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‘Power Settling’ Does Not settle MV-22 crash

“Power settling” into a “vortex ring” state appears to be the proximate, but not necessarily sole, cause of the April 8 MV-22 crash. The issue, however, is far from settled for the Marine Corps, which now must address why the pilots were flying outside of the flight envelope.

Indeed, that question cuts to the heart of Osprey pilot training. The incident, moreover, may have implications for aircraft redesign.

The doomed aircraft reportedly was descending between 1,700 feet and 2,000 feet per minute. That’s more than twice the recommended rate of descent——a maximum 800 feet per minute at a minimum 40 knots of air speed.

“Power settling is the inability to stop a rate of descent when the aircraft begins to settle into a vortex ring state,” says Lt. Gen. Fred McCorkle, the Marine Corps’ deputy chief of staff for aviation. “The vortex ring state occurs when the velocity of the downwash from the rotor is approximately equal to the rate of descent of the aircraft, causing the air to recirculate up and around and back down through the rotor system.”

Simply put, the air circulating down from the rotors is counterbalanced by air circulating upward. The consequent vortex ring results, in effect, in a rotor stall that is neither easily recognizable nor easily reversed. The aircraft, after all, has sufficient power. Thus it is not clear to the pilots why they are descending in a free-fall. Hence the description “power settling.”

Analysts say this perplexing situation can be even more puzzling in the Osprey, which, because of its twin rotor design, can be subject to an asymmetric vortex ring. This would precipitate a wing drop and yaw in addition to free-fall descent. This, in fact, is what happened April 8, service officials say.

In either case, the pilot’s instinctive response is to increase collective pitch for more rotor power. Similarly, to pick up a dropped wing, the pilot’s natural reaction would be to apply opposite stick movement.

Unfortunately, these actions, albeit reasonable and rational, only exacerbate his problems. In fact, that typically is the pilot’s first indication that he is in a vortex ring, analysts say. He increases collective pitch only to see his rate of descent increase.

The pilot, likewise, applies opposite stick movement only to yaw farther to the right or left. He is intensifying the speed of the vortex ring, but doing nothing to eliminate its mortal hold on his aircraft.

The solution is to break out of the vortex ring altogether by increasing forward speed. Partially lowering collective pitch can be helpful here as well.

Simulation Training

Recognition of the problem is key. Yet, it is doubtful that the pilot in command on April 8, Maj. John A Brow, ever knew he was enmeshed in a vortex ring. His flight experience was overwhelmingly in fixed-wing, C-130 aircraft. Power settling, though, is a problem that afflicts helicopters, not airplanes.

Maj. Brow had 85 hours in the MV-22 and 100 hours in the Osprey simulator versus nearly 3,800 hours of total flight time. However, none of his training or experience involved coping with a vortex ring.

“This type of simulation——of external flow-fields and interactions acting upon the body of the vehicle——is not commonly done, beyond basic wind and turbulence effects,” says the Naval Air Systems Command (NAVAIR) at Patuxent River, Maryland in response to a Helicopter News inquiry. But “we are certain that the lessons learned from the upcoming testing will be used to update the simulations.”

An area of concern is incorporated into simulation only when it is identified or anticipated, service officials say.

The best example would be pitch-up with side-slip. “Our simulations did not model this until the problem was encountered,” NAVAIR notes. “Intensive exploration allowed a model to be developed that was good enough for the simulation, and now the Marine and contractor simulations do model pitch-up with side-slip.”

Knowledge Gap

Although a rotorcraft phenomenon, a vortex ring-induced mishap bears some resemblance to a fixed-wing stall-spin accident. Thus the problem should not have been completely foreign to Brow.

Still, with just 280 feet of altitude at four seconds before impact, it is exceedingly unlikely that the pilot had either the time or maneuver room to take effective preventative action, even were he fully aware of his predicament, analysts say.

Clearly, there is a serious “knowledge gap” concerning the Osprey and vortex ring. While the phenomenon affects all helicopters, it may pose a special challenge to the V-22. That is because, as noted, the Osprey’s twin-rotor design can yield a hard-to-read asymmetric vortex ring.

