NTSB Identification: LAX00GA025.
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Accident occurred Monday, October 25, 1999 in SAN JOSE, CA
Probable Cause Approval Date: 03/30/2004
Aircraft: McDonnell Douglas 500N, registration: N904PD
Injuries: 2 Fatal.
: NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this public aircraft accident report.
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 out of its sleeve over the complete range of fore and aft (left to right cockpit pedal) cable movement for some length of time greater than just this flight. The inner cable roughness on the abraded area was in proximity to the sharp edges of the cracked telescoping sleeve ball swivel coupling fitting, and could easily catch or hang up. Two days before the accident, this pilot experienced a yaw control anomaly with this helicopter and he made a precautionary landing at another airport. Over a 2-day period, maintenance technician(s) examined the thruster control system at the precautionary landing location and could not determine the reason for the discrepancy. During the detailed examination of the cable runs and control freedom checks, the maintenance technician did not remove the access panel over the FS113 splitter assembly (the location of the failed telescoping sleeve ball swivel coupling) to fully examine the thruster control cable run. The pilot and the maintenance technician incorrectly attributed the yaw control anomaly to a YSAS (Yaw Stability Augmentation System) failure, and a joint decision was made between the pilot and the maintenance technician to disable the YSAS and ferry the helicopter with the unresolved yaw control discrepancy to the maintenance base for further diagnostic work. At zero or very small sideslip conditions, the nature of the vertical stabilizer design produces a tension load in the thruster control cable, which would pull a failed forward cable telescoping sleeve ball swivel coupling fitting together; at larger right sideslip angles (left yaw), the tension load in the thruster cable decreases and eventually becomes a compressive load, which would tend to assist the movement of the inner cable out of the sleeve. Movement of the anti-torque control pedals changes the fan blade pitch to produce more airflow to the circulation control slots and the anti-torque thruster nozzle. As the pedals are displaced from center toward either extreme of travel, the airflow increases proportionally. The RFM procedures for a stuck thruster condition are incomplete and do not contain procedures to minimize airflow to the thruster nozzle (i.e., neutralize the pedal position at the onset of a stuck thruster condition). The pilot was concurrently flying the HH-60 helicopter for the California Air National Guard. The emergency procedures for an anti-torque failure in the MD500N are diametrically opposed to those for the HH-60. The specific immediate pilot action items for an anti-torque drive system failure in the HH-60 are actions that are expressly prohibited in the MD500N. MDHI and Cablecraft, the cables manufacturer, had been aware of stress corrosion cracking in other coupling fittings in the cable run for 2 years. They failed to expeditiously identify the stress corrosion cracking problem in the telescoping sleeve ball swivel coupling fitting, and failed to change the component specifications in a timely way to prevent the failure and separation of the fitting. The FAA concurred with MDHI assessment that the stress corrosion cracking in various fittings in the cable was not a safety of flight issue and the company was given until January 2000 to solve the cracking problem. The FAA also concurred with MDHI engineers that the stress corrosion cracking in the thruster cable fittings was considered a minor service difficulty and was not determined to be an unsafe condition, which would warrant the issuance of an airworthiness action. This indicates that MDHI and the supervising FAA Aircraft Certification Office inadequately assessed the significance of the stress corrosion problem and the potential consequences of a failure of the telescoping sleeve ball swivel joint. MDHI contends that the position of the jet thruster nozzle as it was found during the wreckage examination indicated right anti-torque pedal control was available during the loss of control and descent to impact. However, witness marks and color transfers establish that the blue main rotor blade flexed downward during the ground impact sequence, severing the tail boom. The center thruster cable was impacted by the blue main rotor blade and forcibly separated at the aft bell cranks and the quick disconnect fitting. The main rotor blade impact with the thruster cable placed a tension load on the cable and would have pulled the telescoping sleeve ball swivel coupling joint back into the outer sleeve, where it was found during the wreckage examination. A sudden tension load on the cable run would move the jet thruster nozzle can, which has no fixed physical limit stops and can rotate 360 degrees on it's mounting. The post impact position of the jet thruster nozzle can was in an over-travel condition induced by the sudden and violent tension load in the cable caused by the rotor blade contact. The post impact position of the nozzle was not considered as a reliable indication of nozzle position before impact. Sound spectrum analysis shows the main rotor transmission was operating at 100 percent until after the Mode C altitude readout spike, and then it rapidly decayed during the descent to ground impact. The twist grip throttle control and the fuel control indices were found at idle. MDHI contends that this indicates the pilot was attempting to deal with a high side governor failure and subsequent over speed of the main rotor, and his resultant actions exacerbated the effects of the stuck thruster condition and caused the ultimate loss of control. Postaccident testing and analytical modeling established that the governor, while exhibiting the wear and tear of a high time component, had an operating capability variance from normal of less than 1 percent and would have had an insignificant effect in main rotor operation. The throttle and fuel control indices position indicates a pilot input in an attempt to arrest the uncontrollable spin. Additionally, anatomical injuries to the right seat passenger and corresponding helicopter interior damage indicates that a violent left rotation spin of the helicopter occurred very early in the sequence and continued until ground impact; the rotation rate was at or near 1 revolution per second.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: 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. Full narrative available
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