Thursday, April 1, 2010
Adapted from 'The Dream Machine: The Untold History of the Notorious V-22 Osprey'
Rotor & Wing contributing editor Richard Whittle has written a history of the V-22 Osprey. The book, which Simon & Schuster will release on April 27, can be ordered now on Amazon.com. Rick agreed to share an excerpt in advance with R&W readers.
The MOTT had done similar missions before. But this time, after the first two aircraft approached the airfield and tilted their rotors upward to land, the second Osprey snapped into a right roll and plowed into the ground with its belly up. It exploded in a fireball that lit the evening sky for miles. The pilots, Major John A. “Boot” Brow and Major Brooks S. “Chucky” Gruber, as well as two enlisted crew chiefs and fifteen infantry riding in the back of their Osprey, were killed instantly. The first Osprey, piloted by Major James M. “Lefty” Wright and Major Anthony J. “Buddy” Bianca, was nearly lost as well. Wright, Bianca, three enlisted crew chiefs and eighteen Marine infantry in the back of their Osprey survived a hard landing that left them shaken.
McCorkle was in a good mood when he returned to the Pentagon press briefing room next, on May 9. He was there to confirm, as CNN had reported five days earlier, that the crash investigators had found nothing wrong with the Osprey. Using charts on an easel, McCorkle described how the accident had unfolded. He didn’t name any of the pilots, but he explained that those flying the first Osprey into Marana, Wright and Bianca, had missed their 3,000-foot checkpoint and begun their approach into the airfield late. Rather than wave off and go around, they had come down “high and hot,” forcing Brow and Gruber to follow in what none of the pilots seemed to sense was a pell mell descent. McCorkle’s third chart traced Brow’s speed and altitude over the last seconds of the flight, as reconstructed from his Osprey’s Crash Survivable Memory Unit, a device that records airspeed, altitude, and other data. “In summary, the data shows that the mishap aircraft was in a high rate of descent at a relatively low forward airspeed,” McCorkle said. “These characteristics can lead to a condition known as power settling, or vortex ring state.”
Here Be Dragons
With that, McCorkle introduced a new, and for many people occult-sounding term into the debate over the Osprey. “Vortex ring state” was esoteric not only to the general public but in the aviation world as well. It was a term from fluid mechanics, an expression even an aeronautical engineer could go a career without hearing very often. Vortex ring state described a condition a rotor could get into as a result of what Navy and Marine Corps pilots called “power settling” and Army and Air Force pilots knew as “settling with power.” What it all amounted to was a way to describe a rotor that was no longer creating thrust and lifting as it should because it was descending into its own downwash too quickly.
|An MV-22B Osprey lands outside the city of Marjah, Afghanistan in February. Photo by Lance Cpl. Justin Loya|
When an aircraft descends too quickly, a pilot’s natural tendency is to add power to arrest the descent, but in vortex ring state, adding power produces the opposite of the desired effect. In a rotorcraft, adding power also adds pitch to the rotor blades, increasing the angle at which they hit the air. Adding power and pitch to a rotor in vortex ring state only creates more disturbed air and pulls it back through the rotor disk faster. Instead of flowing as it should and creating thrust, the air begins bouncing in and around the rotor disk chaotically and the aircraft loses lift even faster. Helicopter pilots are taught about the hazard and told that the best way to get out of it is to reduce power to slow the rotor while flying into “clean air” by tilting their helicopter’s nose forward. That tilts the rotor forward and increases forward airspeed. Without sufficient altitude, though, there may be too little time to do that, so pilots are urged simply to steer well clear of vortex ring state by never descending too quickly at slow speeds. It’s the rotary wing aviation equivalent of an ancient cartographer writing on a map “Here Be Dragons.” Helicopter pilots learn the rule and avoid that edge of the flight envelope out of habit. They simply don’t go there.