Revised Flight Envelope

To address this “knowledge gap,” the Marine Corps plans to conduct special flight tests that will force the Osprey into a vortex ring. These will be conducted at high altitudes, where recovery is possible.

In this way, service officials hope to develop more appropriate flight parameters for the aircraft. Their aim, in so doing, is to prevent the Osprey, once deployed, from descending into a vortex ring.

The Marine Corps contends that had Brow remained within the published flight envelope——a maximum 800 feet per minute at a minimum 40 knots of air speed——he would have averted the vortex ring.

Military flight envelopes, however, constantly are being refined and updated as knowledge of aircraft performance capabilities grows. Concern about the Osprey’s susceptibility to an asymmetric vortex ring no doubt will accelerate this process. Changes in the aircraft’s flight envelope, therefore, can be expected relatively soon.

In fact, because of the accident, the Marine Corps now is requiring that the Osprey fly at least 200 feet apart horizontally and not descend within 50 feet of one another vertically.

Accident Raises Osprey Design Concerns

The MV-22 crash has raised concerns about the Osprey’s design, though the Marine Corps is reluctant to say so, given its heavy investment in the aircraft. Service officials fear that any acknowledgment of a design flaw or inadequacy would give critics of the Osprey the opening they have long sought to kill the program at last.

But military aircraft often are redesigned and improved to address new threats and newfound vulnerabilities. There is no reason to think that the MV-22 should be any different. Certainly, the death of 19 Marines provides the impetus at least to consider this issue.

Specific areas of concern include the Osprey’s twin rotor design, fuel cells and computer software.

Rotor Design

Reworking the twin rotor design is the most daunting challenge, but the one with the most promise, analysts say, since it could prevent formation of an asymmetric vortex ring.

Indeed, power settling reportedly is not much of a problem on the CH-47 and CH-46, which benefit from overlapping tandem rotors. Moreover, when a vortex ring does strike these aircraft, it does so symmetrically, which makes it far more manageable.

To be sure, it is difficult to see how the Osprey could function with overlapping tandem rotors. Any such design surely would impinge on the aircraft’s fuselage when in airplane mode. But some sort of analogous design improvement to avert the more vexing asymmetric vortex ring would be helpful.

Computer Software

Less dramatic but perhaps equally effective would be new computer software that would prevent the pilot from flying outside the flight envelope close to the ground.

Alternatively, the computer could be programmed to alert the pilot to dangerous maneuvers, such as too steep an angle of bank, that might precipitate a vortex ring. The pilot then would be free to accept or override the warning, thereby retaining complete flight control.

(The doomed Osprey reportedly got in trouble when it banked to the right 15 degrees at 280 feet of altitude four seconds before impact.)

This latter option is problematic simply because of the high number of computerized warnings in modern-day aircraft, says Lt. Gen. Fred McCorkle. At some point, he notes, the pilot has little choice but to disregard all such warnings. That point may already have been reached in the Osprey, he says.

Fuel Cells

The MV-22 fuel cells are designed to be crash resistant. However, they are not crash proof——particularly when the aircraft is so close to the ground, pointing nose straight down, in free-fall descent as it was prior to the April 8 crash. In such a situation, the fuel cells are bound to break, analysts say.

The Marine Corps is confident that the Osprey has the best available fuel cells and thus plans no redesign, McCorkle says. Still, he acknowledges that new and more crash resistant fuel cells are scheduled to come online with Production Aircraft Number 19.

Moreover, given the rapid pace of technological advancement, the day may not be far off when the MV-22 is equipped with fuel cells that are truly crash proof, analysts say.

Safe Aircraft

Despite these concerns, the MV-22 is “clearly a much better, much safer and much more capable aircraft” than the Vietnam-era helicopter types (mostly CH-46s) that it will be replacing, McCorkle says.

“Our men and women in uniform deserve the safest, most capable and most advanced equipment that our country can provide for them,” he adds. “The V-22 Osprey tiltrotor aircraft represents that effort.”

The Osprey have accumulated roughly 3,000 hours of flight time and flown for nearly eight years without incident. The aircraft’s two previous mishaps were unrelated to tiltrotor technology.

