Incident Summary
A flight instructor (CFI) overseeing a training session directed a student to perform a full touchdown autorotation. The student began the descent, but during the maneuver allowed the main rotor rpm to decay below the desired range.
Instructor's Actions
The CFI realized that the intended touchdown point was farther than the aircraft's current glide path could achieve. To overcome this discrepancy, the instructor chose to increase collective pitch rather than add engine power, an action intended to stretch the glide distance.
Outcome
The flare, which is the final pitch-up movement before landing, was delayed. When the helicopter contacted the ground, the main rotor blades struck the tail boom. The source does not specify the aircraft type, registration, location, date, or any injuries or damage details.