N747MK

On July 27, 1995, at 1645 hours Pacific daylight time, a Hughes 269C helicopter, N747MK, encountered ground resonance and rolled over during the postflight cool down period at Redlands Municipal Airport, Redlands, California. The pilot was completing a local visual flight rules personal flight. The helicopter, registered to and operated by M.I. Air Corporation, Redlands, California, sustained substantial damage. The certificated private pilot was not injured; however, his passenger sustained minor injuries. Visual meteorological conditions prevailed. The flight originated at the airport at 1545 hours.

The pilot said in the aircraft accident report that after departing the airport with a passenger, he flew in the local area and returned at 1620 hours. The passenger deplaned and another passenger boarded the helicopter. He departed on a local flight and returned at 1645 hours.

During the landing, he felt the helicopter enter the beginning stages of ground resonance. He immediately lifted the helicopter to a 3-foot hover and the ground resonance dissipated. While in the hover, he decided to do some hover pedal turns. During the turn to the right, the pilot noticed that the winds were gusting erratically and he elected to land.

The touchdown was smooth and the initial cool down period was normal at 2,800 rpm. After the engine cylinder head temperature decreased, the pilot reduced the engine speed to 2,000 rpm. During the power reduction, the helicopter began to vibrate and between 2 and 3 seconds later, ". . . the helicopter was in pieces . . . ."

The passenger repeated the pilot's statement as to the sequence of events. She said that during the cool down period the helicopter began to shake, slowly at first, and then violently. The helicopter "tipped over" and came to rest on its right side.

Investigators sent the helicopter's four landing gear actuators and three elastomeric dampers to Schweizer Aircraft Corporation for functional testing and examination. The testing and examination were done under the supervision of a Federal Aviation Administration principal maintenance inspector from the Rochester [New York] Flight Standards District Office.

The inspector reported that the testing was accomplished on September 8, 1995. He said that the actuators were improperly charged and that the right rear actuator was disassembled. The disassembly examination revealed that a required baffle, P/N 269A3167, was not installed.

The inspector also said that the elastomeric dampers were severely overloaded.

A Schweizer Aircraft Corporation engineer said that the baffle, used to deflect the hydraulic fluid from the compressed gasses, is installed on all actuators when manufactured. He said that the company authorized field maintenance personnel to repair the actuators. He said that the baffle was inadvertently not reinstalled. An actuator without the baffle could cause the piston to fully compress and give erratic damping action. The engineer also said that a low charged actuator would exceed the compression limits and affect the phasing of the main rotor system. This condition would lead to ground resonance.