$50 Million and Time
The combined rate of descent and speed at which any particular rotorcraft might go into vortex ring state depends on various factors—the size and shape of its rotors, its flying weight, wind conditions, etc. The rule of thumb for most helicopters, though, is to avoid descending faster than 800 feet per minute when flying forward at 40 knots or less. Where the actual line existed for the Osprey was something the program’s developmental test pilots hadn’t determined. Hundreds of test flights to explore that part of the Osprey’s envelope had been planned. Nolan Schmidt, the program manager and a Marine Corps colonel at the time, told me years later that those tests were scrapped in 1998 to save time and money. The Navy Department was going to cut the Osprey program’s budget for the coming fiscal year by $100 million, Schmidt said. After consulting with the Boeing engineer in charge of flight testing, Philip Dunford, Schmidt said, the program managers decided they could save about $50 million and a lot of time if they didn’t do all the tests planned for the Osprey at high rates of descent. A few tests already done had indicated that the Osprey could safely descend at 1,200 feet per minute below 40 knots airspeed anyway, so Navair and company managers decided to simply adopt the standard rule for helicopters. It was enough to know there were dragons lurking beyond a descent rate of 800 feet per minute at 40 knots or less, so why test? “There was no practical need for any rotorcraft that we knew of to have to exceed that boundary anyway,” one of the Osprey’s developmental test pilots in those days told me. The flight clearance placard setting the limits on how the MOTT could fly the Osprey simply adopted the standard from helicopter flight manuals. “Avoid descent rates of 800 fpm (feet per minute) or greater at airspeeds less than 40 KCAS,” it said, “KCAS” being the abbreviation for “knots calibrated air speed.” Precisely what might happen if a pilot flew beyond that edge of the envelope wasn’t spelled out in the MOTT’s flight clearance placard, but there was no need to go into detail. The rule was labeled a “Warning,” which meant failing to observe it risked loss of the aircraft and death. For pilots, “Warning” is the flight manual equivalent of a skull and crossbones.
At his May 9 news briefing, McCorkle was asked whether the Osprey that crashed had been “descending within the flight envelope.”
“He was descending over a thousand feet per minute,” McCorkle said, noting that the recommended limit was no more than 800 feet per minute at 40 knots or less. Why the pilot had descended so fast was still being studied, McCorkle said. The right rotor of the Osprey, he added, might have gone into vortex ring state only when the pilot moved his control stick to the right four seconds before the crash. “I think that that’s probably what caused it,” McCorkle said.
“Is it correct to say that pilot error is to blame for this accident?” a reporter asked.
“No, it’s not correct to say that,” McCorkle said. “I would really appreciate it if you wouldn’t speculate on that until after the mishap board has reported out. We feel like we have enough information to say that it was not mechanical. There are a lot of things the board is looking at. And why was the pilot at this rate of descent? Why was he in this position at this time? And it would be very inappropriate at this time to say that it was pilot error.”
The main thing McCorkle wanted people to understand was that the crash wasn’t being blamed on a flaw in the Osprey. Developmental test pilots at Pax River were going to do a full range of tests to explore where the edge of the Osprey’s envelope for going into vortex ring state lay, he said. Meanwhile, OPEVAL would go on. “We have waited really a heck of a long time to put the airplane back into the air,” McCorkle said.
The Right Stuff
Once upon a time, test pilots like the famed Chuck Yeager tugged on a helmet and squeezed into a flight suit, strapped on an X-model aircraft, then bet their lives they could find the edge of its envelope without having to punch out or ride the experimental machine into the ground. Those days are long gone. Today’s test pilots still fly to the edge of the envelope. They still need “the right stuff”—steady nerves, quick reflexes, maybe a little swagger in their walk. They still climb into new or modified aircraft and try things no one else has ever done with them. Today’s test pilots, though, no longer routinely fly into the unknown. They don’t even fly as often they work on the ground. A test pilot today is more likely to be found manipulating a keyboard and mouse than a stick and throttle. He—they are still mostly men—is less likely to be sitting in a cockpit than in a meeting with engineers or at a desk, analyzing data and planning flight tests in meticulous detail.
