'Sudden dive' preceded fatal crash
Tue, Jul 27, 2010
The Yomiuri Shimbun/Asia News Network

Testimony by two crash survivors that the helicopter they were riding in suddenly made a vertical dive just before a deadly accident is getting special attention by the Japan Transport Safety Board, as it started its official investigation Monday.

Three aircraft accident examiners from the board began their investigation in the morning by flying over the crash site in a mountainous area of Chichibu, Saitama Prefecture, where a prefectural rescue helicopter crashed Sunday, killing five crew members.

In addition to the board examiners, other investigators were intrigued by the survivors' statement that the helicopter suddenly dove to the ground just before the crash. Officials from the prefectural police headquarters also questioned the two survivors.

The three board examiners spoke with police officials and headed to the accident site aboard a police helicopter shortly after 10 a.m. Before departing, one of the examiners told reporters, "When an accident occurs, traces are always left at the site. We'll first look at facts such as where the aircraft hit [the ground]."

The testimony was given by Sakae Ota, a member of the prefecture's disaster-prevention air team, and Jun Kimura, a firefighter, who had descended from the hovering helicopter via wire ropes just before the crash.

They told officials that the helicopter descended when the two were one meter above the ground, then their bodies suddenly dropped another 50 centimeters and were thrown to the ground. They also said they heard a flip-flop sound that they had never heard before. The survivors said the weather at the time was fine at the accident site and there was no sign it would worsen.

Ota was dispatched from Warabi city firefighting headquarters to the disaster-prevention air team in April last year. Kimura was on loan to the prefectural disaster-prevention air team for three years until the end of March. Because the two are experts on air rescue operations and have considerable experience on rescue helicopters, investigators give great weight to their testimony.

Helicopter experts also noted the pair's testimony that the helicopter fell suddenly and sharply just before landing.

Akira Azuma, professor emeritus at the University of Tokyo and a former aircraft accident examiner, said, "The air becomes thin in high altitudes on hot days."

"In this situation, a [helicopter] engine doesn't have any reserve power," the flight dynamics researcher said. "I assume that they were near the maximum altitude for hovering, causing the engine to lose power. As the aircraft began to fall, it entered an air vortex made by its own main rotor and lost control."

A former helicopter pilot of the Ground Self-Defense Force agreed, saying, "To get out of that kind of situation, the only thing to do is to speed up and fly forward. If the head of the helicopter was facing the mountain, the pilot might not have had a way out."

Prof. Nobumasa Unemoto of Teikyo University, an aerospace researcher who has experience flying helicopters, said the flip-flop sound the survivors heard is not abnormal.

"[That kind of sound] seems to be a phenomenon caused by a change in the number of rotations of the main rotor. But I can't say that this phenomenon itself indicates a problem with the helicopter's body," he said.

- The Yomiuri Shimbun / Asia News Network

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