Much of the debate about the safety of the V-22 stems from the Arizona crash. What happened that April night shocked helicopter experts. The pilots were trying to land when the V-22, which was about 300 feet off the ground, rolled over on its back and plunged into the ground. The Marines' investigation of the crash quickly concluded that the pilots allowed the V-22 to descend too quickly with too little airspeed. As a result, the plane got caught in its own rotor wake, a condition called vortex ring state that causes a rotor to lose lift or to stall. The phenomenon has been blamed for helicopter accidents only rarely and previously received little scientific attention. The official crash report concluded that probably just one rotor underwent vortex ring state and stalled. Within a couple of seconds, that caused the plane to go into the roll, and the pilots had no chance to recover.
It had never occurred to anyone that just one rotor might stall and cause the V-22 to roll. "It's something was that was not on our radar screen at all," said MacDonald, the flight test pilot. (The Marines' handling of the crash investigation is the subject of an inquiry by the Pentagon's inspector general, after a Marine officer alleged improper actions by other officers. A Marine spokesman says the allegations do not affect the conclusion about the cause of the crash.)
In the months after the crash, Bell and Boeing began the special flight tests at Pax River to explore the vortex ring state. That testing was only partially complete when all V-22s were grounded after the December 2000 crash. Bell and Boeing, in their reports to the review panels, say the tests show that the V-22 can recover from a roll if the pilot simply rolls the engines forward to gain speed. But that can take seconds and hundreds of feet of altitude - time and space that even MacDonald concedes may not be available. "If you're close to the ground, it's a bad thing," the Boeing pilot says.
Based on the limited flight testing, the Marines and Bell/Boeing insist that the vortex ring state will not be a problem if the V-22 is flown within the prescribed limits. The V-22 flight manual says pilots should descend no faster than 800 feet per minute when flying at speeds of 40 knots or less - a slow, gradual descent not unlike that of a commercial airliner. MacDonald agrees with that guideline, although he says much more flight testing is needed.
"We're going to do a whole lot more of that when we get back to flying," he said. But the experts who've raised concerns about the V-22 say other test data presented in reports to the review panels strongly indicates that controlling the rate of descent does not eliminate the vortex ring state.
Flight simulator tests show that even with much slower descent, trying to maneuver a V-22 abruptly - making a hard turn or a sudden pull-up to abort a landing - could cause it to roll. Evidence of this was included in a special presentation that NASA's McDonald made to Aldridge in August. That presentation, which has not been made public but was obtained by the `Star-Telegram,' shows that in a simulator test the boundary between safe flight and vortex ring state all but disappears when a maneuver is attempted. Still other simulator tests show that when a pilot attempts an abrupt maneuver, the V-22 may spin out of control in as little as two or three seconds.
That's not much of a margin for error, says Coyle, whose office presented the data to the Blue Ribbon Panel. "A point we made ... was when a pilot is doing a quick stop maneuver, a pull-up, it's because something else is going on," Coyle says. "He's being shot at or he's trying to rescue someone. He's concentrating. If in the midst of that concentration something else occurs, he may not be able to react." In the Arizona accident, the pilot flying the V-22 did not lose control until he tried to turn the airplane to stay in formation.
Boeing's MacDonald concedes that, in theory at least, maneuvers could narrow the safety margin. "We're not sure," he says. "We don't believe it's significant. But we're going to test the hell out of that." When shown the charts that suggest that hard maneuvers could cause the V-22 to stall and roll, several retired Bell test pilots and engineers who worked on the V-22 said the aircraft was not designed for hard maneuvers and should not be flown that way. But Grady Wilson, a combat veteran of the Vietnam War and a former V-22 test pilot for Boeing, said that telling an Osprey pilot he can't maneuver in the heat of battle or even the chaos of military operations is unrealistic.
Wilson agrees that the V-22's maneuverability needs to be thoroughly tested. "As I flew it, we babied it. You had to be really gentle with it." Leishman, the Maryland professor, used a computer to simulate what happens with the rotors of helicopters and an aircraft like the V-22. His findings, in a paper recently published by the American Helicopter Society, show that a tilt-rotor aircraft has more complex aerodynamics than a helicopter. During rapid descent or maneuvers, he believes, it's typical for one rotor to get caught in the other's wake. When that happens the two rotors produce widely varying amounts of thrust.
"The thrust fluctuations lead to the roll response" long before the aircraft should reach vortex ring state, says Leishman, who briefed the NASA panel on his research. Leishman, Coyle and others say the V-22's problems may be related to something less exotic than vortex ring state: a propensity for the rotors to stall easily because they cannot generate enough extra thrust at critical moments to maneuver the airplane. Bell and Boeing engineers, in a written response provided to the `Star-Telegram,' denied that the V-22 is more prone to stall than helicopters. MacDonald says the V-22 will be capable of combat maneuvers. "We do expect people to be under duress in the operational world," he says, "and the aircraft has to be forgiving" of pilot mistakes. "The penalty for a good faith violation of a boundary can't be the loss of an aircraft and the crew," MacDonald says. A Marine spokesman, Capt. David Nevers, says the Marine generals still believe that the V-22 is safe and the right aircraft to replace old troop-carrying helicopters. "Our faith in this aircraft has not wavered," says Nevers, although he adds that the Marines "recognize there is more to learn." MacDonald says the upcoming flight tests, which could take 18 months to two years, will answer the questions and, he believes, show that the V-22 can perform all the duties of comparable helicopters "and then some." The tests to check out the vortex ring state and maneuver issues, are not scheduled to begin until late summer or fall, when a specially modified V-22, heavily instrumented for the purpose is ready to fly.
Bob Cox, (817) 390-7723
[email protected]
Briefings to the Blue Ribbon Panel are available at
www.acq.osd.mil/sts/v22/archives.htm
Professor Leishman's paper on vortex ring state:
www.enae.umd.edu/AGRC/aero.html.
From this link