Going Gray

Feb. 1, 1998

If USAF’s T-38 trainers were cars, most would qualify for “antique auto” license plates. The first production model entered service in 1961. The last rolled off Northrop’s assembly line in 1972. The average age of T-38s still flying for the Air Force is 30 years. Average flying hours per aircraft has passed 12,200 and continues to climb.

The T-38 remains a tough little airplane, but it has not aged altogether gracefully. Cracks resulted in the replacement, in the late 1970s, of lower wing skins with thicker material. Current fatigue problems include small fractures emanating from the lower wing skin fastener holes, cracking in upper cockpit longerons, and corrosion inside the horizontal stabilizer.

In fact, says a major new study of the problem, keeping the T-38s airborne will require the same kind of constant maintenance attention that it would take to keep a fleet of 1967 Chevys in good operating condition. USAF “will continue to face a major challenge to protect the safety and prolong the service life of the T-38 for another 25-plus years,” concludes a new National Research Council report on aging USAF aircraft.

As the NRC found, the T-38 is not an isolated example. Many of the Air Force’s aircraft have surpassed 20 years of age and yet continue to serve as backbones of critical components of the operational force. To varying degrees, all these airplanes can be expected to experience such aging problems as cracking and corrosion.

USAF’s Aircraft Structural Integrity Program and other service efforts have helped keep such problems in check in the past. However, the NRC warns that continued success is not assured, for a number of reasons. These include the Air Force’s maintenance manpower cuts, greater use of contract maintenance and commercial design practices, and possible complacency of Air Force management, says the report.

Keeping older aircraft safe to fly and potent for combat is nothing new for the Air Force, but it is among the top challenges it faces on the eve of the 21st century. [See “The Aging of the Fleet,” June 1996, p. 14.] Even if Congress funds all the new airplanes now planned, the USAF inventory will continue to see a steady increase in the average age of its aircraft for decades to come. The Air Force has never been forced to make do with so many old aircraft.

The report was prepared by a blue-ribbon NRC panel formed specifically to take a hard look at aging- aircraft issues and to recommend a course of action. It is now urging a number of aggressive remedial steps, including more research on cracks and corrosion and establishment of an aging-aircraft “technical czar.” Furthermore, it says, the Air Force should continue to update its durability assessments of all its aircraft models.

Absent such changes, the service’s success in keeping older aircraft sound may become “rather fragile,” concludes the NRC Committee on Aging of US Air Force Aircraft.

The Airplanes

The T-38 may be old, but it is not the true senior citizen of the Air Force. That title belongs to the KC-135 tanker, which was first introduced into service more than 40 years ago. Other venerable aircraft include the B-52H bomber, C-130 airlifter, and T-37A primary trainer, which all first flew operationally 35 to 40 years ago.

The next oldest generation comprises the C-141 and C-5A airlifters, which entered operational service 25 to 35 years ago. The F-15 air superiority fighter, A-10 attack aircraft, and the E-3 Airborne Warning and Control System aircraft clock in at 20 to 25 years of age. In contrast, the F-16 multirole fighter and the KC-10 jet tanker are relative youngsters, having become operational within the past 20 years. The same is true of the F-117 stealth fighter.

Of all these aircraft, the C-141 is the only one whose replacement (the C-17 Globemaster III) actually has gone into production. Other replacement aircraft, such as the Joint Primary Aircraft Training System, or T-6A Texan II (for the T-37A), the F-22 Raptor (for the F-15), and the Joint Strike Fighter (for the F-16), are in varying stages of development. The vagaries of the procurement cycle have made uncertain the delivery schedule of any new combat system. It will be at least 15 years before JSFs are on the ramp in significant numbers, the NRC points out.

The T-38 is not the only USAF model subject to cracks and other aging problems. Both the KC-135 and the C-5A had their lower wing surfaces replaced in the 1970s and early 1980s as a result of worries about widespread fatigue damage.

The C-141 has proved particularly troublesome from a structural point of view. Fatigue damage around the weep holes in lower wing risers has forced the installation of boron composite reinforcing doublers on most of the Starlifters. Further tests have shown that other critical wing parts may reach the end of their life span at around 37,000 flight hours, even though USAF’s current plans call for using the C-141s until they reach 45,000 flight hours and are retired.

According to the NRC panel report, any C-141 flown beyond 37,000 hours now requires “extensive and burdensome” inspections to ensure continued safety. The inspections involve looking at more than 6,000 fastener holes every 120 days. As is the case with the T-38, “the structural management of [the C-141] will continue to be a significant challenge,” says the NRC.

