NTSB Factual Report

On May 15, 2006, approximately 1535 mountain daylight time, a Schweizer 269C helicopter, N2096W, sustained substantial damage following a hard landing while performing an autorotation near Boise, Idaho. The certified helicopter instructor pilot and his student were not injured. The helicopter was registered to and operated by Aviation Specialties Limited of Boise. Visual meteorological conditions prevailed for the local instructional flight, which was operated in accordance with 14 CFR Part 91, and a flight plan was not filed. The helicopter departed from the Boise Air Terminal/Gowen Field (BOI), about 1415.

According to the Pilot/Operator Aircraft Accident/Incident Report (NTSB Form 6120.1) submitted to the NTSB investigator-in-charge (IIC), the instructor pilot reported that approximately 1.3 hours into the flight, at an altitude of 3,900 feet mean sea level (msl) or 1,000 feet above ground level (agl), the instructor pilot, who was occupying the left seat and in control of the helicopter, instructed the student, who occupied the right seat, to reduce the throttle to idle to simulate an engine failure at altitude. The instructor pilot stated that after the throttle was reduced to idle he entered an autorotation, maneuvered the helicopter into the wind, and then noticed the engine had failed. The instructor pilot reported that he decided not to attempt to do an engine restart in flight, and the autorotation was completed to the ground into the wind to a grass field with a slight up slope. The instructor pilot further reported that after the helicopter's skids impacted terrain, it slid backwards approximately 15 feet before the tail boom flexed up and was impacted by the main rotor. The hard landing resulted in substantial damage to the tail boom, all three main rotor blades, and the tail rotor drive shaft. There was no post accident fire. The aircraft was subsequently recovered to a secured area at the facilities of Aviation Specialties Limited, where a further examination by representatives of the Federal Aviation Administration (FAA) would be conducted.

On May 18, 2006, two FAA aviation safety inspectors from the Boise Flight Standards District Office, Boise, Idaho, participated in an engine run. The inspection revealed no anomalies which would have precluded normal operation of the engine. The inspectors reported that while the idle rpm and idle mixture were set for winter temperatures, and that while being adjusted poorly for the altitude and temperature on the day of the flight, the failure of the instructor pilot to follow the cautions in the helicopter's flight manual resulted in the loss of power. (Refer to attached inspector's statement)

According to the Schweizer Aircraft Corporation's Model 269C Helicopter Pilot's Flight Manual, page 4-23, #4-16, ENGINE IDLE AT ALTITUDE, revised 15 June 1994, this section states:

Engine idle speeds at high density altitude may be less than those set at sea level conditions.

WARNING: AVOID THROTTLE CHOPS TO FULL IDLE AT ALTITUDES ABOVE 7000 FEET, TO AVOID POSSIBILITY OF ENGINE STOPPAGE. (Refer to attached Schweizer Aircraft Corporation Pilot's Flight Manual, Normal Procedures, page 4-23)

At 1524, the weather reporting facility at BOI, reported wind 140 degrees at 13 knots, visibility 10 statute miles, sky clear, temperature 33 degrees C, and a density altitude of 5,500 feet.

NTSB Probable Cause

The reduction of the throttle to idle at a high density altitude during a practice autorotation, which resulted in the engine stoppage. Factors contributing to the accident included the instructor pilot's improper planning by not adhering to the flight manual's warning to avoid reducing the throttle to the idle position at high density altitudes, and the uneven terrain.