Robinson Helicopter Company R44 II N423ML accident near Borrego Springs, California

Probable cause

The loss of the helicopter’s drive system for reasons that could not be determined from the available evidence, which led to an autorotation that resulted in a hard landing and rollover on uneven sandy terrain.

NTSB analysis

The pilot of a Robinson Helicopter Company R44 II reported that, after takeoff, he heard a “light thump” and that the helicopter yawed from left to right. Shortly thereafter, the clutch light illuminated; 5 seconds later, the low rotor rpm light illuminated, and the low rotor warning horn sounded. The pilot performed an autorotation by fully lowering the collective and applying aft cyclic to load the main rotor blade system and maintain the airspeed. During the autorotation, the low rotor rpm horn and light continued, and the clutch light went out. The rotor rpm then increased to 100%, and the warning light and horn extinguished. The pilot continued the landing, however while descending through an altitude of 50 ft agl, the wind switched from a headwind to a tailwind, and the low rotor rpm light and horn annunciated again. Due to the helicopter’s low altitude, he was not able to maneuver back into the wind to land. As the helicopter settled through 5 ft agl, it weathervaned, bringing the nose to the left. The pilot used full collective just before touchdown and the helicopter landed hard on its skids on uneven sandy terrain and came to rest on its right side.

The Robinson Helicopter Company R44 II pilot’s operating handbook stated that a drive system failure would be indicated by the low rotor rpm horn and possibly an unusual noise or a yaw to the right or left, all of which the pilot reported. Thus, the available evidence for this investigation indicated that the likely cause of the accident was a drive system loss of power.

The handbook also stated that, if the clutch light illuminates during flight and does not extinguish within 10 seconds, “land immediately if there are other indications of drive system failure” and advised that, under those circumstances, the drive system should be inspected for a possible malfunction. However, postaccident examination of the helicopter revealed no mechanical malfunctions or anomalies that would have precluded normal operation of the drive system. As a result, the National Transportation Safety Board was unable to determine, from the available evidence, why the drive system loss of power occurred.

Text from the NTSB. Open the NTSB docket for WPR18LA133 for the full record.