NTSB Identification: LAX90LA187.
The docket is stored on NTSB microfiche number 43629.
Accident occurred Wednesday, May 23, 1990 in RIALTO, CA
Probable Cause Approval Date: 11/23/1992
Aircraft: HUGHES 269C, registration: N7441F
Injuries: 1 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

THE HELICOPTER ENCOUNTERED GROUND RESONANCE DURING A MAINTENANCE TEST FLIGHT. THE CFI NOTICED AN UNUSUAL VIBRATION DURING STUDENT TRAINING. THE PILOT SUSPENDED THE TRAINING AND INFORMED MAINTENANCE. THE PILOT THEN ATTEMPTED TO DUPLICATE THE VIBRATION FOR ONE OF THE MAINTENANCE MECHANICS. AFTER TWO UNSUCCESSFUL ATTEMPTS, THE PILOT LANDED THE HELICOPTER ON A SLIGHT SLOPE. AFTER ONE SKID TOUCHED DOWN, THE HELICOPTER BEGAN TO VIBRATE. THE PILOT REDUCED THE THROTTLE AND THE VIBRATION BECAME MORE SEVERE. ACCORDING TO THE PILOT, THE HELICOPTER BEGAN TO DISINTEGRATE.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

IMPROPER MAINTENANCE PROCEDURE BY MAINTENANCE PERSONNEL TO TROUBLESHOOT A REPORTED DEFICIENCY AND THE PILOT'S DECISION TO DEMONSTRATE THE VIBRATION, EXCEEDING THE DESIGN LIMITS OF THE HELICOPTER AND INDUCING GROUND RESONANCE. CONTRIBUTING TO THE ACCIDENT WAS THE LOW NITROGEN CHARGE OF THE LANDING GEAR DAMPERS.

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