Accident Overview
A NOTAR (No Tail Rotor) helicopter experienced a loss of control while in a normal descent on the downwind leg for landing. The helicopter entered an uncontrollable left spin and descended to ground impact. The yaw control system failure was attributed to a stress corrosion fracture and separation of a fitting in the thruster control cable, which locked the jet thruster nozzle and prevented the pilot from countering the left yaw.
Mechanical Failure
The anti-torque system thruster control cable contained a telescoping sleeve ball swivel coupling fitting that failed due to stress corrosion. The separation of the swaged retaining lip allowed the inner cable to slide out of the outer sleeve, leading to a fixed nozzle setting. Metallurgical examination confirmed the fitting failure predated the flight, and the inner cable showed severe abrasion consistent with prolonged operation outside the sleeve.
Maintenance and Prior Anomaly
Two days before the accident, the pilot reported a yaw control anomaly and made a precautionary landing. Maintenance technicians examined the thruster control system but did not remove the access panel over the failed coupling. The pilot and technician jointly decided to disable the Yaw Stability Augmentation System (YSAS) and ferry the helicopter for further diagnostics. The source states the maintenance diagnostic actions were inadequate to correctly diagnose the reported anomaly.
Pilot Actions and Training
The pilot was concurrently flying HH-60 helicopters. The emergency procedures for anti-torque failure in the MD500N are diametrically opposed to those in the HH-60. The pilot's negative transfer of anti-torque failure procedures from a conventionally designed helicopter precipitated improper control input in response to the stuck thruster condition.
Regulatory and Manufacturer Oversight
MDHI and the cable manufacturer Cablecraft had been aware of stress corrosion cracking in similar fittings for two years but did not expeditiously identify the problem in the telescoping sleeve coupler. The FAA concurred with MDHI's assessment that the cracking was not a safety-of-flight issue and allowed until January 2000 to resolve it. The source indicates that MDHI and the FAA inadequately assessed the significance of the stress corrosion problem and the potential consequences of a failure.
Flight Dynamics and Wreckage Examination
Ground witnesses observed the helicopter yaw to the right, then suddenly reverse to a rapid left rotation. Radar data showed a spike in Mode C altitude, indicating a large sideslip angle coincident with the pilot's first mayday call. Postaccident examination found that the blue main rotor blade had severed the tail boom during impact, which affected the position of the thruster cable. Sound spectrum analysis showed main rotor transmission operating at 100% until the altitude spike, then decaying. The twist grip throttle and fuel control indices were found at idle; postaccident testing indicated the governor had insignificant variance. The throttle position suggests a pilot attempt to stop the spin. Anatomical injuries to the right seat passenger and interior damage indicate a violent left rotation at nearly one revolution per second.
Flight Manual Procedures
The Rotorcraft Flight Manual (RFM) procedures for a stuck thruster condition were incomplete. They did not contain procedures to minimize airflow to the thruster nozzle, such as neutralizing pedal position at the onset of the stuck condition.