Investigators looking into the fatal crash of a Hughes 500 tour helicopter at Ha'ena hope that finding missing tail pieces will provide clues as to why it crashed.

The helicopter's tail section blew apart in flight, and witnesses saw at least two pieces fall to the ocean. Neither has been found.

The Inter-Island Helicopters aircraft, also designated a McDonnell Douglas 369, with a pilot and four passengers, crashed into the grass at the YMCA's Camp Naue, killing one man.

Passengers and witnesses reported hearing one or two loud bangs before the crash. At the crash scene, the entire tail rotor section was missing.

Rough surf on the north-facing shore has hampered efforts to find pieces in the ocean off Ha'ena Point.

"Police scuba divers searched for the tail rotor output shaft and tail rotor blades for two days, but were unable to locate the missing parts. High surf conditions then prevented divers from continuing the search for the parts. Once surf conditions permit, divers will continue to search the reef and shoals near Tunnel Beach for the missing tail rotor parts," wrote National Transportation Safety Board investigator Jim Silliman in his preliminary report on the March 11 crash.

Pilot Donald Torres told the NTSB that he had inspected the tail rotor section more than once that day without noting anything out of order. Among the inspection results were "no signs of cracks, dings, or delaminating of the surface and leading edges of the tail rotor blades," all nuts properly seated and no visible cracks in any of the parts of the tail rotor assembly.

The helicopter was being flown with the doors off, as is customary.

In some previous Hughes 500 accidents, tail rotors have failed when something from inside the cockpit has blown out and struck the tail rotor blades.

Torres told Silliman that he had checked before the flight to be sure there were no loose objects.

"I viewed the passengers that got on board, jackets were on, zipped up and secured, life-jacket pouches were on and secured, cameras were secured, and they had no hats or bags with them," the NTSB report quoted Torres as saying.

Torres said that after he heard the noise, the helicopter turned to the right and failed to respond to the control pedals. He said he realized that his tail rotor, which helps control the direction of the helicopter's body under the turning main rotor, had somehow failed.

Torres saw the lawn of the YMCA camp and directed the spinning helicopter toward it. He managed to reach the edge of it, but struck a large false kamani tree and landed on a fence, which shattered from the impact.

The helicopter ended up on its side, with one passenger dead and three injured.

Torres was not seriously hurt.

Federal Aviation Administration inspectors reported that rear-seat passengers said that they did not see anything fly out of the helicopter cockpit and did not see any birds that might have hit the tail rotor.

The initial inspection of the remaining portion of the tail rotor gearbox at the NTSB's Materials Laboratory found overstress fractures in remaining metal parts. But no cause for the stress that destroyed the rotor section has been identified.

Michael Gershon, 60, died in the crash.

The three injured passengers were Dania Hansen, 60, of Los Altos, Calif., and Douglas Barton, 60, and his wife Judy Barton, 51, of Newport, N.H.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.