A series of maintenance errors was responsible for the Dec. 7, 2011, crash of a Sundance Helicopters Eurocopter AS350 B2 in the mountains east of Las Vegas, the U.S. National Transportation Safety Board (NTSB) says.
The pilot and all four passengers on the “Twilight Tour” sightseeing flight were killed in the crash, and the helicopter was destroyed.
At an NTSB meeting convened to review the accident, Chairman Deborah Hersman noted that research has shown that the primary category of maintenance error is “failing to carry out necessary actions.
“And that is our finding from this investigation. Inserting a small pin, smaller than a paper clip, and just one small step in a routine maintenance procedure, was the necessary action. The omission of this action was the difference between an uneventful flight and tragedy.”
The flight began at 1621 local time, when the helicopter took off at dusk from Las Vegas McCarran International Airport in visual meteorological conditions with good visibility. The pilot planned to fly to Hoover Dam, about 30 nm (56 km) southeast and then to return to the airport.
The helicopter was not equipped with flight data recorders, and they were not required. However, radar data from the U.S. Federal Aviation Administration (FAA) showed that the helicopter had been level at 3,500 ft, with a groundspeed of 120 kt, until about one minute before impact, and then climbed to 4,100 ft, turned 90 degrees left and slowed. The helicopter descended to 3,300 ft and tracked northeast for 20 seconds before entering a left turn and plunging toward the ground “at a rate of at least 2,500 ft per minute,” the NTSB said in its final report on the accident. Parts of the helicopter were destroyed by a post-impact fire. The wreckage was found in a ravine about 14 mi (23 km) east of Las Vegas.
The NTSB said the probable causes of the crash were Sundance Helicopters’ “inadequate maintenance of the helicopter, including the improper reuse of a degraded self-locking nut, the improper or lack of installation of a split pin and inadequate post-maintenance 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 factors were the mechanic’s and inspector’s fatigue and the “lack of clearly delineated” steps for the maintenance task and the inspection, the NTSB said.
The mechanic who installed the fore/aft servo received his airframe and powerplant (A&P) mechanic certificate in December 2008 and worked on maintaining general aviation airplanes and business jets before being hired by Sundance in June 2011. After his hiring, he received indoctrination training in record keeping, maintenance procedures and use of the Eurocopter manuals, as well as on-the-job training, but he had yet to attend any helicopter-specific training.
His schedule typically included four days of 11-hour shifts, followed by three days off, three days of 12-hour shifts and four days off. Each shift began at 1200.
He estimated that he previously had performed about six fore/aft servo installations before he began work on the accident helicopter on Dec. 6. He had been off duty on Dec. 4 and 5, and initially was scheduled to be off on Dec. 6 as well, but was asked during a telephone call the previous afternoon to report to work. He said that he went to bed about 2200 — four hours earlier than his normal bedtime of 0200 — but fell asleep around 0000 and awoke at 0500, feeling good. He reported to work about 0550 and was assigned the fore/aft servo replacement, which he said he performed without difficulty and without feeling rushed.
The mechanic who inspected the servo replacement had been named a quality control inspector about six months earlier. He also was one of three lead mechanics who directed maintenance tasks when management personnel were not present. He received an airframe and powerplant mechanic certificate in 2002, and had spent about two years in commercial aircraft maintenance and seven years in helicopter maintenance before he was hired by Sundance in 2010.
Like the mechanic, he was not originally scheduled to work on Dec. 6 but was asked during a telephone call the previous day to report to work in the morning. He told accident investigators that he felt rested after about seven hours of sleep. He completed the inspection of the servo replacement work about 1800, near the end of his 12-hour shift.
A review of Sundance maintenance records showed that the inspector, working as a mechanic in June 2011, “failed to properly re-install the chin bubble” portions of the windshield on a helicopter. Sundance determined in a root cause analysis that “the inspector’s perception of the need to expedite the repair to avoid aircraft downtime was a contributing factor leading to this failure.”
The NTSB said its review of maintenance logs for the four months before the accident revealed no discrepancies.
The check pilot, who received an A&P mechanic certificate in 2000 and worked as a mechanic on Robinson helicopters for about six years, was hired by Sundance as a line pilot in 2010. He had 2,400 flight hours, including about 1,400 hours in rotorcraft. Before the accident, he had conducted 10 to 12 flight checks at Sundance.
Flight Control System
The AS350 B2 has a mechanical flight control system, assisted by one hydraulic tail rotor servo and three hydraulic main rotor servos. Of the main rotor servos, two are lateral servos, “which transfer the lateral inputs to the nonrotating swashplate (roll),” and one is the fore/aft servo, “which transfers the fore and aft inputs to the nonrotating swashplate (pitch),” the report said.
A system of control rods, bellcranks and levers transfers the pilot’s collective and cyclic control inputs to the “mixing unit,” where the inputs are delivered “through the appropriate servo control input rod to a servo input rod assembly,” the report said.
Each servo is connected to the main rotor transmission case, the nonrotating swashplate and the servo control input rod. A bolt, washer and self-locking slotted nut connect the servo control input rod to the servo input lever; the nut also is secured with a split pin (also called a cotter pin), which prevents it from unthreading (Figure 1).
The report said that Eurocopter had noted in a certification document that “the loss of control of the fore-aft servo would most likely result in a catastrophic failure of the helicopter.”
