Accident Description
A flight instructor reported that a student pilot was practicing a 180-degree autorotation with a power recovery maneuver. As the student began to flare and raise the collective, the helicopter experienced a dramatic descent and shaking, prompting the instructor to take control. After recovering the helicopter, the instructor landed with forward momentum, causing the aircraft to depart the runway to the right. The instructor noted that the landing did not feel very hard. A postflight inspection revealed that a main rotor blade had struck the tail boom, severing the tail rotor driveshaft. The instructor reported no pre-impact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation.
Safety Information from Publications
Several publications address the phenomenon of rotor blowback and tail boom strikes during autorotations. Robinson Helicopter Company's Safety Notice SN-10 (October 1982, revised June 1994) states that failure to maintain rotor RPM is a primary cause of fatal accidents in light helicopters. It emphasizes that pilots must condition their reflexes to add throttle and lower the collective to recover RPM in emergencies. The notice warns that pulling up on the collective during low RPM can cause rotor stall, leading to blade blowback and potential tail boom strike.
Robinson Safety Notice SN-24 (September 1986, revised June 1994) further explains that rotor stall due to low RPM accounts for a high percentage of helicopter accidents. It describes how asymmetrical stall during forward flight tilts the rotor disc aft, causing the blades to blow back and possibly sever the tail boom. The notice notes that rotor teeter stops may not prevent this.
MD Helicopters' Rotorcraft Flight Manual CSP-600RFM-1 (May 1997, revised July 2011) states that main rotor blowback is present in all rotorcraft and is influenced by aircraft gross weight, flare capabilities, and rotor disk surface area. It warns that excessive blowback from ground contact speeds above 30 knots, low rotor RPM, and high collective pitch can cause the main rotor to contact the tail boom.
The FAA Helicopter Flying Handbook (FAA-H-8083-21, 2012) advises against using aft cyclic to stop forward ground run after an autorotation, noting that it can cause the main rotor blades to strike the tail boom. Instead, lowering the collective slightly during the ground run places more weight on the undercarriage to slow the helicopter. The handbook cautions that holding the helicopter off the surface using rotor RPM can lead to a hard landing and blade flex that contacts the tail boom.
The FAA Helicopter Instructor's Handbook (FAA-H-8083-4, 2012) emphasizes proper collective pitch response upon ground contact to avoid tail boom strikes, highlighting the need for correct rate and timing of collective lowering and adherence to minimum rotor RPM and airspeed requirements.