What happened
A student pilot and flight instructor were conducting training maneuvers involving autorotations with power recovery in a Schweitzer 269C-1. During the maneuver, the helicopter struck the runway with significant force, causing the left skid to collapse and resulting in the aircraft rolling over onto its left side.
The student pilot reported that the main rotor RPM was increasing and attempted to raise the collective. According to the student, the control did not move initially; when it did move, it traveled to the full up position, causing the main rotor blades to stall and leading to the hard impact. The flight instructor stated that after the student raised the collective, he immediately lowered it and added power, but was unable to prevent the hard landing.
The investigation
Investigators examined the flight controls and found that the collective and main rotor swash plate operated free and easy, with no evidence of jamming or mechanical resistance. No loose items were discovered within the helicopter cockpit that could have restricted the movement of the collective lever.
An engineering test pilot from Schweitzer performed a test demonstrating that a momentary pop-up of the collective to the full up position and back down in less than one second would not be sufficient to cause a blade stall. The test indicated that a blade stall would require the collective to be held at the full up position for at least three seconds.