Event Description
A helicopter sustained substantial damage after encountering a severe vibratory event during a liftoff attempt from a sloped surface. The certified flight instructor (CFI) reported that the helicopter was positioned on a slope. The preflight inspection and engine runup were uneventful. The engine was advanced to 3,100 rpm, and the CFI directed the student to level the downslope skid with the upslope skid. As the collective pitch was raised, the helicopter began to vibrate. The CFI responded by reducing engine power and control pressures. He then increased engine speed back to 3,100 rpm. As soon as control pressures were reapplied, the helicopter began to "self destruct." The uphill skid did not leave the surface during either attempted takeoff.
Examination Findings
No unresolved mechanical discrepancies had been noted with the helicopter prior to the accident. An FAA airworthiness inspector examined the helicopter and found no preimpact discrepancies with the main rotor dampers or the skid strut oleos.
Background Information
The FAA Rotorcraft Flying Handbook (FAA-H-8083-21) discusses ground resonance, a phenomenon associated with fully articulated rotor systems. It states that if a helicopter touches down on one corner, the rotor blades may move out of their normal phase. If the rotor rpm is in the normal operating range, the pilot should fly the helicopter off the ground, allowing the blades to automatically realign. If not, the helicopter may self-destruct in a matter of seconds.