The UK Ministry of Defence (MoD) has had to write off a multi-million pound unmanned surveillance drone after a glitch in the software caused the drone to break itself, thinking that it had already landed when it was still in mid-air.

A report released by the UK government enquiry into the incident says that the Watchkeeper WK031 unmanned aerial vehicle (UAV), operated by a civilian crew, was making its approach to land at West Wales Airport in Aberporth on 16 October 2014 when it suddenly took a nose dive from about 10-15ft above the runway.

Due to approaching bad weather, the crew had manually overridden the drone's automatic landing feature to get the UAV to land sooner than initially planned, but the drone suddenly crashed into the ground, causing considerable damage to the fuselage structure, while the vehicle's undercarriage collapsed as it continued to slide along the runway before coming to a standstill.

Although there was no fire and no one was injured in the crash, the UAV was damaged so badly that the MoD was forced to write it off. An investigation was later ordered into the reasons behind the crash. The report stated that the drone's system did not issue any alerts showing anything was amiss, and the crew thought that everything was perfect for landing.

Manual override and faulty algorithms to blame

Watchkeeper was developed in partnership with Thales UK and Israeli drone manufacturer Elbit. Data from the crash was sent to the manufacturer for analysis, and it was discovered that the drone's computer system had incorrectly sensed that the drone had touched ground about 20m (65ft) from the ground, indicating that the computer algorithms had problems in understanding and detecting air turbulence.

The report concluded that although the civilian crew had not caused the accident, besides the flawed drone computer algorithms, there was not enough information about the manual override given by the manufacturer and the crew had lacked situational awareness of the weather, plus the wet runway surface at the airport had caused the drone's laser altimeter to make an incorrect reading about the actual height of the drone.

The Watchkeeper drone fleet is currently being tested and was expected to be deployed in 2017, but the damning report has cast doubt on whether that will actually happen.

Drone software "not fit for purpose"

"I agree with the findings including the cause, contributory factors, other factors and observations. However, the casual statement that the vehicle management system computer (VMSC) functioned 'as designed but not as intended', whilst true, I will express in a more straightforward manner by saying the VMSC logic was not fit for purpose," said Air Marshal Dick Garwood, Director General of the Defence Safety Authority.

"While they might be beyond the purview of this safety-related service inquiry, there are clearly broader lessons for Defence to learn here in relation to how we first procure and then introduce this type of complex capability into front line service."

This is not the first time that a UK Army surveillance drone has crashed – another Watchkeeper drone crash-landed at Boscombe Down in November 2015, following the crew's selection of the manual override controls, according to Garwood.