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ASN Wikibase Occurrence # 35151
Last updated: 24 June 2018
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Type:McDonnell Douglas MD520N
Owner/operator:City Of San Jose Police Dept.
Registration: N904PD
C/n / msn: LN-032
Fatalities:Fatalities: 2 / Occupants: 2
Other fatalities:0
Aircraft damage: Written off (damaged beyond repair)
Location:San Josť, CA -   United States of America
Phase: Approach
Departure airport:Reid Hillview, CA (RHV)
Destination airport:
Investigating agency: NTSB
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. A compilation of ground witness observations revealed that the helicopter began a yaw to the right from normal straight flight, then suddenly reversed direction and entered into a rapid left rotation as it descended to the ground. Radar data showed the flight was uneventful until 11 seconds after completing a left turn to a downwind heading and beginning a normal descent for landing. At this point, a spike was observed in the Mode C altitude readout, indicating that a large sideslip angle had occurred inducing a static system pressure anomaly, and, probably represents the ground witness observed initial yaw to the right. The helicopter's airspeed profile until the onset of the right yaw was normal and within the expected cruise range for this point in the flight. The altitude spike was also coincident with the pilot's first mayday call to the tower controller and strongly indicates that the emergency situation had evolved to a state which alarmed the pilot. The helicopter's NOTAR anti-torque yaw control system utilizes a transmission driven fan with variable pitch blades to supply air to circulation control slots on the tail boom and a pilot controlled directional jet thruster nozzle. The yaw control system has sufficient authority to induce large and prolonged sideslip angles at cruise flight airspeeds. Pilot control of the jet thruster nozzle uses torque tubes from the cockpit anti-torque pedals to a splitter assembly at Fuselage Station (FS)113. Torque tubes transmit pedal movement from the splitter to the fan blade pitch change mechanism to increase airflow in the tail boom as the pedals are displaced from neutral in either direction. A three-part cable transmits motion from the FS113 splitter to the jet thruster nozzle to control its directional orientation. The forward and center cables have a Teflon-coated inner steel control wire which slides back and forth in an outer sleeve. At the FS113 splitter, a telescoping sleeve ball swivel coupling retained by a swaged lip allows for angular displacement of the cable rod end as the splitter assembly rotates. During postaccident examination of the NOTAR jet thruster control cable, a fracture and separation was found in the cable's telescoping sleeve ball swivel coupling swaged lip, which was subsequently identified as caused by stress corrosion. With the retaining lip missing, the ball swivel coupling is not restrained and will allow the inner wire to slide out of the outer sleeve. As the cable moves out of the sleeve (left cockpit control pedal movement), the exposed cable will bow and not transmit any subsequent right pedal movement back to the jet thruster nozzle to counter left yaw. Metallurgical examination found that the fitting's failure and separation preceded this flight by some length of time. The Teflon coating of the inner cable was severely abraded, indicating that it had been operating

Probable 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]
14-Dec-2017 09:41 ASN Update Bot Updated [Time, Cn, Operator, Nature, Destination airport, Source, Narrative]

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