N6960 – ROBINSON HELICOPTER R44 II – 2024-07-13 NTSB Accident Report

On July 13, 2024, about 1035 mountain daylight time, a Robinson Helicopter R-44 II helicopter, N6960, was substantially damaged when it was involved in an accident near Herriman, Utah. The instructor and student pilot were not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight.

The flight instructor reported that he and his student departed from a nearby airport to perform simulated engine out autorotation maneuvers with power recovery. While enroute to the nearby practice area, the instructor discussed, then demonstrated the maneuver with the student shadowing the flight controls. The instructor lowered the collective, applied aft cyclic, right pedal, and reduced the throttle to idle.

As the instructor scanned the instrument gauges, he saw that the engine oil warning light, auxiliary fuel pump caution light, and alternator caution light were illuminated. The instructor attempted to increase engine power, realized the engine had lost total power, and continued the autorotation to an open field. The instructor stated that, as the helicopter settled to the ground, it touched down hard with minimal forward airspeed in a skids-level attitude before it came to an abrupt stop. The main rotor blades subsequently struck the tailboom, and the tail rotor assembly separated. The helicopter came to rest upright. A review of the cockpit video showed that the engine rpm started at 100% before the maneuver. When the instructor reduced the engine power to idle, the engine rpm continued to decrease past idle and continued below 46%.

Examination of the airframe revealed that the skids were slightly displaced, consistent with the hard landing. Flight control continuity from the cockpit to all flight controls was confirmed. The transmission and the overrunning clutch were manually rotated without anomalies. Fuel system continuity was confirmed. Fuel was found in the tanks and within the gascolator.

The engine remained attached to the airframe via the engine mounts. The accessories and accessory cases were unremarkable. No other discernible damage to the engine was noted. An engine test run revealed that the engine throttle needed to be slightly advanced to start the engine and that the electric boost pump had to be set to ON at idle power, or the engine would immediately lose power. According to the engine manufacturer’s troubleshooting guide, this is consistent with an idle mixture set extremely lean. Follow-up examinations and bench testing of the mechanical fuel pump and fuel injection servo revealed no anomalies.

The pilot reported that the helicopter had about a total of 60 airframe hours, and he noticed the helicopter had an engine idle of about 50-52% rpm. He added that he flew the accident helicopter multiple times before the accident flight and did not have any power loss issues or anomalies. The instructor added that he did not report the idle level to maintenance because he did not know the idle setting could be adjusted. A review of the engine logbook entries did not reveal any documentation of idle adjustment after the helicopter was received from the factory or during its most recent annual inspection. According to the mechanic who completed the most recent annual inspection, the engine idle was verified to be within limits, but he could not recall the exact rpm value.

According to the helicopter’s POH, under a NOTE in the Starting Engine and Run-Up procedures, “Idle mixture and speed may require adjustment as conditions vary from sea level standard. Refer to R44 Maintenance Manual for idle adjustment procedures.” The helicopter maintenance manual further stated that an in-service throttle correlation rigging check required an IO-540 engine idle to be adjusted and set to 58% to 62% rpm.

The helicopter manufacturer added that the helicopter rotor system cannot drive a stalled or stumbling engine due to a lighter flywheel, and in combination with the drag of the cooling fan, the engine is likely to stall completely if the engine stumbles at idle. The POH stated that, during a practice autorotation, throttle should be reduced “as desired for tachometer needle separation.” The procedure contained a “Caution” note that stated:

To avoid inadvertent engine stoppage, do not chop throttle to simulate a power failure. Always roll throttle off smoothly. Recover immediately if engine is rough or engine RPM continues to drop.

The manufacturer’s flight safety course recommends that the throttle be reduced just below 80% during a practice autorotation so the governor is deactivated, and to avoid reducing the engine to idle speeds.