The last (1992) accident involved an engine fire that resulted from oil spillage. A 1991 MV-22 crash, likewise, occurred when two of three stabilizing gyros were wired incorrectly.

Human Factors at work in Osprey Crash

Human factors, not pilot error, appear be at work in the April 8 crash of an MV-22 Osprey. To the layman, this is a distinction without a difference since, either way, the pilot bears some culpability.

Pilot error, though, suggests complete culpability by the aviators. The implication is that they intentionally put the aircraft at risk in a burst of reckless machismo, Marine Corps officials say.

But no one suggests that Maj. Brow and his second-in-command, Maj. Brooks S. Grubber, were anything but highly responsible, model pilots.

“As aviators, you know, we all think we’re the best. But I’ve had numerous guys from his squadron tell me, ‘No, he was better than me,” McCorkle says.

“The idea that he was irresponsibly hot-dogging it is simply ludicrous,” adds Marine Corps aviation spokeswoman Capt. Aisha M. Bakkar-Poe.

Automobile Analogy

By way of example consider a car accident that results from hydroplaning. The driver is not exceeding the speed limit. Nonetheless, he spins out of control due to the interaction of his tire with the wet, oily road.

Is the driver at fault? To some degree, of course, yes. But was he careless, reckless and irresponsible? Not necessarily, and, if he has a reputation for good driving, probably not. Most likely, he is the victim of an unusually bad confluence of events to which he only partially contributed.

To be sure, Maj. Brow did exceed the flight envelope. However, he likely had good, mission-related reasons to do so, service officials say. In fact, these now are the focus of the accident investigation.

A taped conversation between the lead and crashed aircraft, in particular, should help to explain Brow’s actions. The use of night-vision goggles while using a head-up display also may have impaired pilot decision-making, investigators say.

Controversy

The dispute over “pilot error” versus “human factors” first arose when Helicopter News reported that “pilot error appears to be an integral part of the April 8 [Osprey] crash.” (See “Pilot Error Likely,” May 5, 2000.)

CNN then followed with a more emphatic declaration: “Investigators believe the crash… was caused by pilot error.” This infuriated the Marine Corps, which put out a statement strongly disputing this assertion.

CNN’s anonymous “‘Pentagon official’ is wrong,” said the Marines. “The fact is we are still looking at several possible causes, such as the effects of operating two aircraft in close proximity to each other in flight, and other heliborne flight characteristics.”

But as one reporter asked at a May 9 Pentagon press briefing: “If the pilot was operating the plane outside of the allowable parameters and there was an accident, then why is there no finding of pilot error?”

McCorkle responded that it is “actually illegal” for him or any other official to speculate about pilot error until the accident mishap board reports its findings. The Marine Corps also does not wish to impugn the integrity and performance of the pilots on the basis of preliminary and incomplete information.

Conclusion

The bottom line: the pilots surely bear some culpability for the accident. However, there appear to be legitimate extenuating circumstances for their actions. These actions, and the context in which they took place, are now being investigated. In addition, the accident likely has other contributing factors, which also are being examined.

Osprey Returning to Flight in Hopes of completing opeval

Marine Corps Commandant Gen. James L. Jones has lifted the temporary pause in MV-22 aerial operations and ordered the service to resume flights in three phases, beginning with the EMD (engineering, manufacturing and development) aircraft. These are flown by developmental test pilots responsible for determining the Osprey’s flight parameters and limitations.

“I feel confident we can put them back in the air. Everything appears to be have been working normally” at the time of the accident, Jones said last week.

The Marine Corps is anxious to return the Osprey to flight so that it can complete operational evaluation (OPEVAL). The service has until June 30 to complete this requirement and has little time to spare.

The second and third phase of the Osprey’s return to flight will involve production aircraft. These will be flown first with essential aircrew only and, later, with troops onboard. Gen. Jones and Air Force Chief of Staff Gen. Michael Ryan have said they will be onboard the initial troop training flight later this month.

The Air Force Special Operations Command is procuring 50 CV-22s, a variant of the Marine Corps MV-22.

During OPEVAL, the Osprey have flown safely more than 700 troops on 40-plus missions, service officials say. The Marine Corps will procure 11 MV-22s in FY00, 16 in FY01 and 360 altogether through 2015.