NTSB Factual Report

The flight instructor reported that the student pilot entered a practice 180 degree autorotation with a power recovery maneuver. He reported that as the student pilot started to flare and raise the collective, the helicopter "fell dramatically and began to shake," at which time he took the flight controls. The flight instructor recovered the helicopter and landed. The helicopter landed with forward momentum and departed the runway to the right. He reported that "it did not seem as though the landing was very hard." A postflight inspection revealed that a main rotor blade had struck the tail boom, severing the tail rotor driveshaft.

The flight instructor reported there were no pre-impact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation.

ADDITIONAL INFORMATION

Robinson Helicopter Company published safety notice, SN-10, dated October 1982, and revised in June 1994, addresses a phenomena referred to as the rotor blowback condition.

The safety notice states, in part:

A primary cause of fatal accidents in light helicopters is failure to maintain rotor RPM. To avoid this, every pilot must have his reflexes conditioned so he will instantly add throttle and lower collective to maintain RPM in any emergency.

Additionally, the safety notice states, in part:

If the pilot not only fails to lower collective, but instead pulls up on the collective to keep the ship from going down, the rotor will stall almost immediately. When it stalls, the blades will either "blow back" and cut off the tail cone or it will just stop flying, allowing the helicopter to fall at an extreme rate. In either case, the resulting crash is likely to be fatal. No matter what causes the low rotor RPM, the pilot must first roll on throttle and lower the collective simultaneously to recover RPM before investigating the problem. It must be a conditioned reflex. In forward flight, applying aft cyclic to bleed off airspeed will also help recover lost RPM.

Robinson Helicopter Company published safety notice, SN-24, dated September 1986, and revised in June 1994, addresses the rotor blow back condition.

The safety notice states, in part:

Rotor stall due to low RPM causes a very high percentage of helicopter accidents, both fatal and non-fatal.

Additionally, the safety notice states, in part:

When the rotor stalls, it does not do so symmetrically because any forward airspeed of the helicopter will produce a higher airflow on the advancing blade than on the retreating blade. This causes the retreating blade to stall first, allowing it to dive as it goes aft while the advancing blade is still climbing as it goes forward. The resulting low aft blade and high forward blade become a rapid aft tilting of the rotor disc sometimes called "rotor blow-back." Also, as the helicopter begins to fall, the upward flow of air under the tail surfaces tends to pitch the aircraft nose-down. These two effects, combined with aft cyclic by the pilot attempting to keep the nose from dropping, will frequently allow the rotor blades to blow back and chop off the tail boom as the stalled helicopter falls. Due to the magnitude of the forces involved and the flexibility of the rotor blades, rotor teeter stops will not prevent the boom chop. The resulting boom chop, however, is academic, as the aircraft and its occupants are already doomed by the stalled rotor before the chop occurs.

MD Helicopters published rotorcraft flight manual, CSP-600RFM-1, dated May 1997, and revised in July 2011, addresses the rotor blowback condition.

The rotorcraft flight manual states, in part:

4-9 Practice Autorotation (4-25)

Main rotor blowback is present in all rotorcraft. Factors that influence the extent of its effect on helicopter operations are determined by aircraft gross weight, flare/deceleration capabilities, and the surface area of the rotor disk. The blowback condition describes the forward portion of the rotor disk being displaced upward, while the rear portion of the disk displaces downward. The blowback condition is exacerbated by ground contact speeds in excess of 30 knots (maximum recommended speed for a running landing), low rotor RPM, and high collective angle pitch setting which causes blade stall over a large portion of the rotor disk. If the resulting blowback is excessive, the main rotor may contact the tail boom. The above factors should be taken into consideration during the ground run-out phase following an autorotation or while performing certain types of anti-torque failure procedures where rotor speed decay occurs faster than forward speed decay.

The Federal Aviation Administration (FAA) published the Helicopter Flying Handbook, FAA-H-8083-21, dated 2012, addresses tail boom strikes during autorotations.

The handbook states, in part:

11 Helicopter Emergencies and Hazards (11-4)

A power recovery can be made during training in lieu of a full touchdown landing. Refer to the section on power recovery for the correct technique. After the helicopter has come to a complete stop after touchdown, lower the collective pitch to the full-down position. Do not try to stop the forward ground run with aft cyclic, as the main rotor blades can strike the tail boom. Rather, by lowering the collective slightly during the ground run, more weight is placed on the undercarriage, slowing the helicopter. One common error is holding the helicopter off the surface versus cushioning the helicopter on to the surface during an autorotation. Holding the helicopter in the air by using all of the rotor rpm potential energy usually causes the helicopter to have a hard landing, which results in the blades flexing down and contacting the tail boom. The rotor rpm should be used to cushion the helicopter on to the surface for a controlled, smooth landing instead of allowing the helicopter to drop the last few inches.

The FAA published the Helicopter Instructor's Handbook, FAA-H-8083-4, dated 2012, addresses tail boom strikes during autorotations.

The handbook states, in part:

12 Helicopter Emergencies (12-3)

NOTE: Numerous tail boom strikes have occurred due to improper collective pitch response upon ground contact and completion of the maneuver. Particular attention must be emphasized as to the proper rate and timing of lowering collective to avoid potential damage to aircraft components. Emphasize the minimum requirements for rotor rpm, airspeed, and trim conditions throughout the maneuver.

NTSB Probable Cause

Abnormal ground contact during a 180 degree autorotation, resulting in a severed tail rotor driveshaft by a main rotor blade. A factor contributing to the accident was the rotor blowback condition due to the aft tilting of the main rotor disk.