What happened
Following its launch, the Insitu ScanEagle X2/X200 experienced a sequence of aerodynamic stalls. The aircraft initially underwent two separate stalls from which it was able to self-recover. However, a third stall occurred at a significantly lower altitude. During this final event, the aircraft lacked sufficient height to recover, leading to a collision with the terrain.
The investigation
The investigation focused on the state of the pitot-static system prior to flight and the subsequent impact of erroneous data on the aircraft's autopilot. Investigators examined the pre-flight procedures, noting that three separate opportunities existed to detect anomalies in the pitot-static system. During the pitot-static function test, a slow rise in True Airspeed (TAS) occurred, which was a symptom of a blockage. While some crew members noticed a slow reduction in TAS following the pressure test, the importance of this sign was not understood. Additionally, while the Ground Control Station (GCS) self-checks indicated a pitot system anomaly, the crew did not observe this alert because they were occupied with other tasks. Furthermore, the GCS failed to provide a specific visual flag or alert regarding the irregularity.
Findings
- A blockage in the pitot-static system was present at the time of launch.
- The autopilot relied on TAS and altitude data to maintain flight; because the airspeed data was unreliable, the autopilot incorrectly interpreted the TAS as increasing.
- In response to this false data, the autopilot increased the aircraft's pitch, leading to the first two aerodynamic stalls.
- The flight profile became unstable, characterized by a 'porpoising' motion with TAS oscillations of up to 30 kt.
- The final stall was triggered by a rapid 30 kt increase in the erroneous TAS data, occurring at an altitude too low for the aircraft to recover.