Incident Overview
An instructor pilot was demonstrating cockpit and runup procedures to a student when the helicopter became light on the skids with operating rotor rpm. Ground resonance subsequently developed.
Cockpit Demonstration
The demonstration involved standard cockpit and runup procedures. As the helicopter became light on the skids, ground resonance began.
Ground Resonance and Response
The instructor bottomed the collective pitch and shut off the engine in an attempt to stop the resonance. However, the ground resonance worsened. According to an FAA handbook (AC 61-13B), the correct procedure for recovering from ground resonance was to lift off.
Consequences
The worsening ground resonance resulted in the tail boom being severed by the main rotor blades. A fuel cell ruptured, and a fire erupted.
Post-Incident Examination
An examination of the helicopter revealed that the left front shock strut was low.