LOS ANGELES, CA, USA
N90230
Bell 205A-1
During an air ambulance flight in the public-use helicopter, the tail rotor and gearbox separated from the helicopter. The pilot autorotated to a forced landing. During the descent over mountainous terrain, the helicopter collided with trees and impacted hard terrain on its left side which crushed inward. The operator's policy required all crewmembers to wear helmets during flight. Helmets were not provided for the two paramedics. During the crash sequence, the passenger seat stanchions and tubing buckled, which resulted in multiple lap belt anchor point separations and the catapulting of crewmembers into the overhead cockpit panel. Safety Board survival factors documentation in conjunction with helmet crashworthiness analysis revealed helicopter impact forces were within human tolerance. The lack of and/or inadequate strength helmets and the lap belt anchor point failures allowed crewmembers' excursions resulting in head trauma. The tail rotor component separations in flight resulted from a fatigue crack originating in the surface of the yoke onto which the tail rotor blades had been attached. In 1996, Bell issued an Alert Service Bulletin (ASB) number 205-96-68, which was designed to measure yoke deformation resulting from adverse in-flight or ground handling operations which imposed excessive bending loads. The test protocol was found problematic in its accuracy due to technical errors in the bulletin and a lack of clarity. City mechanics failed to adhere to all of the ASB's requirements. The bent yoke fractured at a total time in service of approximately 4,113 hours, about 117 hours after its inspection for evidence of deformation. The yoke's stainless steel composition and requisite metallurgical properties were confirmed by the Safety Board. An x-ray diffraction examination of the yoke revealed reduced compressive residual stress in the fracture origin region which allowed operational loads to initiate and propagate the fatigue crack. This significant reduction of the residual stress was likely due to excessive flexure (bending) of the yoke. The initiating event which overstressed and bent the yoke was not identified.
HISTORY OF FLIGHT On March 23, 1998, about 0740 hours Pacific standard time, a Bell 205A-1, N90230, owned by the City of Los Angeles, California, and operated by the Los Angeles City Fire Department (LAFD), experienced the separation of its tail rotor blades and the 90-degree gearbox during cruise flight. A forced landing was initiated, and during the autorotative descent the helicopter collided with trees approximately 1.5 statute miles northwest of its destination, Children's Hospital in Los Angeles, California. The purpose of the flight was to provide air ambulance transportation for a seriously injured passenger. Visual meteorological conditions prevailed, and the LAFD was monitoring the helicopter's flight progress. The public-use helicopter was operated under the provisions of Title 14 CFR Part 91. The helicopter was destroyed upon impacting the terrain, and the commercial certificated pilot and one crewmember were seriously injured. Three additional crewmembers and the passenger sustained fatal injuries. The local area flight originated from the Van Nuys Airport, California, about 0722. Upon dispatch, the helicopter flew about 7 miles northeast and landed about 0731 at the Stonehurst Elementary School playground, near the scene of an automobile traffic accident. At this location, the trauma patient (passenger) was loaded into the helicopter. The pilot took off about 0733. Using the air traffic control call sign "Lifeguard Fire Three," the pilot flew in a south-southeasterly direction past the Burbank Airport while climbing and leveling off between 1,900 and 2,100 feet mean sea level (msl). Recorded radar track data indicates that the helicopter attained a 104-knot average ground speed. Between 0737:17 and 0737:22, while cruising over a heavily wooded mountainous area known as Griffith Park, an event occurred which was manifested by an increase in the helicopter's average ground speed to about 118 knots. Seconds later, between 0737:26 and 0737:45, the helicopter's average speed reduced to between 76 and 84 knots, while descending through its last recorded altitude of 1,400 feet msl. Thereafter, three other pilots and two ground witnesses reported hearing radio transmissions about a helicopter in an emergency or going down in Griffith Park. About 0739 one of these pilots, who was communicating with the Burbank Air Traffic Control Tower, reported hearing an emergency transmission about a helicopter crashing. About 0740, two news media helicopter pilots reported hearing