NTSB Identification: FTW95IA397.
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Incident occurred Monday, September 18, 1995 in LAFAYETTE, LA
Probable Cause Approval Date: 04/29/1996
Aircraft: MBB BO-105C, registration: N91070
Injuries: 2 Uninjured.
NTSB investigators used data provided by various sources and may not have traveled in support of this investigation to prepare this aircraft incident report.
While over 'Fast Lane,' a visual check point on the highway approach to the Lafayette Regional Airport, the helicopter 'abruptly and violently pitched nose down.' It took both pilots pulling aft on the cyclic to level the helicopter. Once control was re-established, the PIC saw that the 'hyd block' light was illuminated and the Mast Moment Indicator (MMI) limit light was on steady. The helicopter was landed without further incident. Examination and testing of the Tandem Hydraulic Unit found the System 1 hydraulic pump leaking internally. A nick and an abrasion were found in the edge of its longitudinal control slider. Metallic chips were found in the hydraulic high and low pressure filters, and one chip was found in a groove of the longitudinal control slider. The electrical portion of the 'jam-proof system,' which should activate the back-up hydraulic system two in case of a control slider jamming, only worked randomly. The connector was heavily corroded and the magnetic core of the solenoid valve had an internal short circuit.
The National Transportation Safety Board determines the probable cause(s) of this incident to be: a short in the 'jam proof system' solenoid valve which prevented activation of the backup hydraulic system and resulting in a cyclic hardover. Factors were the contaminated hydraulic fluid and the resulting jammed actuator (longitudinal control slider). Full narrative available
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