In a development that could affect drone operations in the UK, the Air Accidents Investigation Branch (AAIB) has recommended that the local civil aviation authority revises its visual line of sight rules (VLOS), following an accident in which a police drone crashed into a house in Poole two years ago.

In making this recommendation, the AAIB is of the view that it is not enough for a drone pilot to just keep the unmanned aerial vehicle (UAV) within his visual line of sight; and has therefore called for the CAA to additionally require pilots “to be able to demonstrate that at the distance they are flying, they can manoeuvre (the drone) rapidly to avoid a collision and can also land the unmanned aircraft safely following a loss of position-holding without reference to video or telemetry.”

The recommendation was made in a report released on Thursday, following a two-year investigation, which also concluded that a battery connection problem was one of the causes of the accident.

The investigation wanted to know how the DJI Matrice 210 quadcopter belonging to Dorset Police suddenly stopped following commands and flew several metres beyond its pilot’s visual line of sight before crashing onto the first-floor balcony of the house.

No one was injured in the crash, but the investigators expressed worry that there might have been casualties had anybody been on the balcony when the drone went down.

The AAIB made three safety recommendations to the drone’s manufacturer, DJI, and one to the Civil Aviation Authority over flying guidance.

“The DJI Matrice M210 … was being used for a police operation over the city of Poole,” the report said. “The remote pilot was working with an observer who had a slave controller.

“At 11:08 hrs (on November 19, 2020) the remote pilot obtained a wind forecast at 400 ft of 24 mph from the north-west using a UAS weather forecast app. At 11:17 hrs, a flight towards the south-west was carried out with no issues.

The two batteries were replaced and then at 11:45 hrs the Unmanned Aircraft (UA) took off again. Standard control checks were carried out at a height of 10 m before climbing to 120 m (400 ft) and flying south-east towards a target location that was 500 m away. The remote pilot reported that he maintained a good visual sight of the UA and referred to his controller for flight and aircraft information.”

Sadly, at this time, the drone itself was about to run into problems of its own. At 120m in the air, it encountered winds stronger than it could withstand – and there was something wrong with the battery set-up.

“(The pilot) then noticed two messages on the controller screen: one stating ‘Battery communication error’ and then another stating ‘Fly with caution, strong wind’. He noted that one of the batteries was showing 97 percent state of charge (SOC) while the other battery SOC was decreasing faster than normal. The pilot tried to fly the aircraft back towards him, but it did not appear to be moving any closer.”

Meanwhile, one of the two batteries continues to show a rapid power drainage; it was at 58 percent capacity now, while the other stayed static at 97 percent.

And still, the drone would not respond to the command for it to return; worse it was moving past the 500m that had been set on the flight app as the distance to maintain visual contact with the pilot.

“The pilot was now very concerned and activated the ‘Return to home (RTH)’ feature on the controller, but it did not appear to engage despite being operated multiple times. RTH then appeared to activate but the aircraft did not move any closer. The pilot then switched to ‘Sport Mode’ as per their emergency procedure which he expected would give him a top speed of 51mph, allowing a greater ability to overcome the headwind.

“This cancelled the RTH feature so he pressed RTH again, but it would not re-engage.

“The remote pilot asked the observer to try engaging it using his slave controller, but this did not work either.

By this time, neither the pilot nor the observer could physically see the aircraft, their only contact with it being the dot on the map that showed that their drone was still in the air, heading slowly towards Poole Park boating lake.

Then all of a sudden, all communication was lost.

“The pilot and observer packed their kit and drove to the last location of the aircraft shown on the map display. When they arrived in the area of the last position, the controllers regained communication with the aircraft and displayed its GPS coordinates.

“They found the aircraft on a first-floor balcony of a house. There was no one at the front of the property but there was light foot traffic along the path by the lake, about 30m from the aircraft’s location. The aircraft’s right leg had snapped at the mounting bracket, three propeller blades had shattered, and one propeller had detached but was located next to the aircraft.

The investigation revealed that shortly after take-off one of the UA’s two batteries had disconnected which resulted in its maximum speed being restricted.

It also reached the following conclusions regarding what happened.

• Battery 1 became disconnected shortly after take-off which reduced the UA’s maximum pitch attitude and maximum speed.
• The pilot did not notice that the ‘battery communication’ message included the words ‘land as soon as possible’.
• When the wind measured by the UA exceeded the manufacturer’s wind limit the alert message to the pilot advised him to ‘fly with caution’ instead of to ‘land as soon as possible’.
• The wind at 400 ft was stronger than forecast and at times above the UA’s restricted maximum speed so the pilot could not fly it back towards him.
• The wind speed calculated by the UA was not displayed to the pilot on his controller app so he did not know that the wind limit had been exceeded.
• After communication was lost, the UA entered an auto-land mode but it was unable to avoid colliding with a wall.

Investigators were also worried that the drone’s response to encountering high winds at altitude was to warn the pilot to fly with caution, instead of advising him to land the drones as soon as possible, since the limit had been exceeded.

In this light, the AAIB made some recommendations to DJI:

• It is recommended that DJI amend the DJI Pilot and DJI GO4 apps to warn the remote pilot when the wind limit has been exceeded and that the UA should be landed as soon as possible.
• It is recommended that DJI amend the DJI Pilot and DJI GO4 apps so that an aural alert is triggered when alert messages relating to safety of flight appear.
• It is recommended that DJI amend the Matrice 200 series user manual to provide information on the pitch attitude limiting system, including the new maximum speed which results from the limit, and the battery level at which it triggers; and communicate this change widely to pilots and operators.