On 24 October 2016, the operator (pilot) of a Lockheed Martin Stalker eXtended Endurance (XE) unmanned aerial system, configured in vertical take-off and landing (VTOL) mode, prepared to conduct a demonstration flight at Mount Disappointment, Victoria.

The previous day, the operator had loaded the latest autopilot software onto the aircraft’s main autopilot. One mission goal was to demonstrate the use of a particular radio frequency for the command and control communication link. The ground radio equipment incorporated a narrow beam width directional antenna.

The operator programmed a simple mission: launch segment to a first waypoint and one primary waypoint, which was a coordinate centred on the launch location, then a landing pattern and an alternate landing pattern.

The crew then conducted a physical inspection of the aircraft and ensured all was mechanically correct. All pre-flight checks were also completed normally. The operator then commanded the aircraft to launch. All four vertical lift rotors were energised and the aircraft lifted off the ground. At about 20 ft above ground level (AGL), the aircraft paused in accordance with normal procedures, to conduct an airborne check.

The aircraft then climbed to about 300 ft AGL, which was the programmed transition altitude but did not transition to forward flight. As the aircraft climbed above the ground antenna, it flew into a null in the antenna pattern above the antenna where communication between the GCS and the aircraft was interrupted.

The aircraft hovered, and after a few minutes, the operator commanded ‘abort’ from the GCS. At that time, the operator observed that communication with the aircraft had been lost and repositioned the antenna to point directly at the aircraft. After observing continued lost link indications for about 30 seconds, the operator enabled the alternate radio datalink, restoring communication with the aircraft. However, the previously selected ‘abort’ command was unavailable for re-issue after the initial selection.

About 11 minutes after launch, the aircraft’s power failed, the vertical propellers stopped, and the aircraft pitched about 95 degrees nose down, descended vertically and collided with the ground.

The aircraft sustained substantial damage.

Aviation Short Investigations Bulletin – Issue 59