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More woes for Osprey.
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V-22 Special Report Main Page
Rotorcraft's hydraulics leaks go back nine years
MARINES' OSPREY HAD FLIGHT INCIDENT IN 1992 SIMILAR TO DECEMBER CRASH
Inside the Pentagon, May 17, 2001 -- Pilots test-flying the Marine Corps' V-22 troop transport experienced a flight incident in 1992 involving conditions similar to those that led the troubled rotorcraft to crash last December, killing all four service members aboard, Inside the Pentagon has learned. A rupture in the hydraulics system providing vital fluid to the left engine nacelle -- which governs the critical tilt of the Osprey rotor blades -- occurred in both incidents, further aggravated in each case by faulty logic in the computer that largely controls the rotorcraft.
Although problems with the V-22 hydraulics and its computer system have been in the spotlight in recent weeks, until now it was not widely known just how long chafing of the hydraulics lines -- which leads to potentially critical leaks -- has plagued the aircraft.
The V-22 is a technologically complex aircraft that flies like a fixed-wing turboprop plane, but takes off and lands like a helicopter. Marine Corps leaders say the tiltrotor aircraft will be vital for quickly ferrying large numbers of troops in and out of battle areas.
The Marines' official investigation of last year's North Carolina crash, released last month, describes a series of events in which electrical wire bundles chafed a hydraulic system pipe within the Osprey's engine nacelles, leading to a serious rupture and leak.
A highly experienced flight crew acted fast to correct the problem -- using the rotorcraft's computer system, as they were trained -- but their repeated button-pushing backfired. A previously undiscovered glitch in the software made the V-22 impossible to sustain airborne, and just 30 seconds after the first sign of a hydraulic system leak emerged, the rotorcraft plunged into a dense forest seven miles north of the Marines' New River, NC, home base.
According to archival incident reports obtained by ITP, such hydraulic problems go back at least as far as May 1992, when the V-22 -- then in “full-scale development” -- experienced an in-flight leak nearly identical to the one that precipitated the crash five months ago.
Sources close to the program say the V-22's engineering and airframe design have not changed appreciably in the ensuing years, although prime contractor Bell-Boeing has improved the cockpit and further developed the software. Claims by some Osprey advocates that the design of the current production model of the V-22 is wholly different from the earlier FSD version are sharply disputed.
The Marine Corps was unable to provide comment on this story by press time (May 16).
An early incident
On May 27, 1992, a V-22 “experienced an incident” at Boeing Helicopters' New Castle, DE, flight test center as the crew was hovering in helicopter mode at about 50 feet and getting ready to land the aircraft, according to a June 4, 1992, incident report signed by the pilot, Patrick Sullivan. “The No. 1 hydraulic system pressure line to the left inboard swashplate actuator chafed through and caused a severe hydraulic leak,” the report stated.
When the V-22 is in helicopter mode or transitioning to fixed-wing mode, pilots control the aircraft by varying the tilt of the rotors, using the swashplate actuators. To keep functioning throughout a flight, the actuators require a constant influx of hydraulic fluid, maintained at high pressure. In the V-22, each of two engine nacelles receive hydraulic fluid from both the No. 1 and No. 2 pumps. The No. 3 pump provides fluid to lower the landing gear and lower the ramp that drops out the V-22's tail to offload Marines, but it also serves as a backup in case of a failure in the No. 1 or No. 2 lines, according to experts on the rotorcraft.
A May 27, 1992, report on the same incident, signed by the flight engineer, attributed the chafing of the No. 1 line to a nearby “electrical connector.”
During the flight, a sensor in the aircraft detected the leak in the No. 1 hydraulic system, and the onboard computer directed the rotorcraft's No. 3 backup hydraulic pump to augment the left actuator with fluid.
But a quirk of the V-22's “triple-redundant” hydraulic system is that the No. 1 and No. 3 pumps actually share about 6 feet of piping as they near each actuator, according to V-22 experts. In this incident, the rupture occurred in a section of pipe shared by the two pumps, so the fluid from the No. 3 hydraulic system sprayed out of the very same hole, according to Sullivan's report.
However, the computer initially failed to alert the aircrew to the depletion of No. 3 hydraulic fluid reserves because of another idiosyncrasy of the computer software “logic” at the time of the incident, when the V-22 was still very much in development. Because the Osprey was coming in to land at the time, its gear was down, and the landing gear uses considerable quantities of the No. 3 system's hydraulic fluid. As a result, “if the landing gear is down . . . the No. 3 system is not monitored for excessive rate of depletion,” the Boeing test-pilot explained in the report.
It took 28 seconds for the V-22 computer to react to the No. 3 line problem, closing the appropriate valves to stop the loss of pressure from that pump. “But by then, the No. 3 system quantity [of fluid] was essentially depleted. Any fluid remaining in system No. 1 [was also] then depleted,” reads the report.
