On April 27, 2004, at 1440 Pacific daylight time, N61415, a Schweizer 269C-1 helicopter, encountered a severe vibratory event during the initial run-up at the Van Nuys Airport, Van Nuys, California. Group 3 Aviation was operating the helicopter under the provisions of 14 CFR Part 91. The certified flight instructor (CFI) and the private pilot undergoing instruction (PUI) were not injured; the helicopter sustained substantial damage. Visual meteorological conditions prevailed for the local instructional flight that was originating at the time. The CFI said that during the run-up, the magneto checks produced a rough running engine. At 2,000 rpm he attempted to correct the roughness by leaning the mixture. The helicopter then began shaking from side-to-side, indicating ground resonance. Due to the low rpm, the CFI elected to close the throttle rather than lift the helicopter from the ground. The ground resonance increased and neither the PUI nor the CFI could hold onto the controls. The CFI then pulled the mixture control and the engine stopped. According to the PUI, he was performing the run-up check when the right magneto failed the check twice. The CFI assumed the controls and attempted to troubleshoot the problem. A small vibration developed into a violent bucking. The PUI then assisted the CFI in shutting down the helicopter. A witness submitted a statement. While walking along a fence he heard a loud noise. He turned and saw a helicopter violently shaking fore and aft. The tailboom impacted the ramp, then the blades started to break apart. The landing gear collapsed, and the movement of the helicopter ceased. The skid damper assemblies were examined and tested for charge at a maintenance facility in the presence of the National Transportation Safety Board investigator-in-charge. The landing gear dampers are pressurized with nitrogen gas. Technicians pressurize the dampers in accordance with the current operating temperatures and environments at the time of the maintenance period. The dampers assist in absorbing the vibration consistent with helicopter operations. In post accident tests, the rear dampers had a lower charge than that specified in the maintenance manual for the helicopter. The forward dampers were within limits. The main rotor dampers were examined with no mechanical anomalies noted. The last inspection on the helicopter was a 100-hour inspection approximately 61 hours prior to the accident. In table B-2 of the Schweizer Handbook of Maintenance Instructions, Section 2, page 2-8, item 30 on the 100-hour inspection checklist states the following: "Forward and aft landing gear dampers (LH [left-hand] side and RH [right-hand] side) for operation and conditions, and for damper extension with full fuel tank (Basic HMI, Section 12). Visually check dampers for leakage; replace if leakage is obvious or extension is not within limits (Basic HMI, Section 12). According to FAA publication FAA-H-8083-21, "Rotorcraft Flying Handbook:" "Ground resonance is an aerodynamic phenomenon associated with fully-articulated rotor systems. It develops when rotor blades move out of phase with each other and cause the rotor disc to become unbalanced. This condition can cause a helicopter to self-destruct in a matter of seconds.... If the rpm is low, the corrective action to stop ground resonance is to close the throttle immediately and fully lower the collective to place the blades in low pitch. If the rpm is in the normal range, you should fly the helicopter off the ground, and allow the blades to automatically realign themselves. You can then make a normal touchdown...."