WASHINGTON — The National Transportation Safety Board has asked a private helicopter tours safety group to implement four changes to address safety problems found in investigations of a 2007 fatal crash in Hawai'i and a 2002 nonfatal accident in Arizona of Heli-USA aircraft.

The NTSB recommendations are aimed at beefing up maintenance, inspection monitoring and mechanics' knowledge of helicopters.

The NTSB wrote to the Tour Operators Program of Safety "to urge your organization to take action on the safety recommendations."

"The safety board is vitally interested in these recommendations because they are designed to prevent accidents and save lives," the NTSB wrote.

Tim Cudny, chairman of the safety group, said he was aware that recommendations would be coming from the NTSB.

"We're kind of honored that they are recognizing us as an entity that does set higher standards for safety and something like this is easy to implement," Cudny said. "We basically do a lot of this stuff as it is."

The safety board said the recommendations stem from investigations of two Heli-USA helicopter crashes, including one on March 8, 2007, at Princeville Airport on Kaua'i. Investigators found that a critical flight control device came loose before the crash, which killed four people and seriously injured three others.

The other accident near Peach Springs, Ariz., in 2002, involved an identical Aerospatiale helicopter that had a hydraulic failure leading to a hard landing, slightly injuring a passenger and doing substantial damage to the aircraft.

"The investigative findings from these two accidents revealed safety issues related to Heli-USA's ineffective maintenance, inadequate quality assurance programs, model-specific maintenance training and the Federal Aviation Administration's lack of surveillance ... to identify maintenance nonconformance," the NTSB said.

There were 165 air tour accidents between Jan. 1, 1996, and Oct. 1, 2007, with 88 involving helicopters, the NTSB said.

Of the 71 helicopter accidents in which causes and contributing factors have been determined, 25 involved mechanical failures or malfunctions, the NTSB said.

Membership in the tour safety group, based in Seal Beach, Calif., is voluntary and made up mostly of large tour operators. Its 11 members represent about 50 percent of the total air tour capacity in the United States.

The members' operating practices are audited for management, aircraft equipment, pilot training, maintenance and ground support.

Heli-USA is not a member of the group and the company did not respond to requests for comment on the NTSB recommendations.

The NTSB recommended that the safety group analyze the effectiveness of its members' inspection and maintenance programs for quality and safety. The recommendations also call for the group to expand its safety audit program to ensure that operators implement a maintenance quality assurance program.

The NTSB also wants the group to work with the FAA to develop these programs. And the board wants the safety group to establish requirements for formal, model-specific helicopter maintenance training for mechanics and to examine their members' education programs annually.

The NTSB investigators found that in the 2007 Hawai'i accident, the helicopter crashed because one of three control devices known as a "rotor control servo" became detached from its mounting bolt. The investigations also found a "severely worn" locking washer and improper torque on the locking nut used to secure the device.

"It is probable that maintenance personnel did not properly torque the locking nut when the servo was installed," the investigators said. "The accident is still under investigation and a probable cause has not yet been determined."

The NTSB investigators said the helicopter had undergone several "extensive and detailed inspections" after the servo was installed — the most recent only six flight hours before the accident — and the problem should have been found since it was "visually apparent."

"Because there was no effective quality assurance program in place at Heli-USA, maintenance errors were not detected, which led to accidents," the NTSB said. "Furthermore, the board found that most air tour operations do not have any quality assurance programs."

In the Arizona accident, investigators found that a hydraulic pump failed because the mechanics failed to apply enough lubrication during the pump's installation.

"Review of maintenance records revealed that the hydraulic pump had been installed less than 75 hours before the accident flight," the NTSB said.