Robinson Helicopter Company R44 II N544BS accident near Nashville, Tennessee

Probable cause

The pilot’s failure to maintain rotor rpm during an autorotation after a perceived partial loss of power, which resulted in a hard landing. Contributing to the accident was radio frequency interference with the governor controller, which resulted in an uncommanded decrease in throttle and the perceived loss of engine power.

NTSB analysis

The pilot took off from his departure airport on a cross-country helicopter flight. The pilot reported that, when the helicopter was about 1 mile north of the airport and near a group of high-powered transmission broadcast towers, the engine started running roughly and that he felt an uncommanded decrease in engine power and movement of the twist grip throttle. The pilot initiated an autorotation, and the helicopter landed hard, which resulted in substantial damage to the tailboom. Postaccident examination of the engine revealed no mechanical anomalies that could have precluded normal operation.

According to a flight instructor based at the departure airport, he had encountered a similar uncommanded decrease in engine power on multiple occasions while flying east of the same broadcast towers in the same make and model helicopter. During each encounter, the pilot felt the movement of the electronic governor through the throttle twist grip and was able to override the governor by advancing the throttle. Further, Federal Aviation Administration research revealed that high-power broadcast AM radio transmitters likely affect the governor circuit on this helicopter and can result in governor control that reduces the engine speed regardless of the pilot's power input. Thus, it is likely that radio frequency interference increased the engine speed signal frequency as the pilot flew the helicopter near the transmission towers. The governor controller then engaged the governor motor to decrease engine power, which the pilot perceived as a loss of power.

About 15 years before the accident, the manufacturer had issued a safety notice to address previous reports of radio interference. The safety notice stated that flying near broadcast towers may result in "erratic operation of the governor," which may cause the governor to roll the throttle to the idle or open position. The notice also instructed pilots to keep one hand on the collective and throttle and to refrain from flying near broadcast towers. The pilot reported that he was familiar with the safety notice but that he had not read it since he purchased the helicopter about 7 years before the accident. The pilot should have been able to recognize that the engine was still producing power and overridden the governor by moving the twist grip throttle, and he should have been able to maintain rotor rpm while executing the autorotation, which would have resulted in a softer landing; however, he did not do so.

Text from the NTSB. Open the NTSB docket for ERA14LA311 for the full record.