Investigators found the fore/aft main rotor servo control input rod in the wreckage, disconnected from the input lever; its connection hardware was not found.
Sundance Helicopters had 22 helicopters and 50 pilots at the time of the accident and, during the off-season when the crash occurred, averaged 35 tour flights and 40 shuttle flights a day to Grand Canyon West Airport. In 2011, the company operated 31,350 flight hours and transported more than 200,000 passengers.
All maintenance, except for component overhauls, was performed at the operator’s maintenance base in Las Vegas.
The last maintenance on the accident helicopter — which included a 100-hour inspection and replacement of the tail rotor servo, the engine and the main rotor fore/aft servo — was completed the day before the crash. The fore/aft servo was replaced with a new unit, and the mechanic said after the accident that he had no difficulties with the installation.
Eurocopter, in its Standard Practices Manual, specifies that a locking nut may be reused only if it is “not excessively damaged,” is hard and cannot be tightened by hand.
The mechanic said that, when deciding whether any nut could be reused, he removed it, cleaned it and inspected it for damage, then threaded it onto the bolt “to see if it will thread all the way down, and if he is able to turn the nut down to where the shank is visible, he replaces the nut,” the report said. In assessing the nut on the accident helicopter, “he deemed the hardware airworthy,” the report added.
After the accident, Sundance directed maintenance personnel to replace the self-locking nut and other connection hardware at the next scheduled inspection of all of its helicopters with more than 5,000 flight hours and then at 5,000-hour intervals. The company also said that the nut must be replaced with a new nut any time an input rod is disconnected from a servo.
Most Likely Scenario
The report noted that, “because proper functioning of the control input rod is necessary for takeoff,” it was clear that the bolt and the control input rod were in place when the accident flight began. Laboratory analysis revealed no evidence, however, that the connection hardware was present at impact.
“Based on the evidence,” the report added, “the NTSB concludes that the most likely explanation for the in-flight loss of control is that the fore/aft servo bolt disengaged in flight, which resulted in the separation of the control input rod to the fore/aft servo’s input lever, rendering the helicopter uncontrollable.”
The report said accident investigators evaluated several scenarios to determine how the connection hardware became disengaged and concluded that the most likely explanation was that “the self-locking nut became separated from the bolt, allowing the bolt to work its way out of the joint due to normal in-flight vibratory forces.”
The report noted a Eurocopter letter that said that, because the nut had two locking devices — the self-locking feature and the split pin — it was designed to remain tight. Even if the split pin had not been in place “behind an airworthy self-locking nut,” the nut should not have loosened as long as it was properly torqued, Eurocopter said.
Sundance inspected all of its AS350s after the accident and found all fore/aft servo connection hardware was properly connected and safetied. At the same time, the connection hardware was examined on the main rotor servos of all Sundance helicopters with at least 5,000 flight hours; the examinations showed that about half of the self-locking nuts on the 13 helicopters that had been inspected by January 2012 had no locking capability, the NTSB said.
On two helicopters, self-locking nuts “could be easily and fully tightened or loosened on the accompanying bolts with finger pressure,” the NTSB said, adding, “This indicates that the nuts … were not suitable for reuse.”
This prompted the NTSB’s conclusion that, “at the time of the accident, Sundance Helicopters was not following Eurocopter and FAA self-locking nut reuse guidance, which led to the repeated improper reuse of degraded nuts on its helicopters,” the report said.
The Sundance General Maintenance Manual requires independent inspections of specific maintenance procedures, including procedures that require use of a split pin, by a designated company quality control inspector.
The inspector who checked the work on the fore/aft servo installation said he found no problem during his inspection of the accident helicopter.
The company’s check pilot, who conducted a postmaintenance check flight the morning of the accident, told accident investigators that he “saw nothing in the left [main gear box] cowl area during his preflight inspection that indicated that the helicopter was not in a condition to fly.”
The report added, “The NTSB concludes that the mechanic, inspector and check pilot each had at least one opportunity to observe the fore/aft servo self-locking nut and split pin; however, they did not note that the split pin was installed improperly or not present.”
Maintenance Human Factors
Although maintenance personnel were under no time pressure to complete their work, the mechanic and the quality control inspector both met criteria for “susceptibility to the debilitating effects of fatigue,” the report said. “Because both the mechanic and the inspector had insufficient time to adjust to working an earlier shift than normal, they were experiencing fatigue. … In addition, the mechanic had an inadequate amount of sleep and the inspector had a long duty day, both of which also contributed to the development of their fatigue.”
Although the report said that fatigue alone could not explain the maintenance errors, it noted the NTSB’s “longstanding concerns about the effects of fatigue on maintenance personnel.”
In particular, the NTSB cited the extended duty time that contributed to the inspector’s fatigue, adding that it “continues to believe that establishing duty-time limitations is a key strategy to reducing the risk of fatigue-related errors in aviation maintenance.”
The report included several safety recommendations to the FAA, including one that called for the establishment of duty-time regulations based in part on start time, workload, shift changes, circadian rhythms and adequate rest time.
This article is based on NTSB Accident Report AAR-13/01, “Loss of Control; Sundance Helicopters Inc.; Eurocopter AS350-B2, N37SH; Near Las Vegas, Nevada; December 7, 2011.” Jan. 29, 2013.