Turbulence Is Blamed in Osprey Crash

The V-22 Osprey crash that killed 19 Marines last month occurred when unexpected turbulence caused the rotor on one side of the novel aircraft to lose lift, which tilted the plane dangerously to the right and quickly threw it into a fatal descent, Pentagon officials now believe.

There is no indication of recklessness by the pilot, Marine Maj. John A. Brow, who died in the April 8 crash at a civilian airstrip in Marana, Ariz., about 15 miles northwest of Tucson, a Pentagon official familiar with the inquiry said. Rather, by tentatively attributing the accident to “human factors,” he said, “we mean he was getting into flight parameters he hadn’t been in before.”

The V-22 is an unusual aircraft that takes off and lands like a helicopter but can tilt forward its two 38-foot rotors and fly horizontally like a conventional airplane. The Marine Corps is scheduled to publicly disclose some of what investigators have learned about the dusk crash at a Pentagon news conference Tuesday.

The accident occurred in the last moments of the V-22’s flight across Arizona from the Marine Corps base at Yuma, at the western edge of the state, as part of a Marine test that involved two V-22s flying in close formation. The V-22 had changed from airplane to helicopter mode, which it does by tilting its rotors so they point upward. It was trailing the other V-22 by about 200 feet and was descending rapidly, at a rate of about 1,700 feet a minute, the official said.

At an altitude of about 250 feet, it apparently encountered turbulence under its right rotor that neutralized that rotor’s ability to lift the aircraft, a second official said. Investigators at first believed the turbulence came from the lead aircraft but now think the first aircraft was too far ahead to have caused it.

“It’s called ‘power settling,’ ” this official said, explaining the prevailing theory of what caused the crash. “It happens when you’re in helicopter mode and the turbulence going up through the rotor equals the turbulence above the rotor, so you have no lift.”

It is not clear what happened at that point, but the V-22, which like most helicopters is top-heavy, probably began to pitch over, the first official said. “Once it gets to a certain degree of angle of bank, it actually starts to pull itself over, because the rotors are pulling you that way,” he said. “That’s the nature of helicopters.” He added that there is no indication that this is a problem unique to the Osprey.

The plane, almost on its back with its nose pointed down, then flew almost straight down, hitting the ground two to three seconds after flipping, the Pentagon official said. Everyone aboard was killed.

Whether the 19 Marines died on impact or in the subsequent fire remains unclear. Investigators are looking at the degree to which the aircraft’s fuel tanks were ruptured by the crash. That is significant because making the fuel cells sturdier could require some redesigning that could delay production of V-22s and reduce the fuel capacity and the range of the aircraft. Thus far, Marine Corps aviation officials have insisted they have seen no indication that any redesign is necessary.

Defense Department technicians are still analyzing the flight data, and the investigators’ conclusions are not final, the first official noted. “It’s still a theory,” he said of the explanation that turbulence threw off the pilot and led the aircraft to flip, “but it’s the only one that makes a lot of sense.”

The Marine Corps plans to announce at the Tuesday briefing that it will conduct tests, starting later this week, at the Patuxent River Naval Air Station in Maryland to gauge this and other theories on the crash.

In recent days, the families of the Marines lost in the crash have been informed of these tentative conclusions, this official said. “They’re still in a state of shock,” he said. “They’re appreciative, but it is still very painful to them.”

The V-22, which is built jointly by Bell Helicopter Textron Inc. and Boeing Co., is a controversial aircraft in part because three have crashed. In the previous lethal crash, seven people died in July 1992 when an Osprey plunged into the Potomac River near Quantico Marine base in Virginia.

The plane also has attracted some skepticism in Congress because of its price tag. According to the Congressional Budget Office, the V-22 costs at least $60 million each, which is expensive compared with conventional helicopters. The Marines plan over the next 10 years to replace all their troop-carrying helicopters with the V-22, ultimately buying about 360 of them.