Accident at Las Vegas, Nevada, 7 Dec 2011 (N37SH)

Accident narrative

On December 7, 2011, about 1630 Pacific standard time, a Eurocopter AS350-B2, registration N37SH, crashed in mountainous terrain about 14 miles east of Las Vegas, Nevada. The helicopter, operated by Sundance Helicopters, Inc., was conducting a "Twilight tour" sightseeing trip under 14 Code of Federal Regulations Part 135. The pilot and four passengers were killed, and the helicopter was destroyed by impact forces and a postimpact fire.

### The flight

Visual meteorological conditions with good visibility and dusk light prevailed at the time of the accident. The flight originated from Las Vegas McCarran International Airport about 1621, with an intended route to the Hoover Dam area and return.

The 31-year-old pilot held a commercial pilot certificate and had accumulated 3,174 total flight hours, including 1,360.6 hours as pilot-in-command in AS350 helicopters. He had flown 1.4 hours earlier that day on a tour to the Grand Canyon in the accident helicopter.

Radar data showed the helicopter following the prescribed tour route easterly, then turning southeast toward the Hoover Dam, flying level at 3,500 feet with a groundspeed of about 120 knots. About one minute before impact, the helicopter unexpectedly climbed to 4,100 feet, turned about 90 degrees to the left, and slowed. The radar data then showed the helicopter descending to 3,300 feet and tracking a northeasterly course for about 20 seconds before entering a left turn and descending at a rate of at least 2,500 feet per minute. The last radar target was recorded about 1/8 mile from the accident site.

### What the investigation found

The helicopter crashed in a ravine between Henderson, Nevada, and Lake Mead, impacting the terrain in a nose-low attitude on a northeasterly heading. A large amount of the structure forward of the horizontal stabilizer was consumed by postimpact fire.

The Eurocopter AS350-B2 is equipped with three hydraulic servos installed on the main rotor: two lateral servos and one fore/aft servo. At the accident site, the fore/aft main rotor servo's control input rod was found disconnected from the input lever. The connection hardware—a bolt, washer, self-locking nut, and split pin—was not found during comprehensive examinations of the wreckage. The control input rods for both lateral servos and the tail rotor servo were found properly connected.

The fore/aft servo's control input rod was found in two pieces, with separation points consistent with overload and impact damage. Examination of the upper input rod end revealed no visible elongation of the bolt hole. Microscopic examinations of the attachment area did not reveal any marks indicating that the connection hardware was present at the time of impact. However, resolidified aluminum and plastic found covering the attachment holes indicated that the hardware was not securing the control input rod at the time of the postimpact fire.

The four servos were examined by Meggitt Control Systems. All were severely damaged by impact forces, but the examinations did not reveal any indications of jamming or obstructions. The investigation also found no evidence of a preimpact loss of hydraulic pressure.

The Board concluded that the bolt connecting the control input rod to the fore/aft servo's input lever must have been present at the start of the flight to allow normal flight up until the catastrophic upset. The Board concluded that the most likely explanation for the in-flight loss of control is that the fore/aft servo bolt disengaged in flight, resulting in the separation of the control input rod and rendering the helicopter uncontrollable.

### Maintenance and hardware

The day before the accident, the helicopter underwent a 100-hour inspection, which included the replacement of the main rotor fore/aft servo. Federal regulations require that any removable fastener whose loss could jeopardize safe operation must incorporate two separate locking devices. For the fore/aft servo input rod, the first locking device is a self-locking nut, and the second is a split pin.

Eurocopter and Federal Aviation Administration guidance state that a self-locking nut should not be reused if it can be tightened by hand or cannot meet minimum prevailing torque values. The investigation found that Sundance Helicopters was not following this guidance. Examinations of other helicopters in the company's fleet revealed that about half of the self-locking nuts checked had no locking capability. The NTSB also measured the peak tightening and loosening torque on new test nuts, finding that after 10 cycles, the average torque values dropped by almost half.

The Board evaluated four scenarios to explain how the connection hardware disengaged: the bolt was sheared by forces, the bolt fractured, the nut and split pin were never installed, or the self-locking nut separated from the bolt. Shearing was ruled out because there was no elongation of the clevis holes. Fracturing was ruled out due to the low hardness of the components and a lack of history of such failures. The Board noted it was unlikely the hardware was never installed, as the helicopter had flown safely for 3.5 hours prior to the accident flight and the missing hardware would likely have been noticed during preflight checks.

The Board concluded that the fore/aft servo bolt most likely disengaged because the split pin was installed improperly or not at all, and a self-locking nut that was either degraded or not torqued was used, allowing the nut to unthread and separate from the bolt.

### Inspections and human factors

Following the maintenance on the day before the accident, a quality control inspector inspected the work. The morning of the accident, a check pilot conducted a Before First Flight check. The Board concluded that the mechanic, inspector, and check pilot each had at least one opportunity to observe the hardware, but they did not note that the split pin was installed improperly or not present.

The paperwork used for the 100-hour inspection listed the servo replacement task but provided only one place for the inspector to sign off for the overall installation, rather than individual places for each step. The Board concluded that the use of work cards with clearly delineated steps can help prevent and trap errors in the performance and verification of maintenance tasks. The investigation also noted that Sundance Helicopters did not meet Tour Operators Program of Safety (TOPS) audit requirements because the company's director of maintenance misinterpreted the standards regarding the training and qualifications of mechanics.

The investigation evaluated the work schedules of the mechanic who installed the servo and the inspector who checked it. Both were contacted on their off-duty days to report to work on the day of the maintenance, starting their shifts about six hours earlier than their normally scheduled shifts. The mechanic had slept for about five hours the night before, and the inspector completed the inspection after being awake for about 14 hours.

The Board concluded that because both men had insufficient time to adjust to working an earlier shift than normal, they were experiencing fatigue. The Board concluded that both the mechanic's and the inspector's performance probably were degraded by fatigue, which contributed to the improper securing of the hardware and the incomplete maintenance inspection.

### Probable cause

The National Transportation Safety Board 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.