VAN NUYS, CA, USA
N8873F
HUGHES 269A
THE PILOT ENCOUNTERED GROUND RESONANCE AFTER LANDING. THE HELICOPTER HAD TOUCHED DOWN LIGHTLY ON ITS LANDING GEAR SYSTEM AND BEGAN TO OSCILLATE FROM SIDE TO SIDE. THE HELICOPTER CONTINUED TO OSCILLATE UNTIL ONE SIDE OF THE HELICOPTER LANDING GEAR STRUCTURE FAILED. THE HELICOPTER THEN ROLLED ON ITS SIDE. THE FOUR LANDING GEAR DAMPERS AND TWO MAIN ROTOR DAMPERS WERE FOUND OUT OF LIMITS. THE ACCIDENT FLIGHT WAS THE FIRST FLIGHT AFTER A 50-HOUR INSPECTION WAS PERFORMED. DURING THE INSPECTION, THE LANDING GEAR DAMPERS SHOULD HAVE BEEN CHECKED FOR PROPER EXTENSION. THE LANDING GEAR AND MAIN ROTOR DAMPERS SHOULD ALSO BE CHECKED BY THE PILOT DURING HIS PREFLIGHT PREPARATION OF THE HELICOPTER.
On March 16, 1995, at 1057 hours Pacific standard time, a Hughes 269A, N8873F, encountered ground resonance after landing at the Van Nuys Airport, Van Nuys, California. The certificated private pilot received minor injuries. The helicopter was destroyed. The helicopter was being operated by the pilot/owner as a solo instructional flight. Visual meteorological conditions prevailed. Witnesses reported to Federal Aviation Administration (FAA) inspectors that the helicopter had touched down lightly on its landing gear system and began to oscillate from side to side. The helicopter continued to oscillate until one side of the helicopter landing gear structure collapsed. The helicopter then rolled on its side. According to the FAA, the pilot holds a private pilot certificate with ratings for single- and multiengine airplanes. The pilot was pursuing a helicopter rating and was conducting a solo flight under the supervision of a certified flight instructor. The pilot had accrued about 60 hours of flight experience in helicopters, of which 1.7 hours were on the accident flight. The four landing gear dampers and three main rotor dampers were sent to manufacturer facilities for testing. According to the manufacturer, the four landing gear dampers and two of the three main rotor dampers were found out of limits. A copy of the manufacturer's report is attached as part of this report. A 50-hour inspection was accomplished by an airframe and powerplant mechanic on March 3, 1995, about 1.7 flight hours before the accident. The handbook of maintenance instructions for the helicopter details specific inspection items during the 50-hour inspection. The first item on the inspection checklist indicates a daily inspection should be performed as part of the 50-hour inspection. During the daily inspection, the landing gear dampers are checked for proper extension. The Handbook of Maintenance Instructions instructs maintenance personnel to check and service landing gear dampers during a daily inspection. The handbook also states, "ground resonance can result if helicopter is operated when the damper extension, type of oil, and/or oil-to-air ratio in the damper is incorrect." The landing gear and main rotor dampers are also part of the pilot preflight inspection of the helicopter.
the operation of the helicopter with improper adjustment of the landing gear and main rotor dampers due to the inadequate maintenance inspection of the helicopter by maintenance personnel and an inadequate preflight inspection by the pilot.
Source: NTSB Aviation Accident Database
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