NTSB Factual Report

On September 25, 2004, at 1549 Pacific daylight time, a Schweizer 269C-1 helicopter, N61413, encountered a severe vibratory event after landing at the Bob Hope Airport, Burbank, California. Group 3 Aviation was operating the helicopter under the provisions of 14 CFR Part 91. The certified flight instructor (CFI) and private pilot undergoing instruction (PUI) were not injured; the helicopter sustained substantial damage. Visual meteorological conditions prevailed for the local instructional flight, and no flight plan had been filed. The helicopter departed from the Van Nuys Airport, Van Nuys, California, about 1515.

According to the operator, the PUI was taking refresher training in the helicopter. The PUI performed a normal approach; however, the touchdown was harder than normal. With the helicopter situated on the ground, the CFI began to discuss the touchdown with the PUI, and the helicopter started to vibrate. The CFI said he did not have time to react prior to the helicopter self-destructing.

The three main rotor dampers and four landing gear dampers were examined at the Schweizer Aircraft Corporation on October 12, 2004, in the presence of a Federal Aviation Administration (FAA) inspector from the Rochester, New York, Flight Standards District Office. The results of the examination noted that the damage to all of the dampers was consistent with a ground resonance event. The two aft landing gear dampers tested slightly low but near minimum operating condition. The two forward landing gear dampers contained fluid in excess of the specified levels. When examined, the pistons were in the fully extended condition, and the fluid level was 1.025 inches when measured from the top of the piston to the fluid. The dampers were tested and they were out of compliance with design data, as well as the Schweizer Handbook of Maintenance Instructions (HMI) and production assembly instructions. It was noted in the report that dampers in this condition will not compress or attenuate airframe motion and vibrations or prevent ground resonance.

The last inspection on the helicopter was the 100-hour completed 4.5 hours prior to the accident flight. The total time on the airframe was 6,298.7 hours. The landing gear dampers were serviced on January 12, 2004, at a total airframe time of 5,750.1 hours.

In the Schweizer HMI, Part III, Section 5, the procedures for reassembly, charging, and leakage check on the landing gear damper assembly are discussed. On page 5-2 it noted that the fluid level for the 269A3150-19 dampers (the forward dampers) should reach 0.5-inch below the top of the piston. The fluid level for the 269A3150-21 dampers (the rear dampers) should be 1.40 inches below the top of the piston. When the fluid is added, the piston is to be compressed, not extended. A warning, also on page 5-2, states the following:

"Incorrect fluid levels, improper pressure, or inoperable valving will deteriorate the damping capabilities of the landing gear dampers. These conditions may result in ground resonance and destruction of the helicopter. Follow all instructions in the HMI carefully, to ensure safe helicopter operation."

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...."

NTSB Probable Cause

the failure of maintenance personnel to correctly service the forward landing gear strut dampers in accordance with the manufacturer's maintenance instructions.