Each step in a modern test flight is scripted on a “test card,” which is carried on a clipboard strapped to the pilot’s knee for easy reference while airborne. After a flight, the pilot writes a detailed report, which he and the engineers will go over in yet another meeting. The report may rely in part on tape recordings of observations the pilot spoke into a microphone as he flew. It will certainly rely on data gathered during the flight by onboard instruments that instantly and constantly record not only the machine’s altitude, speed, pitch, yaw, roll, and other motions but its every moan and groan. The instruments radio the data in “real time” to a platoon of engineers on the ground whose job is to assess the results as they come in and watch for signs of danger. The engineers will warn the pilot to knock it off if they see him flirting with trouble. The era when the military was willing to risk a pilot’s life or a multimillion-dollar aircraft in a test faded into history with the advent of computerized flight simulators. Today, pilots “fly” dangerous tests first in SUV-sized machines that mimic real flight with astounding fidelity, yet without ever leaving the ground. Equipped with the same cockpit as the aircraft to be flown, the best of such computerized simulators can project on a screen beyond a mock windshield a virtual-reality display of nearly any geographic location as seen from any altitude. Runways, buildings, mountains, rivers and trees are depicted faithfully, in living color, the way they would appear from an aircraft in a climb, a bank, a dive. Some simulators ape the motions of flight faithfully enough to produce vertigo and airsickness. Based on calculations fed into their computers, simulators can be programmed to replicate how an aircraft will behave in almost any circumstance. Flying the aircraft itself is the only way to prove the calculations were correct, but as a rule, test pilots today aren’t betting their lives when they go out and fly.
The Osprey’s High Rate of Descent tests were an exception to that rule. The pilots who flew them would venture into that region on the map marked “Here Be Dragons.” You could die there.
The HROD tests would be dangerous because vortex ring state was largely a mystery. The phenomenon was hard to describe, much less predict, partly because so little research had been done on it in the six-decade history of the helicopter. Beyond that, no one had ever devised an entirely reliable way to predict all the air flows created by rotors. Like the weather, there was a nearly infinite number of variables that could affect them—the configuration of the aircraft, the speed and direction of the wind, the aircraft’s altitude, the rotor’s angle of attack. The flows around an airplane wing, which moves into clean air constantly, can be predicted with great accuracy. The flows around a rotor, which moves in a circle but might be stationary or moving forward or backward or sideways as it rotates, are devilishly more complex. This makes engineering rotorcraft an art as well as a science. The pilots and engineers in the Osprey program were certain the aircraft’s high disk loading made it far less likely to go into vortex ring state than critics assumed. The only way to prove it, though, was to take an Osprey aloft and go find its vortex ring state boundary. Based on calculations and the tests done in 2000, the engineers and pilots were confident an Osprey could get out of vortex ring state simply by tilting its rotors into clean air. If they were wrong, though, someone could die trying.
The Osprey suddenly snap-rolled right more violently than Macdonald had ever seen it do. Instinctively, he pushed the stick left as the roll began but the Osprey didn’t respond.
Tom Macdonald, as the Osprey program’s chief test pilot, decided he should be the pilot-in-command for each of the dozens of flights planned in the second series of HROD tests. His copilot most often would be Steve Grohsmeyer of Boeing or Bill Leonard of Bell, though Macdonald always would have the controls. Only the two pilots would be aboard; there was no reason to risk more lives by having engineers or crew chiefs in the back.
The tests began only six months after the Osprey’s return to flight partly because engineers had to come up with a piece of hardware to accurately measure the Osprey’s forward speed at less than 40 knots. Then, from November 2002 to July 2003, Macdonald and his copilots did sixty-two HROD tests, flying a total of 104 hours over a restricted flight range between the towns of Cambridge and Salisbury on Maryland’s eastern shore. Macdonald would climb to 10,000 feet, then tilt the rotors up into helicopter mode and slow to a target airspeed. As Macdonald held it there for a bit, a challenge in itself, the copilot would put his hands on his own set of controls, just in case. To duplicate the way the Osprey that crashed at Marana had been flying, Macdonald would lower the landing gear, tilt the nacelles back to 95 degrees, and start reducing power. The Osprey would begin to drop. Macdonald would let it fall until they hit a target rate of descent, then adjust the power to hold the Osprey at a steady sink rate. At that point, still descending, Macdonald and his copilot would talk into their microphones about the Osprey’s stability and handling, the quality of the ride, and whether they heard anything unusual. They were looking for ways a pilot might detect the onset of vortex ring state before getting into it. They repeated the flight profile over and over during those months, taking their measurements at every 500-foot increment of sink rate at a series of ever slower airspeeds.