NRC descriptions of the aging problems of other Air Force aircraft can begin to sound like a roll call of the wounded. B-52 heavy bombers are prone to fatigue cracking in flap tracks and to cracking in aft body skins. A-10 aircraft—whose low-level evasive maneuvers subject them to three times the level of stress originally assumed by its designers—can develop cracks in wing upper skin, some parts of their main landing gear, and engine nacelle hanger frames.

The E-3A AWACS, based on old Boeing 707 commercial airframes, contains many parts made from a corrosion-susceptible 7000-series aluminum alloy. F-15E dual role fighters, fully loaded with munitions on pylons beneath their wings, can produce in-flight shock waves which eventually damage some of their own skin panels.

No Exceptions

Even in relatively new aircraft, design mistakes can produce structural problems. Fatigue cracking and corrosion on the B-1B appear to be minimal so far, for instance, but it turns out that the location of the B-1B tail, just above the exhaust wake of the engines, causes problems. The placement helps achieve high-performance turns at low velocity—but it also places the tail within the engine’s high-acoustic-noise envelope, and it has caused some fatigue cracking fairly early in the airplane’s life.

USAF aircraft models designed before the early 1970s—when military specifications dealing with damage tolerance in aircraft design were tightened—have all been the subject of Durability and Damage Tolerance Assessments. These tests, performed by contractors and Air Force logisticians, identify critical areas where wear and tear cause fatigue damage, determine the limit of such damage the airplane can safely endure, and lay out safety inspection requirements.

The NRC aging-aircraft committee strongly recommends that many of these DADTAs be updated. Highest priority in this regard should be given to the A-10, F-16, U-2, and T-38, it reports. “In general, an update about every five years is appropriate,” says the report.

Still, not even the most rigorous inspection procedure can prevent the onset of aging problems. As flight hours increase, cracking, corrosion, and other damage will inevitably occur on all aircraft models.

Operational changes, such as better fuel management, reduced pressurization, and more extensive flight restrictions, can help ease the aging process. “For aircraft that are approaching their economic service limit, these options should be considered to allow time for modification or replacement acquisition programs,” says the NRC.

However, the Air Force has no clear-cut standard for the point at which keeping an aging airplane in the air is no longer cost-effective. Another major NRC recommendation is that the Air Force significantly improve its estimates of the probable economic service life of its aging-aircraft systems.

“Lack of these tools frustrates the ability of Air Force planners to establish a realistic timetable to phase out a current system and to begin planning for replacement systems,” says the report.

The Process

In December 1969, at Nellis AFB, Nev., an Air Force F-111 fighter experienced a catastrophic wing failure and crashed after it pulled up sharply after firing a rocket. The swing-wing aircraft had clocked only about 100 hours of flight time when the crash occurred. Eventually, investigators determined that the cause of the accident was a forging defect in the wing pivot—a problem almost impossible to detect via normal inspection methods.

To protect the fleet as it matured, Air Force science advisers recommended that all F-111s be subjected to a special, low-temperature load test developed for the Apollo space program. During the next 25 years, 11 F-111s failed the difficult test, which is conducted at minus 40 degrees Fahrenheit.

All of those aircraft would probably have broken up in the air. But thanks to the load test and the attention of engineers, logisticians, and inspectors, no F-111s since 1969 have crashed as a result of structural failure.

“The F-111 history truly represents a major success story for the Air Force structural integrity program,” concludes NRC’s study.

As this example suggests, the Air Force’s system for ensuring the structural safety of its aircraft is a rigorous one. The 1970s saw the introduction of DADTAs for older aircraft as well as the advent of the Air Force Structural Integrity Program and damage tolerance requirements for new designs. Ever since that time, service losses of aircraft to parts failure have been minuscule.

The NRC report says, “The failure rate for all weapon systems that are maintained using the damage tolerance approach is one aircraft lost due to structural reasons in more than 10 million flight hours.”

This success has been based on a number of interlocking factors, among them: rigid enforcement of ASIP regulations by the Air Force’s system program offices and the Air Logistics Centers; technical oversight of the whole area by an experienced, standing Air Force committee through the mid-1980s, plus periodic reviews by Scientific Advisory Board committees; developing of competent examiners at the ALCs able to perform damage tolerance analyses and judge contractor’s work; and money for DADTAs and R&D funding the field.