a call from Fire Three that he was experiencing an emergency over Griffith Park. This transmission was followed 20 seconds later with the statement "gonna put it in Griffith Park." Two air support police officers, who were in hangars, heard the following transmissions at 0740: "Fire 3 we have an emergency." During this approximate time, two hikers were on a mountain trail in Griffith Park. They reported hearing two "bangs" and observing components depart the southerly flying helicopter as it passed west of the Griffith Park Planetarium. Additional witnesses reported seeing the helicopter descend down a canyon and collide with a series of trees before crashing in a partial clearing. INJURIES TO PERSONS Two of the six persons onboard the helicopter survived. No one on the ground was injured. PERSONNEL INFORMATION Air Operations Unit Management, Duties & Responsibilities. The Air Operations Unit (AOU) of the LAFD is headed by a commander (manager) who, on a daily basis, is physically located at the airport unit. The commander reports to superior personnel located in the department's downtown Los Angeles headquarters. In brief, the commander is responsible for helicopter operations including planning, scheduling of training, maintaining records, implementing department orders and communicating with his headquarters management. The commander does not possess a pilot certificate. The AOU is staffed on a 24-hour basis with LAFD helicopter pilots. According to the AOU commander, the pilots are responsible for the safety of the helicopter in which they are flying. They are in command of the helicopter, and have the authority to veto any proposed operation, which in the pilot's opinion, would be unsafe. (See the extract from the Air Operations Manual for the statement of pilot responsibility.) Flight Crewmembers. On the accident flight, the helicopter crewmembers consisted of one pilot, two helitacs and two paramedics. The helitacs are trained to serve as helicopter crewmembers. In part, they are responsible for overseeing the safety of the working environment including providing guidance to ground personnel. The paramedics perform emergency medical aid to the patient, if required. Pilot. A review of the pilot's personal flight record logbook indicates that he began primary flight training in July 1990, and he received a private pilot certificate in November, 1990, with an airplane single engine land rating. In June 1993, he began rotorcraft flight training, and 4 months later he was issued a commercial pilot certificate. The pilot subsequently was issued a certified flight instructor certificate with rotorcraft privileges. In October 1995, after principally training in the Robinson R22 and the Bell 206 helicopters, the pilot received his first flight in the Bell 205A-1 (accident) helicopter. The pilot continued receiving LAFD flight training in the helicopter, and the following year he completed the checkout process. By the accident date, the pilot had approximately 1,865 total flight hours, of which about 1,440 hours were flown in rotorcraft. His total experience piloting the Bell 205A-1 helicopter, and his experience flying this model during the 90-day period preceding the accident, were 234 and 15 hours, respectively. Between 1997 and 1998, the pilot received refresher training in emergency procedures including touchdown autorotations and tail rotor failures. On August 4, 1997, the pilot passed an Federal Aviation Administration (FAA) administered proficiency flight check evaluating his knowledge and skill as an air carrier (FAR Part 135) pilot. AIRCRAFT INFORMATION Certification and Operations Base. The FAA issued the newly manufactured transport category helicopter, serial number 30221, a standard airworthiness certificate on March 12, 1976. On May 10, 1976, the FAA registered the helicopter in the name of the City of Los Angeles. The helicopter was physically based at the LAFD's Van Nuys Airport Air Operations Unit, which is adjacent to the Los Angeles City Helicopter Operations and Maintenance Facility. The Los Angeles City Director of General Services and maintenance facility management reported that the helicopter was maintained in accordance with FAA regulations including Bell's service bulletins. Helicopter Modifications and Utilization. The Los Angeles City maintenance participant reported that the helicopter's interior had never undergone a major modification or overhaul. An external, belly-mounted, water tank had been installed on the bottom of the helicopter. According to the LAFD, the helicopter was principally used for fire-fighting (water drops) and other activities such as flight and swift water rescue training. Secondarily, it