LAX00GA025

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

1.1 History of flight

On October 25, 1999, at 1537 hours Pacific daylight time, a MD Helicopter, Inc. (MDHI) 500N helicopter, N904PD, collided with a city street in San Jose, California, following an in-flight loss of control during an approach to the San Jose International Airport. The helicopter, operated by the City of San Jose Police Department as a public-use flight under the provisions of 14 CFR Part 91 of the Federal Aviation Regulations, was destroyed during the collision sequence. The airline transport pilot and nonrated passenger were fatally injured. Visual meteorological conditions prevailed and a company flight plan was filed. The flight originated about 1524 from Reid-Hillview airport and was en route to San Jose International Airport.

While on a routine patrol on October 23, 1999, the pilot and a police observer, who was flying the aircraft, experienced an uncommanded yaw of about 10 degrees to the right while practicing stuck pedal maneuvers. In his statement, the observer reported that after ascertaining that neither of them had made a control input, the pilot took control and made a precautionary landing at the Reid-Hillview airport. The observer said that the pilot reported the pedals felt mushy, but he had control authority. The next day a contract maintenance technician arrived and was unable to find a discrepancy in the anti-torque control system (see Aircraft Information for a more detailed discussion). The pilot and technician believed the problem was in the Yaw Stability Augmentation System (YSAS), and it was deactivated in accordance with the Rotorcraft Flight Manual (RFM) procedures in order to fly the helicopter back to the maintenance base at San Jose International. Prior to departing on the accident flight, the pilot made several left and right pedal turns while in a hover.

A review of the air-ground communications tapes from the San Jose International Air Traffic Control Tower (ATCT), revealed that the pilot contacted the facility about 4 miles east of the airport and requested landing at taxiway "Victor." The pilot was cleared to crossover the airport midfield at or above 1,000 feet agl for a left downwind. He was given the wind, altimeter setting, and transponder code. The pilot read back the instructions. Runways 30R, 30L, and 29 were in use. The pilot stated he was going to Aris Helicopters and was cleared to land on taxiway "Victor" paralleling the active runways. His next transmission at 1535:21 was "Police 1, mayday, mayday," and then again "mayday, mayday" 3 seconds later. Witnesses reported seeing the helicopter yawing erratically then descending rapidly while spiraling.

Controllers on duty in the ATCT observed the helicopter while on downwind for runway 29. One controller observed the helicopter "in erratic flight . . . (then) . . . plummet to the ground." Another controller observed the helicopter "approximately 1 to 1 1/4 mile base leg when it just fell out of the sky." Another controller assumed that the pilot was performing an autorotation and queried the others about the maneuver. The last controller observed the helicopter in a tight spiral on the left downwind and alerted the supervisor.

Twenty-one ground witnesses were identified and interviewed. Of the 21 witnesses, 15 observed the helicopter spin, although only 4 were able to quantify the direction as a "counter clockwise" rotation. Eight of the witnesses said they heard unusual engine sounds and they variously described them as "whining" or "choking/sputtering." One of the witnesses reported a "tighter and tighter" spin as the helicopter descended, while another said it was a "tight 180-degree turn." One witness said the tail "appeared to be bouncing back and fourth," while a second witness observed that the helicopter "started to rotate [one direction] and reversed direction," then spiraled straight down.

1.1.1 Radar Derived Flight Path

Recorded radar data in the form of a CDR Editor listing was obtained from the San Francisco Bay Terminal Radar Approach Control (TRACON). The data was recorded and processed by the TRACON's ARTS IIIA system using a surveillance antenna at the Moffett Federal Airport, located about 8 miles northwest of the accident site. The antenna rotates clockwise at a 4.7-second sweep rate. The data reviewed was the time of each secondary beacon return, the recorded target position, the Mode C altitude, ground track heading, and ground track speed. According to the facility, the ARTS IIIA system computes target ground speed and ground track heading based on a smoothing algorithm, which averages the last several data points.

In addition, recorded plot data was obtained from the San Jose International Airport noise abatement monitoring office. The facility obtains a direct feed of radar data from the Bay TRACON ARTS IIIA system and incorporates sound microphone decibel data to form an integrated picture of the tracks of departing, arriving, or over flight aircraft at the airport. The raw recorded radar data is processed in a computer program, Airport Noise and Operations Monitoring System (ANOMS), written by Lochard Company. The ANOMS program also uses a smoothing algorithm to compute and present target ground speed and track heading information.

Recorded raw radar data (time, position, and Mode C altitude) was also processed in a computer program Tactical Mapping by ERM, Inc. This program computes target point-to-point airspeed and heading without smoothing algorithms.

The tabular and graphically plotted radar data from all three sources is appended to this report.

Review of the data disclosed that the secondary beacon return was tracking west on a heading averaging 270 degrees at a Mode C reported altitude of 1,100 feet and a ground speed, which varied from 104 knots to 119 knots. As the target track passed over the airport midfield, the ground track heading turned left over the time frame 1534:16 to 1535:12 from 268 degrees to 126 degrees. The computed rate of turn was 2.4 degrees per second. Over the same time interval, the ground speed decreased to 115 knots. According to the air-ground communications tapes, 11 seconds after achieving the 126-degree heading, the pilot broadcasted the first "mayday" call. During this 11-second time interval, the Mode C reported altitude decreased to 900 feet as the ground speed decreased to 103 knots. Coincident with the "mayday" call, the Mode C reported altitude jumped from 900 to 1,100 feet in one 4.7-second radar antenna sweep interval, followed in the next 4.7-second sweep with a decrease to 800 feet. Over this same 9.4-second time frame, the ground speed decreased to 80 knots. The last secondary beacon return was recorded at 1535:31. Point-to-point computation of the ground track speed over the last four 4.7-second antenna sweeps yielded speeds of 79.7, 72.8, 56.3 and 23.2, then ground impact.

1.5 Personnel information

Review of the Federal Aviation Administration (FAA) Airman Certification records disclosed that the pilot held an airline transport pilot certificate with a multiengine land airplane rating. In addition, he held commercial pilot privileges for single engine airplanes land and sea, and rotorcraft-helicopter. The pilot held an instrument rating for airplanes and helicopters. He also held a flight instructor certificate with ratings for single and multiengine airplanes, rotorcraft-helicopter, and instruments. The most recent second-class medical certificate was issued to the pilot on October 30, 1998, and contained no limitations.

According to San Jose Police Department (SJPD) records, the pilot had accumulated a total flight time of 2,586 hours, consisting of about 1,215 hours of military helicopter flight time, and about 1,266 hours in the MDHI NOTAR (No Tail Rotor) helicopter. In the preceding 90 and 30 days, the pilot had flown 88 and 31 hours, respectively, in the 500N helicopter