The NRC believes that recent acquisition reforms, plus budget and manpower cuts, “have all adversely affected these factors.”

Needed: A “Czar”

Aggressive action is needed to counteract this deterioration and prevent more problems in the future, says the NRC report. For instance, the study recommends appointing a single knowledgeable and experienced technical leader—an old-airplane “czar,” so to speak—to oversee aging-aircraft engineering and R&D activities.

The Air Force’s technical oversight should be bolstered through establishment of a resources group to examine personnel deficiencies in aging-aircraft research fields and a number of working groups to provide a technical link from basic research through solution implementation.

The NRC believes some engineering tasks should receive higher priority. These include corrosion control programs. The spectacular 1988 accident of an Aloha Airlines 737, in which the aircraft lost much of its forward fuselage skin but managed to survive, focused much civilian and military attention on this pervasive problem. The NRC, however, said civilian airlines continue to surpass the Air Force in the degree to which they inspect their individual airplanes for corrosion.

In particular, the Air Force may need to do more in the field of Stress Corrosion Cracking, according to the NRC’s investigators. SCC is a dangerous, hidden kind of corrosion, difficult to detect visually because it occurs within the very grain of the material.

“The committee recommends that the Air Force include an assessment of the vulnerability of each of their aging aircraft to structural failure caused by SCC” in their overall DADTA vulnerability updates, according to the NRC report.

When it comes to aging aircraft, the corrosion of airframe structures constitutes the single most costly maintenance problem for the Air Force, running up to $3 billion a year in repairs.

Corrosion can occur in any number of forms, from SCC to pitting corrosion, galvanic corrosion (caused by mild electrical currents), and plain old general corrosion. Among its primary causes in Air Force aircraft are use of older, corrosion-prone aluminum alloys and exposure to such corrosive environments as humid air, salt water, and latrine leakage. Corrosion damage is typically discovered by visual inspection. However, because a fair amount of corrosion damage on older aircraft can be hidden from sight, much of it can go undetected.

In line with its view that early detection of corrosion and new control techniques should receive a high Air Force priority, the NRC suggests a number of operational needs. These include:

New, more environmentally friendly protective coatings to replace hazardous materials now being phased out.

Better ways of finding hidden corrosion without disassembling aircraft.

Better understanding of rates of corrosion.

Installation of dehumidified aircraft storage or development of techniques to dehumidify susceptible areas of particular aircraft. Studies have shown that reduction of relative humidity 30 to 40 percent would significantly reduce the corrosion of stored aircraft.

Unlike corrosion, fatigue cracking—another main problem of aging—is a direct result of use and will eventually occur in all aircraft. It is divided into two types: low-cycle fatigue (typically caused by flight maneuver and gust loads) and high-cycle fatigue (caused by vibration from mechanical, aerodynamic, or sound sources).

Low-cycle fatigue can be exacerbated by changes in aircraft use, such as addition of new armaments or the introduction of new flying tactics. Understanding the implications of such changes on wear and tear is a top priority, according to the NRC.

Similarly, changes in an aircraft’s basic configuration can affect high-cycle fatigue wear, by placing new parts in the path of vibratory energy or altering the flow of shock waves along airplane exteriors. Identification and elimination of sources of high-cycle energy, where possible, is a key technical issue in this area.

Actually finding fatigue damage poses a critical problem. In some cases cracks that can degrade a structure can be as small as a few hundredths of an inch in width. Existing Non-Destructive Inspection methods for finding small cracks, such as eddy currents and ultrasound inspections, are tedious, time-consuming, and expensive. They are also less reliable when applied to the interiors of aircraft sections, such as the inner layers of a wing or fuselage joint or in interior structural members, such as stringers.

“Among the greatest NDI challenges is to develop methods that can reliably, rapidly, and cost-effectively determine, without fastener removal or disassembly, if an aircraft has widespread fatigue cracking,” says the NRC committee.

Finally, the Air Force needs a better understanding of the implications of some of its repair methods, says the NRC panel. Generally, aged-structure repairs now consist of reinforcement doublers that are bolted or bonded over damaged areas. In recent years the Air Force has favored bonded composite patch repairs, which save weight and are easier to mold into complex shapes. Further research might improve this repair practice—and it could provide predictions for the life span of composite patches, which today are only rough estimates.

Peter Grier, the Washington bureau chief of the Christian Science Monitor, is a longtime defense correspondent and regular contributor to Air Force Magazine. His most recent article, “The Pentagon Prescribes Tricare,” appeared in the January 1998 issue.