- The Washington Times - Tuesday, April 30, 2002

The Air Force has discovered a tail stress problem in the nation's most expensive jet fighter and will delay testing as it considers changes to the F-22 Raptor's airframe.
Air Force officials said that last fall they found that certain high-force flight maneuvers put unacceptable stress on the F-22's distinctive twin tail.
Such stress, known as tail buffet, is common in two-fin jet fighters. But excessive stress, such as that experienced by the test aircraft in certain maneuvers, can lead to early fatigue and a need to replace the fin structure.
The Air Force is optimistic that the problem will be fixed before the first production stealth fighter costing $200 million leaves the Lockheed Martin assembly line in Marietta, Ga.
The problem comes at a crucial time for the $67 billion program. Defense Secretary Donald H. Rumsfeld has ordered a review of the F-22, and other expensive weapons, to determine whether to purchase fewer planes and use the savings for other projects.
On Capitol Hill, a few military staffers are talking of cutting the F-22's procurement rate until the tail stress is corrected. The Raptor, the Air Force's top procurement priority, is billed as the world's most advanced air-dominance fighter, with composite materials that enable it to evade radar. Congressional staffers want to ensure that the fixes do not increase the plane's radar cross-section.
Engineers have already designed two fixes: reinforcing a rudder hinge and adding titanium spars to the tips of each tail to strengthen them. Contractors incorporated the changes into in-production planes in December. But additional testing shows that those fixes may not be sufficient.
Last week, the Air Force assembled a 10-member "red team" of private and government aviation experts to analyze the Air Force fixes.
Marvin R. Sambur, the assistant Air Force secretary for acquisition, said the stress issue should not jeopardize the Air Force's plan to buy 339 airplanes.
But he said he will have to delay the beginning of operational testing scheduled for April 2003. He said the "initial design needed to be structurally augmented, structurally changed."
"We've actually reported this to the third-floor [Pentagon policy-makers] and to members of Congress that perhaps that structural change that we had implemented might need to be augmented," Mr. Sambur said yesterday. "But we don't consider any of these to be showstoppers."
Mr. Sambur said the stress occurs in a small part of the F-22's entire flight envelope.
"There is what we call a 'thumbprint' of a small area in which they will not be able to fly," he said. "We're actually trying to be very conservative because we want to make sure this is a plane that's going to do everything we say it does."
A former Navy test pilot said yesterday that he is not surprised by the stress problem in a twin-tail jet fighter. He said the Air Force faces the tricky task of finding fixes that do not add weight or make the plane more visible to radar.
"Whatever they do, it's going to slightly decrease performance," he said.
The most likely correction will be to insert two, 8-inch-high "fences" on each side of the fuselage to direct airflow away from the tails.
"Basically what you're looking at is a little bump to just change the airflow," Mr. Sambur said. "So we're not saying this is not an issue. We're saying it's not a showstopper issue."
Maj. Gen. John D.W. Corley, Air Force director of global-power programs, said he was confident that fences would not degrade the plane's ability to evade radar.
Edward C. Aldridge, undersecretary of defense for acquisition, in August approved the F-22's transition into low-rate production. The first eight planes will be used for testing and training and will not have any of the tail-stress fixes. The corrections will be incorporated into the first production planes, whose major components are now being made, officials said.
The F-22 is scheduled to be ready for combat in 2005.
Mr. Rumsfeld said during a tour of bases in Central Asia during the weekend that with the F-22 "the big debate is not whether, but how many."
The F-22 was born in 1983 as the Advanced Tactical Fighter. It has gone through a number of delays and cost increases. The plane's avionics suite is hampered by software problems.
But the Raptor has a number of backers in Congress, who defeated an effort in the House two years ago to suspend production.
Congress funded 10 F-22s in the current budget. The Air Force wants 23 fighters next year and 26 the year after that.

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