was used as an air ambulance, although it had not been configured with any equipment for such usage. During the accident flight, emergency medical equipment for the care of the patient that the LAFD had required, by policy, to be onboard was not carried. (See the L.A. County Prehospital Care Policy Manual for the list of required equipment absent from helicopter.) Tail Rotor Design and Yoke Straightness. The Bell Helicopter participant reported that the company had designed the tail rotor assembly of the helicopter with two tail rotor blades. The blades are bolted to a yoke that holds them together. The yoke assembly is mounted onto the output drive shaft of the tail rotor's 90-degree gearbox, which rotates the yoke. The yoke is referred to as a flex-beam yoke. A portion of the yoke is referred to as the "flexure." This portion accommodates movement or flapping of the tail rotor blades during in-flight rotation. Additionally, the yoke can flex under certain ground operations, and when exposed to adverse environmental conditions. Bell personnel verbally reported to the National Transportation Safety Board investigator during a March 26, 1998, telephone conference, that when bending loads are applied to the yoke which exceed its design strength, its relative "straightness" may be altered, and the yoke's anticipated infinite service life will be reduced. Bell Helicopter initially placed a retirement life limit of 4,000 hours on the yoke. Bell personnel further reported that following a review of the service history and fatigue evaluation data for this model yoke, and with FAA approval, the yoke's retirement life was increased to 5,000 hours. This action occurred in October 1989. Alert Service Bulletin. Bell issued an Alert Service Bulletin (ASB), number 205-96-68, dated August 1, 1996, which was pertinent to the yokes installed in all model 205A-1 helicopters between serial number 30001 and 30228, having a time since new greater than zero hours. In summary, Bell indicated that if the yoke encountered adverse bending loads during specific ground handling or in-flight conditions, it could flex and become deformed. Bell provided the following description of events about which it was concerned: "...When not turning, the tail rotor yoke flexure is susceptible to static overload if it is loaded by external bending forces. Examples of bending loads include high wind gusts (such as those from prop blast), improper ground handling (where the tail rotor blade has been used as a hand hold), improper feathering bearing removal (where the yoke assembly is not properly supported when pressing out bearings), or a static ground strike of some type (such as it being struck by a vehicle). An overload may also occur dynamically during a power-on or off sudden stoppage incident or hard landing." In the ASB, Bell recommended that the tail rotor blades be secured when exposed to wind gusts in excess of 45 knots. It also issued the following warning statement: "Do not exceed load and/or deflection limits during tiedown procedures for tail rotor. Maximum load allowable at blade tip is 50 lbs." Bell opined that an undamaged yoke "is reliable for its full retirement life." Notwithstanding the yoke's "reliability," Bell designed the ASB to ferret out if the aforementioned conditions had occurred. It prescribed a procedure combining a historical record review, performance of a dimensional (straightness) inspection check and subsequent recurring inspections for evidence of excessive flapping. The requisite yoke examination procedures directed mechanics to use a Bell-supplied tool, called a "fixture set," during the dimensional testing, and to follow specific procedures described in the ASB. (See the Alert Service Bulletin for complete details, including a drawing of the yoke and the dimensional testing equipment.) Yoke History, Maintenance and Trunnion Flapping Stops. A review of the helicopter's maintenance records and pilot squawk sheets indicates that since manufacture, the helicopter was maintained and operated by Los Angeles City personnel. The City indicated that it had performed annual inspections and maintenance in follow-up to FAA requirements, manufacturer's recommendations, and pilot squawks. The accident yoke, P/N 212-010-744-5, S/N JIDA-2193, was installed by Bell in 1975 upon manufacture of the helicopter. It remained in use until June 16, 1993, when it was removed for overhaul at a total helicopter (and yoke) time of 3,996.7 hours. The helicopter continued in service with another yoke. On August 2, 1996, the accident yoke was removed from storage and Bell's ASB, Part 1 record review, and the Part 2 dimensional (straightness) check were