Pitot system blockage results in unmanned aircraft accident

An unmanned aircraft’s collision with terrain highlights the importance of confirming the significance of any unexpected observations during pre-flight checks, an ATSB investigation has detailed.

Insitu ScanEagle X200 unmanned aircraft system (UAS)

The 9 January, 2019 accident occurred after the Insitu ScanEagle X200 unmanned aircraft system (UAS) was launched to conduct ‘beyond visual line of sight’ aerial survey work in the Woleebee Creek area of Queensland. The operating crew consisted of two pilots and two ground crew.

Shortly after launch, the X200 pitched up and then entered an aerodynamic stall. It self-recovered, but the pilots then received an alert indicating an airspeed sensor failure. The X200 continued to fly to its first programmed waypoint, oscillating as it passed out of sight. While the flying pilot was executing the emergency procedures checklist, the X200 entered a second aerodynamic stall. Once again, the UAS self-recovered, but then entered a third aerodynamic stall and collided with terrain. The aircraft was destroyed.

During the pre-flight check, the crew had not identified erroneous airspeed indications...

“A post-accident inspection of the wreckage identified a partial blockage to the aircraft’s pitot system,” said ATSB Director Transport Safety Stuart Macleod. “The pitot system provides data to the autopilot which is used to maintain controlled flight.”

The ATSB investigation found that the blockage in the pitot-static system resulted in the autopilot receiving unreliable airspeed data. During the pre-flight check, the crew had not identified erroneous airspeed indications, and the ground control station had not flagged them. This led to the X200 eventually entering an aerodynamic stall at a height that was insufficient for recovery.

“This occurrence highlights the importance of confirming the significance of any unexpected observations during the pre-flight checks, to minimise the risk of the aircraft departing with an unserviceability,” Mr Macleod said. “In addition, providing pilots and ground crew with the reasoning behind specific checks and procedures can enhance their ability to identify anomalies and perform the appropriate corrective actions in a timely manner.”

As a result of this accident, the manufacturer took several safety actions to prevent a recurrence, including providing support to pilots and ground crew in identifying anomalies, both on the ground and in-flight. All the operator’s pilots underwent refresher training with a focus on identification and emergency procedures regarding incorrect indications.

Additionally, updates to the operational software are intended to ensure that any spurious on-ground anomalies will be accompanied by an alarm, to prevent X200s being launched with any unidentified issues in the future.

Read the report AO-2019-004: Collision with terrain involving an Insitu X200 ScanEagle unmanned aircraft system (UAS) near Woleebee Creek, Queensland, on 9 January 2019

Last update 15 August 2019