Loss of power and control due to electronic speed controller failure, Temple Newsam, Leeds, Yorkshire, 16 March 2019.
The DJI Matrice 210 small unmanned aircraft was being operated commercially to record video footage of an outdoor athletics event. The pilot started to position the aircraft back towards the landing site due to an increase in the rainfall. The pilot then saw the aircraft “wobble” slightly and as it neared the landing site it flipped over before descending rapidly to the ground from a height of about 3 m (10 ft). No one was injured. During the accident flight the aircraft had been operating at heights of up to about 30 m (100 ft) near to, and above people on the ground. This investigation reviewed other similar accidents and the risk of injury to people on the ground. Two safety recommendations are made to the UK CAA.
Sixteen Matrice 200 incidents have been reported to the AAIB between December 2017 and July 2019
Failure of DJI Matrice 210 SUA at Temple Newsam
The manufacturer stated that the accident had been caused by the failure of the No 4 ESC, but no information was made available as to why this component had failed.
When the No 4 ESC had failed, a No 3 ESC ‘link down’ message was also recorded. The manufacturer did not confirm what this message meant, or if this ESC had also failed.
Subsequent inspection of the aircraft found that moisture had entered the No 3 ESC chamber and residue was present on the ESC circuit board. No evidence of moisture was found on the other ESC’s.
Since owning the aircraft from new, the pilot had followed the manufacturer’s guidelines to prevent moisture entering the aircraft and during the accident, the No 3 motor arm had remained intact. It is, therefore, more likely that the moisture entered the No 3 ESC chamber during the last three minutes of the accident flight when the rainfall increased rather than during the previous operation or following the accident.
The lower section of the ESC chambers adjacent to the antennas was not sealed and this could have provided an entry path for rainwater. The IP 43 accreditation testing was also not performed with the propellers installed and so it is possible that these tests did not fully reflect in-service operation in rain. It is also possible that rainwater entered the unit due to rainfall exceeding the manufacturer’s limitation of 10 mm/h or that the IP 43 protection had degraded over time. Although it was not confirmed if moisture ingress caused this accident, information shows that other Matrice 200 series accidents have been caused by moisture entering the aircraft. The manufacturer’s analysis also showed that 27% of accidents were attributed to a loss of propeller motor propulsion that were for reasons other than a fault with the ESC. The manufacturer did not provide guidance on ascertaining if the rainfall exceeded limitations, or the duration that the IP 43 protection may remain in place. It is therefore possible that pilots of the Matrice 200 series could operate the aircraft in rain without knowing that it could result in the loss of control of the aircraft due to moisture ingress.
The following Safety Recommendation is therefore made:
Safety Recommendation 2020-001
It is recommended that the Civil Aviation Authority notify users of the DJI Matrice 200 series of the possibility of moisture entering the aircraft when operating in rain and that this could result in a sudden loss of control of the aircraft.
Operation of UAS over persons
The ANO allows a UA with a mass of up to 20 kg to be flown over assemblies of up to 1,000 people as long as the aircraft maintains a height of at least 50 m.
UA’s such as the DJI Matrice 200 series rely upon their propulsion system for lift. If propulsion is lost, aircraft of this type typically fall vertically to the ground. From a height of 50 m, the descent would take about three seconds, which would provide limited time for a pilot to warn people and for them to take avoiding action if possible.
The risk of injury to a person on the ground if struck by a falling UA is high, with the DROPS analysis indicating that a UA with a mass of only a few kg falling from a height of just several metres could result in a serious or even fatal injury.
In accordance with the ANO, a person must not ‘permit an aircraft to endanger any person’ and may only fly the aircraft ‘if reasonably satisfied that the flight can safely be made’.
It is therefore up to the operator or remote pilot to decide if flying a UA over people will endanger them.
However, there is no guidance available from the CAA on how to make that assessment. This could include consideration of standards of safety, reliability, UA mass and type, the operational environment and whether any secondary safety systems are fitted. IR (EU) 2019/947 is due to come into force in July 2020 and will require that a UA operating in the open category with a mass of more than 250 grams must not be flown over ‘uninvolved persons’.
If operating a UA in the specific category, the operator will need to comply with mitigating safety actions to prevent injury to people. However, these actions are not due to be published by the EC until 2021 which leaves an unresolved hazard prior to publication.
The following Safety Recommendation is therefore made:
Safety Recommendation 2020-002
It is recommended that the Civil Aviation Authority specify the conditions that must be met for an unmanned aircraft to be flown safely over people.
The DJI Matrice 210 crashed whilst operating in rain.
The manufacturer stated that the accident had been caused by a fault with the No 4 motor ESC. Moisture was also found on the No 3 motor ESC, but it was not established if this contributed to the accident.
However, information showed that other Matrice 200 series of accidents involving aircraft operating in rain had been caused by ESC failures due to moisture ingress.
Other types of DJI Matrice 200 series failures included contamination of the motors by fine particles that the IP43 rating did not provide protection against. Failures of the Matrice 200 series aircraft resulted in a loss of power and control, with the aircraft typically falling vertically to the ground.
This poses a risk of injury to people on the ground which is not mitigated by the current UK regulations or the published guidance material. To address this, two Safety Recommendations have been made to the CAA.