UAS Visual Observer reported that after liftoff the UAS began climbing rapidly and was at risk of exceeding 400 ft. AGL. The UAS was landed and it was learned the autonomous flight plan was created using meters instead of feet.

Narrative

While conducting an operational flight with an optical payload; the UAS; VLOS flight; climbed through its maximum planned altitude of 390 ft AGL. The UAS took off normally; from the beach; under the full autonomous mode using Mission Planner. At the transition altitude of 150 ft AGL; over the ocean; the aircraft accelerated into full forward flight. It became apparent the that the aircraft was climbing to an altitude that would exceed 400 ft AGL. The entire flight climb out occurred over the ocean. The pilot manipulating controls used the computer and Mission Planner to direct the aircraft back toward the beach and take off area. A pre-programmed waypoint at an altitude of 150 ft was selected and the aircraft responded. One visual observer crew member stood ready to take manual control as required. This was not required. The aircraft return to the take off point and landed. A debriefing occurred following the flight. The consensus of opinions was that since the controller laptop was operating Mission Planner for the first time it defaulted to metric. The aircrew changed the units to Imperial. The Mission Planner interface showed Imperial Units; however; the aircraft waypoints were loaded in Metric units. In the future while using Mission Planner; if the units of measure are changed then Mission Planner should be shut down after the change. The aircraft should be powered down. Then both Mission Planner and the aircraft should be restarted. This will clear all cached data. These procedures will be added to the checklist. Mission Planner.Aircraft X with DAA (Detect-and-Avoid) system. Control link ZZZ MHz.