Sullivan commented after-the-fact that “the leak detection/fault isolation logic appeared to function as designed,” implying that the computer programming itself needed to be changed. The software “did not correctly analyze the leak after [the] No. 3 system was powering the left No. 1 swashplate actuators, which allowed the No. 3 quantity to be depleted,” he wrote in the incident report. Initially the aircraft computer had advised him to return to home base, only subsequently raising the level of alarm and instructing him to “land as soon as possible [at the] first available field.”
“Had the leak been analyzed in a more timely fashion with respect to [the] No. 3 system, the No. 3 system would have been retained,” he wrote. “The redundancy management of the system must be improved.”
Being so close to the ground when the in-flight emergency began, the veteran pilot was able to safely land the V-22 without further damage, according to a source familiar with the incident. But Sullivan and six others would die a month and a half later when another Osprey he was piloting crashed in Quantico, VA. The July 20, 1992, accident was attributed to a leak of transmission fluid into an engine. When all four V-22 crashes are taken into account, 30 crew members and passengers -- mostly Marines -- have perished.
Fast-forward eight and a half years to Dec. 11, 2000. The in-flight emergency leading to the North Carolina crash plays out in hauntingly similar fashion, albeit with some notable differences.
Same leak, same pipe
Flying at 1,600 feet, the V-22's aircrew “experienced a hydraulic system No. 1 failure . . . caused when a hydraulic line, located in the left nacelle area, was weakened by chafing [from a] wire bundle, and ruptured under pressure,” reads the Marine Corps' official accident investigation report, released April 5. “The No. 3 hydraulic system normally serves as a backup to certain flight control systems (in this case the left swashplate actuator).”
Up to this point, “these are identical failures,” said one source close to the V-22 program, interviewed this week on condition of anonymity.
But it appears that the V-22 program learned the lessons of the early flight tests in this regard because here the two incidents diverge: The computer now cuts off hydraulic fluid from the No. 3 hydraulic system when a leak is detected in the common piping. “In that [hydraulic system No. 3] uses the same tubing as [No. 1], it could not continue providing fluid to the swashplate actuators since this leak could not be contained (this would eventually lead to the loss of [No. 3]),” the December accident investigation report observes. “The hydraulic leak detection/fault isolation logic worked as designed, resulting in an appropriate system response that prevented [hydraulic system No. 3] from switching into the damaged hydraulic tube.”
But a new problem emerged. The computer software's “isolation logic generated a condition where the swashplate actuators in the left nacelle were now being powered by only one hydraulic system (No. 2) while the swashplate actuators in the right nacelle were being powered by two hydraulic systems (No. 2 and No. 3),” according to the accident report. “This resulted in the left swashplate actuators moving at a slower rate than the right swashplate actuators.”
Marine Corps investigators found that this hydraulic-system anomaly alone “would not normally have caused an aircraft mishap,” according to Maj. Gen. Martin Berndt, commanding general of the 2nd Marine Expeditionary Force, who briefed reporters on the V-22 accident report on April 5. But the hydraulics failure prompted “a series of rapidly cascading warning indicators” to light up in the cockpit, and the pilot responded with the “published procedure,” namely to “press the primary flight control system reset button.”
But a “software anomaly caused significant pitch and thrust changes in both prop rotors,” said Berndt. “Because of the dual hydraulic failure on the left side, the prop rotors were unable to respond at the same rate.” Repeated pressing of the reset button -- by a pilot who undoubtedly did not realize the accepted procedure was actually making the situation worse -- forced the V-22 into wildly uncontrollable movements that some have compared to a bucking bronco, which, said Berndt, “eventually stalled the aircraft.”
While the manageable problem that affected the test pilot in 1992 was ultimately corrected, computer programmers had inadvertently introduced a new, more serious lapse in logic into the V-22: A serious hydraulic line leak can lead to an imbalance of fluid between the two rotors, and is compounded by a computer reset process that further destabilizes the rotorcraft.
Déjà vu times two
“What's remarkable here is not only a hydraulics failure [in 1992] that's similar to the December crash, but a software failure that made the incident worse, as in the December crash,” said Philip Coyle, who until last January served as the Pentagon's top official for testing military equipment, in a May 14 interview.
By the time Coyle finished overseeing the V-22's eight-month operational evaluation last July, he had documented 177 safety-related failures in the Osprey's “flight-critical subsystems,” InsideDefense.com reported Feb. 8. Among those were 39 hydraulic power system failures with potential safety implications, far outnumbering the problems found in other V-22 subsystems and alone representing nearly a quarter of the dangerous failures.
Including failures not related to safety, Coyle identified a total 723 malfunctions in critical V-22 subsystems, with hydraulics comprising 170 -- again nearly 25 percent -- of those.
