What happened
On 18 September 2019, a Velos Single Rotor unmanned aerial system (UAS) was performing aerial work over Hangingstone Hill in Dartmoor, Devon. After completing an automated survey mission, the pilot transitioned to manual control to return the aircraft to the Take Off and Landing Site (TOLS).
During this return leg, the pilot observed the aircraft descending. In an attempt to regain altitude, the pilot increased the collective input, but the aircraft failed to climb. When the pilot subsequently reduced the collective and adjusted the cyclic control to decrease forward speed, the UAS experienced a violent upward pitch and fell to the ground from a height of approximately 30 feet. The aircraft sustained damage beyond economical repair.
The investigation
An analysis of the flight data by the manufacturer revealed that three minutes before the conclusion of the flight, the left Electronic Speed Controller (ESC) malfunctioned, causing both the ESC and the associated motor to shut down. This failure was accompanied by a one-second short circuit across the batteries, which dropped the battery voltage below minimum operational levels.
Following the loss of the left motor, the remaining right motor attempted to compensate by increasing its output. This increased the electrical load, further draining the battery voltage. As the batteries reached a critically low energy state, the governor could no longer maintain the necessary rotor speed. Eventually, the voltage dropped to a point where the batteries could no longer support the required power, causing the rotor speed to fall and the aircraft to pitch up uncontrollably.
Findings
- The primary cause of the accident was an ESC failure that resulted in the loss of one motor.
- The remaining motor's increased power demand, coupled with a short circuit, caused a rapid depletion of battery voltage.
- The pilot was flying via visual line of sight and FPV monitors, but the telemetry screen containing critical system alerts was positioned behind the pilot and out of his direct field of view.
- Had the telemetry been monitored, the pilot might have identified the ESC malfunction and the declining battery voltage in time to perform an emergency landing.