The Panel concluded that the cause of the crash was aerodynamic stall due to inaccurate air speed being used by the FCS within the VMSC. This air speed was calculated by the VMSC, which was reacting to inaccurate pitot total pressure measurements caused by pitot blockages from flying in cloud and precipitation over a prolonged period.
The FCS response to the erroneous air speed included pitching oscillations and FEP manoeuvres, over speeding and unstable flight near-stall speed, which may have caused physical damage to the left V-tail.
The Panel identified the following causal factor:
a. Aerodynamic stall due to inaccurate airspeed being used by the Flight Control Software within the Vehicle Management System Computer.
The Panel identified the following contributory factors:
a. Flight in cloud and precipitation over a prolonged period of time.
b. Pitot blockages, resulting in inaccurate total pressure measurements.
c. Vehicle Management System Computer logic for disqualifying sensor readings and computing air speed.
d. The design of the Air Data System limited the UA's ability to fly in cloud and precipitation.
e. The conduct and supervision of the trial.
f. The DO may have over-estimated the UA's ability to fly in cloud and precipitation, which led to inaccurate limits being set for the MFTP.
g. Deviating from Flight Reference Card guidance for a caution not considered to be 'latched'.
h. Lack of rigorous post flight data analysis.
The Panel identified the following aggravating factors:
a. The non-recovery of the UA.
b. The carriage of the radar payload.
(end of excerpt)
Click here for the full report (134 PDF pages) on the UK Government website.
(EDITOR’S NOTE: The report faults both the Ministry of Defence and Thales for their handling of the Watchkeeper program, and for the way these trials were managed.
Air Marshal Susan Gray, director-general of the Defence Safety Authority, criticised the Ministry of Defence and Thales for their "incomplete level of detailed technical understanding" regarding the drone and its systems.
To date, the British military has crashed five Watchkeepers, including two in quick succession during 2017, which led to a months-long grounding of the entire fleet, The Register reported April 15.
“Those two crashes, of which WK042 was one, were not publicly revealed until a senior Navy officer blabbed about them within earshot of a sharp-eared reporter, prompting accusations of an MoD cover-up over the troubled multimillion-pound programme,” it added.)