Raytheon COBRA N602RN incident near Whetstone, Arizona

Probable cause

A total off of engine power resulting from an overheated piston, and the pilot's subsequent failure to send a proper command to the UAS. Contributing to the accident was the operation of the engine at high power for an extended period with a lean fuel setting.

NTSB analysis

Note: This case was reclassified from an accident to an incident as a result of applicable revisions to 49 Code of Federal Regulations Part 830.2, as amended at 75 FR 51955, Aug. 24, 2010. The case was previously identified under accident number DEN08FA160.

An unmanned aerial system (UAS) was flying a pre-programmed flight profile to test a mapping camera and telemetry package when it lost engine power. During the subsequent attempt to force land the aircraft on the runway, the UAS entered a commanded right banking turn and the aircraft descended,impacted desert terrain, and was destroyed. The aircraft had flown for approximately 39 minutes. During approximately 30 minutes of the flight, the aircraft's engine was operated at a high constant power setting of 5,300 RPM. An examination of the engine showed that one of the pistons overheated, expanded and rubbed against the cylinder wall, stopping the engine. The needle valves on the carburetor were set at a very lean fuel mixture which would contribute to the damaged piston. An examination of the autopilot software showed that during the attempt to put the vehicle into automatic landing mode, an improper command was sent by the operators.

Text from the NTSB. Open the NTSB docket for DEN08IA160 for the full record.