The National Transportation Safety Board is calling for establishing new duty-time regulations covering maintenance workers involved in on-demand, fractional and all repair station operations, along with those involved with scheduled carriers.

The recommendation is one of three new ones issued today (Jan. 29) following the safety board’s investigation of the Dec. 7, 2011 crash of a Eurocopter AS350-B2 helicopter operated by Sundance Helicopters on a sightseeing trip outside Las Vegas. The pilot and four passengers were killed.

The safety board cites inadequate maintenance as the probable cause of the accident, and finds that mechanic and inspector fatigue played a contributing role.

The helicopter, N37SH, had undergone a 100-hr. inspection the day before the accident and the fore/aft servo was replaced. But investigators believe that the input rod and fore/aft servo became disconnected shortly before the crash, leading to a loss of control. The fore/aft servo is connected to the input rod by a bolt secured by a self-locking castellated nut and a split pin.

The maintenance technician indicated that the work had been accomplished as required, and a company inspector approved the work. NTSB believes a bolt had been in place connecting the servo with the input rod, since control of the aircraft would not have been possible otherwise.

But the safety board believes the self-locking nut became separated from the bolt, likely because it was worn. The safety board further believes that the split pin, the second securing device, either was not installed or had been improperly installed.

Investigators found that both the mechanic and inspector had been called in to work on a shift that they had been scheduled to be off, and hours earlier than they normally work.

Both were on duty before 6 a.m., and each had worked longer than 12-hr. shifts. Both knew proper procedures for correct installation, the safety board notes. “If the work shifts of the maintenance personnel had been consistent, a major source of their fatigue could have been mitigated,” the NTSB says.

“In too many investigations we have seen the tragic results of human error, and we’ve long known about the safety challenges unique to maintenance – tasks performed at night (when our bodies are programmed to rest), long duty days, shift changes and interruptions, the disassembly and reassembly of complex systems, time pressures and more,” says NTSB Chairman Deborah Hersman.