Associated Press
Investigators examine the wreckage of a sightseeing tour helicopter from Las Vegas that crashed near Lake Mead.

LAS VEGAS — Federal regulators on Tuesday recommended that aircraft mechanics and inspectors get enough rest and use checklists to guide their work after a 13-month investigation of a Las Vegas sightseeing helicopter crash that killed five people when a crucial bolt came loose.

"A checklist can be a crucial reminder and especially helpful when we are tired or distracted or new to a job," National Transportation Safety Board Chairwoman Deborah Hersman said after the five-member board reached its conclusions in Washington, D.C.

"Look what checklists have done for safety in the cockpit," Hersman said. "They can make similar lifesaving contributions on the hangar floor. They are a backstop to human error."

The board cited "inadequate maintenance" for the December 2011 crash of the Sundance Helicopters Inc. chopper during a scenic twilight tour of Hoover Dam and the Lake Mead reservoir on the Colorado River. Investigators found a crucial bolt may have been reused too many times and improperly installed, with the mistake not found during inspections.

Sundance CEO Bob Engelbrecht issued a statement Tuesday promising to study the NTSB report, review the board findings and recommendations, and "look to further improve our processes and procedures."

The board accepted 12 findings and made three recommendations based on its conclusion that a crucial lock nut worked loose in the rotor mechanism during flight, causing the aircraft to wobble into an uncontrollable climb and fall into a rocky ravine some 14 miles east of Las Vegas. Most of the six-seat Aerospatiale AS-350 helicopter was pulverized to small bits and burned.

Killed were a honeymooning couple from India, Lovish Bhanot, 28, of Gurgaon, and Anupama Bhola, 26, of New Delhi; a couple from Utica, Kan., who were celebrating their 25th wedding anniversary, Delwin and Tamara Chapman, both 49; and the pilot, Landon Nield, 31.

Nield was a devout Mormon who grew up in Wyoming and Utah, and had married six months before the crash.

"I know what happened to my brother can't be changed," Nield's sister, Angalena Adams said Tuesday. "But I hope Sundance has fixed the problems that were wrong," she said.

"The choices they make to overwork their workers can affect other people's lives," said Adams, who monitored the NTSB hearing from her home in Cedar City, Utah.

Gary Robb, a Kansas City, Mo., lawyer who filed wrongful death lawsuits on behalf of the four passengers, wasn't immediately available for comment.

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Sundance was acquired for $44 million in December by Air Methods Corp., a publicly traded Denver-based emergency air medical transportation company. Sundance has a fleet of 22 helicopters based at McCarran International Airport focusing mostly on Grand Canyon helicopter tours. It also provides firefighting, air lift, natural resource surveying and photography operations.

In his statement Tuesday, Engelbrecht maintained that the company had an excellent safety record and made safety its top priority.

"Based upon the investigation to date and our own internal reviews, we have already initiated a number of safety improvements," he said.