WASHINGTON - The April crash of the Marine Corps' new V-22 Osprey aircraft in Arizona that killed 19 Marines was caused by "human factors," investigators said yesterday, primarily pointing to the pilot's rapid descent and slow engine speed, which caused the plane to stall and veer out of control.
Moreover, the pilot of a second V-22 that was flying ahead of the doomed aircraft contributed to the fiery crash, investigators said, because he did not follow the proper flight path and instead approached the landing zone at a steep angle while descending rapidly.
The pilot who crashed was "chasing" the lead aircraft when he spun out of control, the investigation concluded.
Marine investigators found no evidence of mechanical "failure or malfunction" in the crash of the V-22, a "tilt-rotor" plane that takes off and lands like a helicopter and can fly like a fixed-wing aircraft.
Some critics have questioned the safety of the plane, but the Marines said they have confidence in the craft and are pushing ahead with the $37 billion program to build 360 V-22 Ospreys to replace the Vietnam-era CH-46 helicopter.
The Air Force plans to buy 50 Ospreys. Eleven Ospreys are scheduled for delivery to the Marines this year.
The Osprey crashed during a training run at Marana Airport near Tucson. Although the Marines halted flights in April, they resumed exercises with the plane in early June.
Lt. Gen. Fred McCorkle, head of Marine aviation, said investigators found that a 10- to 15-knot tailwind was pushing both aircraft forward as they came into the airport, causing the pilots to reduce speed to avoid overshooting the landing zone.
Rather than abort the landing and circle for a second attempt, the lead pilot attempted to land. Neither crew "recognized the dangerous potential of the flight profile," the report said.
McCorkle said both the pilot and co-pilot of the lead plane were disciplined for contributing to the accident.
Although both are still authorized to fly, neither can command an aircraft for six months and each will have to requalify for that position, the general said. Neither the pilot nor co-pilot was identified.
The plane that crashed was piloted by Maj. John Brow, who McCorkle has said was recognized as one of his squadron's best pilots.
But Brow brought the Osprey in for its approach to the airport at 2,000 feet per minute - more than double the maximum safe descent rate of 800 feet per minute, investigators said.
Brow's rapid rate of descent and slow engine speed caused it to stall. The Osprey then lost lift and flipped over in the air before it crashed into the ground nose-first.
It was the worst aviation disaster for the Marine Corps since 22 were killed in a helicopter crash in South Korea in 1989.
Gen. James L. Jones, the Marine Corps commandant, said in a statement: "We have learned a great deal from this accident. The tragedy is that these were all good Marines joined in a challenging mission. Unfortunately, the pilots' drive to accomplish that mission appears to have been the fatal factor."
Investigators recommended that Marine pilot training emphasize the dangers of high rates of descent at slow engine speeds while continuing to study "recovery techniques" needed in the event of a stall.
Finally, investigators recommended a "warning system" to alert pilots to unsafe rates of descent and insufficient engine speeds.
McCorkle said the Marines will look into such a system, but he has been told by aviation experts that it would be "difficult to develop." He noted the Marines' fixed-wing aircraft have no such warning system.
McCorkle, who flew helicopters during the Vietnam War, said it would be more effective for a pilot to keep an eye on the flight indicators. "To me the warning system is 800 feet per minute on your ... rate of descent when you're below 40 knots," he said.
The April crash was the third for the Osprey since 1991, when a malfunction caused the plane to crash, although no one aboard died. In 1992, seven were killed when an Osprey crashed after it developed engine failure.
Some have questioned the safety of the plane's hybrid design, which combines a helicopter and an airplane.
As late as two years ago, the General Accounting Office, the investigative arm of Congress, said the plane's design "has not been stabilized."