As predicted, they found it hard to put the Osprey into vortex ring state. The boundary for a helicopter was a sink rate somewhere around 800 feet per minute at 40 knots or less of forward speed. Macdonald had to let the Osprey sink at least 2,500 to 2,600 feet a minute at 40 knots before it would near vortex ring state. Even at slower forward speeds, the boundary was a sink rate of around 1,700 feet per minute. He and his copilots weren’t actually trying to lose control of the Osprey, they were just trying to find the edge of the envelope, the point where they felt vortex ring state coming on. They got there a number of times without incident. “The thrust would begin to oscillate,” Macdonald told me. “A little up and down motion would settle into the aircraft that we weren’t commanding.” The Osprey would rock a little bit in some cases, and at the lowest speeds they tested, as slow as 10 knots forward speed while plunging toward the ground, the pilots would hear “this eerie howling and audio sound of the air rush,” Macdonald said. “We’d just get really silent and quiet.” Eleven times during the tests Macdonald flew, one of the rotors went into vortex ring state and the Osprey did a sudden, uncommanded roll—seven times to the right, four times to the left. Each time, Macdonald was able to recover by pushing the thumbwheel switch to tilt the nacelles forward and put the rotors into undisturbed air. Two seconds was usually all it took. Those seconds, though, gave Macdonald and his copilots some hairy moments.
Sink Rate, Sink Rate
The hairiest was on July 17, 2003. The tests already had achieved their primary goal—charting the Osprey’s envelope for vortex ring state—a couple of months earlier. Based on that envelope, the program had developed warning devices for the Osprey to alert pilots when they were flirting with vortex ring state. One was visual, a red light on the control panel in front of each pilot that would flash SINK, SINK if an Osprey exceeded a safe rate of descent. The other was a recording of a woman—a device pilots call a “bitching Betty”—who would be heard in the pilot’s headset saying “sink rate, sink rate” in an urgent monotone if the limit were exceeded. The HROD tests were continuing, though, because Navair was trying to satisfy key critics that the tests had been adequate. That July 17, Macdonald and Grohsmeyer were flying 7-10 knots and descending at more than 2,300 feet per minute, well beyond the vortex ring state envelope already established. They were dropping like a rock while a special test device alternated the lateral tilt of each rotor to see if that would prevent vortex ring state. Macdonald was on the radio with an engineer on the ground when the Osprey suddenly snap-rolled right more violently than Macdonald had ever seen it do. Instinctively, he pushed the stick left as the roll began but the Osprey didn’t respond. By the time Macdonald recognized what was happening, the Osprey was flying on its side, left wing up and right wing down, and spiraling toward the ground ever faster. Macdonald pushed the thumbwheel to tilt the nacelles forward as fast as they would go, all the way to airplane mode. The seconds it took seemed like minutes, but finally he regained control and straightened the Osprey out. For a moment, there was silence in the cockpit and over the radio. Macdonald, Grohsmeyer and the engineers on the ground knew the pilots had just had a close call. If the Osprey had rolled all the way upside down in helicopter mode, there was no telling what damage they might have done to the aircraft—or whether they could have regained control at all.
After Macdonald and Grohsmeyer landed, the program office and the pilots decided they had taken the HROD testing of the Osprey far enough. The Osprey’s boundary for vortex ring state had been established beyond any doubt, and studied more than that of any rotorcraft in the world.
Three months later, at a dinner in Los Angeles, the Society of Experimental Test Pilots gave Macdonald its highest honor, the Iven C. Kincheloe Award for “outstanding professional accomplishment.” The citation said Macdonald had flown “every flight in a test program where no test pilot has been before.” Chuck Yeager was in the audience that evening. He shook Macdonald’s hand.