Incident Overview
During a refresher training flight, a helicopter was performing a maneuver to practice procedures for a stuck (fixed) right pedal. The training pilot aligned the helicopter with the runway heading at an airspeed between 55 and 65 knots. The instructor pilot held the right pedal to induce a 10-degree right yaw. Touchdown was estimated to occur at approximately 30–40 knots.
Sequence of Events
A run-on landing with the fixed right pedal was made to the 260-degree taxiway. At or near touchdown, the low rotor rpm horn was on, but the rotor speed was not noted. The collective was positioned at some point above half travel, and the cyclic was slightly forward of neutral. Shortly after touchdown, both pilots felt the main rotor strike the tail boom. Directional control was lost, and the helicopter rotated about 220 degrees nose right before coming to rest upright.
Weather Conditions
The wind reported at the airport about 14 minutes after the incident was 330 degrees at 6 knots. The pilots reported that the wind was 310 degrees at 6 to 8 knots with gusts to 12 knots.
Manufacturer Analysis
The manufacturer indicated that main rotor/tail boom contact from blowback of the main rotor may result from forward velocity and low or decaying main rotor rpm (advance ratio) due to a high collective position during the ground run-out phase following a touchdown above 30 knots. The blowback condition is exacerbated by a high angle pitch setting, which causes blade stall over a large portion of the rotor disk. Blowback is present in all helicopters but may be more pronounced in certain models due to greater gross weight, reduced flare and deceleration capabilities, and increased rotor blade surface area.
Flight Manual Caution
The helicopter's Rotorcraft Flight Manual contains a caution for practice autorotation landings: avoid conditions of rotor rpm (Nr) less than 60 percent with headwinds across the rotor greater than 30 knots during touchdown autorotations. Such conditions during touchdown and the subsequent ground slide can lead to excessive rotor blowback, reduction in blade tip to tail boom clearance, and subsequent damage to the aircraft. To avoid these conditions, the manual recommends reducing collective pitch after touchdown (surface conditions permitting) and minimizing ground run.
Crew Statements
According to the flight crew, there were no mechanical discrepancies with the helicopter. The flight purpose was refresher training, and the specific maneuver at the time of the incident was training in the procedure for a stuck right pedal.