On December 7, 2011, about 1630 Pacific standard time, a Sundance Helicopters, Inc., Eurocopter AS350-B2 helicopter, N37SH, operating as a "Twilight tour" sightseeing trip, crashed in mountainous terrain about 14 miles east of Las Vegas, Nevada. The pilot and four passengers were killed, and the helicopter was destroyed by impact forces and postimpact fire. The helicopter was registered to and operated by Sundance as a scheduled air tour flight under the provisions of 14 Code of Federal Regulations (CFR) Part 135. Visual meteorological conditions with good visibility and dusk light prevailed at the time of the accident, and the flight operated under visual flight rules. The helicopter originated from Las Vegas McCarran International Airport, Las Vegas, Nevada, about 1621 with an intended route of flight to the Hoover Dam area and return to the airport. The helicopter was not equipped, and was not required to be equipped, with any on-board recording devices.
The accident occurred when the helicopter unexpectedly climbed about 600 feet, turned about 90° to the left, and then descended about 800 feet, entered a left turn, and descended at a rate of at least 2,500 feet per minute to impact. During examination of the wreckage, the main rotor fore/aft servo, one of the three hydraulic servos that provide inputs to the main rotor, was found with its flight control input rod not connected. The bolt, washer, self-locking nut, and split pin (sometimes referred to as a "cotter pin" or "cotter key") that normally secure the input rod to the main rotor fore/aft servo were not found. The investigation revealed that the hardware was improperly secured during maintenance that had been conducted the day before the accident. The nut became loose (likely because it was degraded) and, without the split pin, the nut separated from the bolt, the bolt disconnected, and the input rod separated from the linkage while the helicopter was in flight, at which point the helicopter became uncontrollable and crashed.
The safety issues identified in this accident include the following:
- Improper reuse of degraded self-locking nuts. Although Eurocopter and the Federal Aviation Administration (FAA) publish guidance on the reuse of self-locking nuts, the investigation revealed that Sundance Helicopters' maintenance personnel were reusing nuts that did not meet the criteria specified by Eurocopter and FAA guidance. Federal regulations require that any removable fastener whose loss could jeopardize the safe operation of the helicopter must incorporate two separate locking devices. For the Eurocopter AS350-B2 helicopter, the first locking device is the self-locking nut and the second one is the split pin. The improper reuse of a degraded self-locking nut on critical flight control components is a safety hazard because it negates the safety benefits of one of the two required locking devices. Reinforcing the importance of maintenance personnel properly following manufacturer and FAA self-locking nut reuse guidance would help ensure that nuts without locking ability are not being reused. In response to the National Transportation Safety Board's (NTSB) concerns on this matter, the FAA Safety Team (FAAST) published a general aviation maintenance alert on its website at http://faasafety.gov in November 2012 and distributed the alert to over 239,000 mechanics and pilots via its e-mail registry. The NTSB provided accident case study data related to maintenance errors to FAAST for inclusion in its inspection authorization (IA) renewal training. This information will be included in IA renewal training clinics throughout the United States in 2013. In addition, the FAA will publish an Aviation Maintenance Alert (Advisory Circular 43-16A) in early 2013 related to helicopter maintenance issues.
- Maintenance personnel fatigue. Both the mechanic and the quality control inspector, who inspected the maintenance work completed the day before the accident, were likely fatigued during the December 6 shift, in part, because they had insufficient time to adjust to working an earlier shift than normal. The mechanic and inspector's performance was degraded by fatigue, which contributed to the improper securing of the fore/aft servo connection hardware, the improper installation of the hydraulic belt, and the inadequate postmaintenance inspection of the accident helicopter, respectively. If the work shifts of the maintenance personnel had been consistent, a major source of their fatigue could have been mitigated.
- Need for work cards with delineated steps. In addition to the degradation of the mechanic's and inspector's performance due to fatigue, the maintenance documentation did not clearly delineate specific inspection steps, which can allow these tasks to be more vulnerable to error through human factors. Using work cards that clearly delineate the steps to be performed and critical areas to be inspected to support both the maintenance and inspection tasks is one way to mitigate inadvertent errors of omission in the performance and verification of maintenance tasks, especially tasks involving critical flight controls.
- Lack of human factors training for maintenance personnel. Although the FAA has developed substantial guidance for the industry about maintenance human factors and has repeatedly stressed the importance of human factors training, there is still no requirement for human factors training for maintenance personnel involved in air carrier operations. The NTSB notes that on May 21, 2012, the FAA issued a notice of proposed rulemaking (NPRM) proposing that repair stations operating under 14 CFR Part 145 provide human factors training relevant to aviation maintenance for employees performing maintenance, preventive maintenance, alterations, or inspection functions. However, if adopted, the rule would not require maintenance personnel working for Part 121 and 135 operators or Part 91 Subpart K programs to receive human factors training. This investigation has shown that providing human factors training, including training on the causes of fatigue, its effects on performance, and actions individuals can take to prevent the development of fatigue, to all maintenance personnel would help reduce the likelihood of human error in aviation maintenance.
The NTSB determines that the probable cause of this accident was Sundance Helicopters' inadequate maintenance of the helicopter, including (1) the improper reuse of a degraded self-locking nut, (2) the improper or lack of installation of a split pin, and (3) inadequate postmaintenance inspections, which resulted in the in-flight separation of the servo control input rod from the fore/aft servo and rendered the helicopter uncontrollable. Contributing to the improper or lack of installation of the split pin was the mechanic's fatigue and the lack of clearly delineated maintenance task steps to follow. Contributing to the inadequate postmaintenance inspection was the inspector's fatigue and the lack of clearly delineated inspection steps to follow.
As a result of this investigation, the NTSB makes safety recommendations to the FAA, reiterates a previous recommendation to the FAA, and reclassifies a previous recommendation to the FAA.