the separation of electrical contacts.
- Thermal damage to the aircraft's battery connector led to the structural failure of the connection pins.
- A progressive increase in electrical resistance occurred as the number of functional contact pins decreased, generating enough heat to melt the thermoplastic composite material.
- Evidence of scorching on the battery from the first flight suggests the degradation process began earlier in the day, but the damage was not detected during pre-flight or inter-flight inspections.
Safety action
- The manufacturer has updated the Sky Mantis Operating Manual to explicitly require pilots to check that both the aircraft and battery contacts are clean and undamaged during pre-flight inspections.