accomplished. The Los Angeles City participant reported that, following the completed examination, the records indicated the yoke was returned to storage as a usable part. On July 25, 1997, the accident yoke was removed from storage and reinstalled on the helicopter. Between August and October, 1997, the yoke's trunnion assembly flapping stops, P/N 212-010-738-001, were visually inspected five times for deformation. (According to the inspection process listed in Part 3 of the ASB, if the stops were found bent, the yoke had been exposed to excessive bending (flapping) loads and must be discarded.) On January 12, 1998, the helicopter received an annual inspection at a total time of 5,097.0 hours, and the flapping stops were again examined for deformation signatures. None of the maintenance records indicated that the flapping stops were bent. The helicopter continued in service until the accident at which time the helicopter and accident yoke had total times of about 5,114.6 and 4,113.4 hours, respectively. In summary, the accident occurred about 886.6 hours prior to the yoke's 5,000-hour mandatory retirement life. At the time of the crash, the yoke had accumulated about 116.7 hours since undergoing the ASB 205-96-68 Part 2 inspection for straightness, and about 17.6 hours since it was last visually examined pursuant to the ASB for evidence of excessive tail rotor flapping. METEOROLOGICAL INFORMATION The closest aviation weather observation station is at the Burbank Airport, about 6 miles from the accident site. During the 1-hour period before and after the accident, Burbank reported 20 miles visibility, clear sky, and calm wind. AIDS TO NAVIGATION According to FAA records of facility operation, all electronic aids to navigation pertinent to the helicopter's route of flight were functional. COMMUNICATION The Burbank Air Traffic Control Tower was the only FAA facility that recorded communications from the accident helicopter. Burbank's audiotape was reviewed by the FAA and LAFD participants. In summary, they reported that nothing unusual was noted with either the pilot's communications, which sounded routine, or in the background noise during the transmissions. None of the airborne or ground witnesses reported having recorded the accident pilot's transmissions. WRECKAGE AND IMPACT INFORMATION Three separated components from the helicopter were found along the flight path leading to the main wreckage. The two separated tail rotor blades, with the associated yoke onto which they were bolted, and the 90-degree gearbox were found about 1 mile north of the main wreckage. The second rotor blade was found on a road. All of the components were within 0.1 mile of each other. Along a southerly track within about 220 feet north of the main wreckage, severed tree trunks and felled limbs were noted over the down sloping terrain. The estimated elevation of the topped trees was 701, 667, 641, and 638 feet msl. About 75 feet northeast of the main wreckage a portion of one main rotor blade was observed suspended by tree limbs, about 60 feet above ground level. The main wreckage was found east of the intersection of Red Oak and Fern Dell Drives, at an elevation of about 588 feet msl. The helicopter was observed in the following approximate attitude: 26 degrees nose (pitch) down, 125 degrees left bank (slightly upside-down) and on a southeasterly magnetic heading of 130 degrees. The helicopter was partially straddling a concrete retaining wall separating a grassy parkway and an asphalt footpath, west of the Fern Dell Creek. A visual examination around the perimeter of the wreckage revealed no evidence of skid marks or any secondary impact crater. The entire right side sliding door, with its attached emergency egress window panels, was found separated from the helicopter. According to the LAFD, upon responding to the crash scene its personnel were unable to slide the door open. To gain entrance they ultimately pulled the entire door off the side of the cabin. The Safety Board investigator noted that the locking pins, which secure the exit windows to the doorframe, were found in the locked position, a
An excessive bending load applied to the tail rotor blade assembly of an undetermined origin which produced a fatigue crack, the separation of the assembly, and a forced landing. Factors were the lack of suitable terrain to perform a forced landing, the manufacturer's unclear maintenance bulletin instruction and procedures which facilitated the operator's inadequate inspection for the yoke's straightness, and the inadequacy of restraint systems and protective equipment.
Source: NTSB Aviation Accident Database
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