The hydraulics problems became a primary focus for the “Blue Ribbon Commission” appointed after the December crash by then-Defense Secretary William Cohen. The panel chairman, retired Marine Corps Gen. John Dailey, told the Senate Armed Services Committee on May 1 that he views repeated hydraulics problems as a key “production issue.” As such, hydraulics were a factor in the commission's recommendation to reduce production to minimum levels until problems can be analyzed and corrected.
Dailey cited the V-22's use of titanium, rather than stainless steel, for the hydraulics lines, which allows them to be lighter but also makes them more vulnerable to ruptures as a result of chafing. And the use of a 5,000-pound-per-square-inch hydraulics system -- as opposed to a 3,000 psi system used in many aircraft -- also cuts weight by allowing for a smaller actuator assembly and less fluid reserves. Each of the two main hydraulics systems contains less fluid than the amount of milk most Americans probably keep in their refrigerator.
But a side effect is that the very high pressure increases leaks in the seals, Dailey and other program experts observe. “It's really tough to hold that kind of pressure inside the actuators,” said the source close to the V-22 program.
“These are production issues that have arisen as a result of the dense packing of the nacelle, primarily, with hydraulic lines, wire bundles, and fasteners that hold these in place,” Dailey testified earlier this month. The 5,000 psi hydraulic system, he added, “is the wave of the future,” providing the V-22 “the opportunity to use smaller actuators, so you can get a more compact packing or density of the nacelle, and provides advantages.”
Still, he implied that the combination may not yet be quite right. “They're using titanium lines, which are very strong and light, but they're also more brittle than stainless steel and more subject to chafing,” Dailey said. “And so when these fasteners do not hold these parts in the proper position, they rub against each other, and that's what happened in one of the accidents.”
Although the Marine Corps accident investigators concluded the hydraulics failure alone -- without the subsequent computer software problem -- likely would not have been responsible for the V-22 crash last December, there is little doubt among those who have reviewed the program that the hydraulics problems add increased cost and higher operational risk for the Osprey.
What has not been widely recognized is how long hydraulics problems have seriously affected the rotorcraft. The accident report on the North Carolina crash cites a finding that hydraulic-line chafing had been identified as a problem as far back as June 1999. The June 1992 incident report documents the problem was identified at least seven years earlier, and anecdotal reporting suggests leaks in the actuator seals have been routine since the first aircraft was built.
At his April 5 press conference, Berndt was asked why fixes in the system were not made, given they had been “discovered” in June 1999. “As a matter of fact,” Berndt responded, “April a year ago today, [in] 2000, a service bulletin was issued by Bell-Boeing that identified problems with chafing and hydraulic lines. And the squadron started to inspect those lines. Unfortunately, the affected line that had some role in this mishap was not one of the lines identified to be serviced.”
And yet the rupture in the No. 1 hydraulic line providing fluid to the actuators in the left nacelle, seen in the December crash, appeared to roughly duplicate the location of the rupture in Sullivan's aircraft in 1992. According to one former Boeing employee, interviewed on background last week, the prime contractor's early approach to the program was, “We've got to sell this son-of-a-***** first; we'll fix it later.”
Hydraulics problems and other failures were quietly put on a back burner for years in the confidence the government would pay to fix them later on, under “engineering change proposals,” this source said.
One Boeing test pilot resigned in late 1992, citing just such problems in the company's V-22 program. “We promote the 'good old boy' who's been there the longest and will follow the mold,” states the pilot in a Dec. 15, 1992, resignation letter, obtained by ITP. “Now, in a period of 14 months, we have crashed three aircraft and killed seven people.” (His tally includes Sullivan's crash, plus two non-fatal crashes of the Osprey and the Apache helicopter.)
A software conundrum
If tightly packing a vulnerable hydraulics system into the V-22 nacelles is akin to stuffing 50 people inside a Volkswagen without injuring any of them, perhaps the corollary for the V-22's software dilemma is finding the proverbial needle in a haystack -- then removing it without disturbing any of the surrounding straw. The rotorcraft's more than 1 million lines of computer code “is so complex that I'm not sure they have the capability to chase it and make sure it can be fixed,” said the Osprey expert interviewed last week. “And if you fix one thing, it affects another.”
Coyle similarly described the magnitude of the programming challenge. “There may be other software errors lurking,” he told ITP. “So you have to go back and 'pull the string' to review all the software instructions and how they would function in various emergency situations.” He said the computer hardware must undergo a detailed engineering analysis as well.
Berndt told reporters last month that the December crash raised two important questions, one pertaining to “the adequacy of testing on the flight control system and the associated software,” and the other “to the placement of the V-22 hydraulic lines and wire bundles within the nacelles.”
Observers familiar with the 1992 incident agree that those two questions are vital, but note that history might have played out differently had they been raised nine years earlier. -- Elaine M. Grossman
© Inside Washington Publishers
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