Federal crash investigators say mechanics and inspectors should use work checklists to avoid maintenance errors like one they say caused a fatal sightseeing helicopter crash outside Las Vegas in December 2011.
The National Transportation Safety Board ruled Tuesday in Washington, D.C., the Sundance Helicopters crash was caused by a crucial locking nut working loose in the rotor mechanism.
That caused a loss of control before the crash near Lake Mead killed the pilot and four tourists.
Officials think a fatigued mechanic improperly installed the nut during maintenance before the crash.
They note the mechanic had been called in early to do the work.
But the helicopter also passed inspections prior to the crash.
"This investigation is a potent reminder that what happens in the maintenance hangar is just as important for safety as what happens in the air," said NTSB Chairman Deborah Hersman.
The Eurocopter AS350, operating as a "Twilight City Tour" sightseeing trip, crashed in mountainous terrain about 14 miles east of Las Vegas. The helicopter originated from Las Vegas McCarran International Airport at about 4:21 p.m. with a planned route to the Hoover Dam area and then return to the airport.
After the trouble began, the helicopter climbed roughly 600 feet, turned about 90 degrees to the left, descended about 800 feet, began a left turn, and then descended at a rate of at least 2,500 feet per minute to impact, the NTSB report stated.
Killed were two honeymooners from India, a couple from Utica, Kan., and the 31-year-old pilot.
Copyright 2013 Associated Press. All rights reserved. CBS 5 (KPHO Broadcasting Corporation) contributed to this report.