Drone Manufacturer: Velos
Drone Model: Helicopter
Country: United Kingdom of Great Britain and Northern Ireland
Pilot Qualifications: Approved by Public Safety Agency or Company
Pilot Flight Experience: 533 Hours
Link to External Information About This submission: https://www.gov.uk/aaib-reports/aaib-investigation-to-velos-single-rotor-uas-registration-n-a-180919
File Uploaded: velos_single_rotor__uas_registration_na__0420
The single rotor UAS was being flown back to its landing site when it pitched up, uncommanded, and lost control. Assessment of the UAS flight data identified that an ESC fault caused one of the UAS’s motors to run down. The increased load required to maintain the aircraft in flight depleted the battery power rapidly, resulting in the loss of control. The telemetry screen, that would have alerted the pilot of the ESC failure, was not in the pilot’s field of view.
History of the flight
The single rotor UAS had completed a 13-14 minute automated survey flight in an area of the Dartmoor national park. Once the survey had been completed the pilot took control of the UAS and flew it back towards the Take Off and Landing Site (TOLS). During the return flight the pilot noticed that the aircraft was slowly descending, so he increased the collective input in an attempt to climb back to the original height. This did not have an effect, so he gently reduced the collective input and pulled back on the cyclic control to reduce forward speed. As he did so the UAS pitched up violently and then fell to the ground from approximately 30 ft agl.
The Velos single rotor UAS helicopter (Figure 1) is capable of carrying varied deployable and fixed payloads such as survey equipment and cameras.
Two individual Electronic Speed Controller (ESC) controlled motors combine to provide power to a 1,950 mm diameter main rotor. In the event of a single ESC or motor failure, the UAS can fly using the remaining functional ESC and motor. The controls are the same as a conventional helicopter with a collective control which changes the pitch of all the main rotor blades simultaneously to control vertical movement, and a cyclic control which controls the pitch of individual blades to provide pitch and roll control, a yaw control provides directional control through a variable pitch tail rotor. The total mass of the UAS involved in this accident was 19.8 kg.
The pilot was operating the UAS by visual line of sight, supplemented by two First Person View (FPV) cameras on the aircraft which transmitted forward and rearward video streams to two monitors positioned in front of the pilot. Another monitor, which displayed the aircraft telemetry, was positioned behind the pilot and was not directly in his line of sight.
Recorded data from the accident flight was downloaded and interpreted by the UAS manufacturer. The manufacturer determined that the UAS was returning to the TOLS after completing its desired activity when the accident occurred. It was toward the end of the flight and the batteries were in a low energy state.
The data showed that three minutes prior to the end of the flight the left ECS malfunctioned. This resulted in it and the left motor shutting down, for the remainder of the flight. As the malfunction occurred, a short circuit was recorded across the batteries. This lasted for approximately one second and reduced the battery voltage to below their minimum operational level.
After the loss of the left motor, the right motor was commanded to increase its output. This increased the right motor current load, which in turn reduced the battery voltage further1 . To maintain the main rotor speed the governor started to compensate for the lowering voltage by increasing the current demand.
Approximately one minute before the loss of control, the right governor was demanding 100% power to maintain the required rotor speed, however the available voltage diminished to a level where the batteries no longer had sufficient charge to maintain it. The rotor speed then dropped below that able to sustain flight, resulting in the aircraft pitching up.
It was not possible to determine the cause of the ESC failure; however, it should have been possible for the UAS to land away from the TOLS in the event of a single ESC failure. The pilot flying the UAS was operating alone in visual line of sight, supplemented by using FPV and was not monitoring the telemetry which was shown on a separate screen positioned behind him as he flew the aircraft. Had he been monitoring the telemetry screen he may have noticed cautions regarding the ESC failure and the rapidly diminishing battery charge which should have prompted the pilot to land the UAS immediately. The position of the screens had been optimised for the planned mission but did not allow easy viewing of vital information which could have prevented the accident.