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ASN Wikibase Occurrence # 35151
Last updated: 21 January 2017
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Type:McDonnell Douglas MD520N
Owner/operator:San Josť Police
Registration: N904PD
C/n / msn: LN032
Fatalities:Fatalities: 2 / Occupants: 2
Other fatalities:0
Airplane damage: Written off (damaged beyond repair)
Location:San Josť, CA -   United States of America
Phase: Approach
Departure airport:Reid Hillview, CA (RHV)
Destination airport:(SJC)
Investigating agency: National Transport Safety Bureau (NTSB) - United States of America
As the NOTAR (No Tail Rotor) helicopter entered a normal descent on downwind for landing, control was lost and the helicopter entered an uncontrollable left spin as it descended to ground impact. A stress corrosion fracture and separation of a fitting in the anti-torque system thruster control cable resulted in a fixed jet thruster nozzle setting and precluded the pilot from controlling the left yawing rotation. RFM (Rotorcraft Flight Manual) procedures provided inadequate information for the pilot to understand the anti-torque system and apply proper corrective action to minimize the effects of the stuck thruster condition. FAA and the manufacturer failed to recognize the implications and significance of a known stress corrosion cracking problem and take appropriate preventative measures in a timely manner. Maintenance diagnostic actions were inadequate to correctly diagnose a yaw control system anomaly reported by the pilot 2 days prior to this flight. The pilot's negative transfer of anti-torque failure procedures from a conventionally designed helicopter precipitated improper pilot control input in response to the fixed jet thruster nozzle condition. CAUSE: The pilot's in-flight loss of control due to the failure and separation of the forward thruster control cable telescoping sleeve ball swivel fitting, which resulted in a stuck thruster and the entry into an uncontrollable yaw/spin. Also causal was the mechanics improper maintenance actions during diagnostics to determine the cause of a yaw control anomaly in that he failed to remove an access panel over the FS113 splitter to fully and completely examine the thruster control cable. Factors in the accident were: (1) the incomplete emergency procedures/system explanations in the RFM for a stuck thruster condition; (2) the pilot's negative transfer of emergency procedures from the HH-60, which likely induced him to make incorrect inputs to throttle, collective, and the anti-torque controls during the onset of the stuck thruster condition; and (3) MDHI and the cable manufacturer's failure to expeditiously diagnose and correct the stress corrosion cracking problem in the forward thruster cable ball swivel fitting.


Revision history:

24-Oct-2008 10:30 ASN archive Added
28-Feb-2013 08:09 TB Updated [Aircraft type, Cn, Operator, Location, Nature, Source]
02-Mar-2013 11:34 TB Updated [Nature]

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