Airframe and Powerplant

Inevitable Aging

September 1, 2003

Researchers delve into the effects of time on aircraft

Should U.S. pilots be concerned when a foreign air safety agency issues airworthiness directives (ADs) that apply to their airplanes? Does a foreign AD that establishes a life limit on a vital airplane component, such as a wing, merely signal that foreign aviation safety agencies are overly conservative, or should U.S. owners regard these ADs as harbingers of (expensive) future U.S. ADs? These and other questions about how general aviation airplanes age are being explored in a college laboratory in Wichita.

In 1988, the forward-upper fuselage of an Aloha Airlines Boeing 737 blew away during an interisland flight, leaving a gaping 18-foot-long hole aft of the cockpit. Although one member of the crew died when she was ejected during the explosive depressurization, the airplane landed safely with no other fatalities. Although it never flew again, that 737 will be remembered for its role in prompting the government and the aviation industry to learn more about how airplanes age.

The Aloha Airlines accident spurred the U.S. Congress to create the Aging Aircraft Safety Act of 1991 (AASA). In the 12 years since then the industry and the FAA have learned a great deal about how corrosion and metal fatigue affect airplanes. As a result the FAA will soon enact a rule titled "Aging Aircraft Safety." This rule will require that the FAA inspect all airplanes with more than 10 passenger seats that are operated commercially under FAR parts 121, 129, and scheduled 135 after 14 years in service. Operators of these airplanes will be required to create, and submit for FAA approval, damage-tolerance based inspection programs in order to continue flying.

The cumulative forces that caused the fatigue damage to Aloha 737 — the strains of pressurizing the cabin, the high number of pressurization and operational cycles (a takeoff and a landing is a cycle) each day, and flight times that increase like the latest Harry Potter novel piles up sales — are so far removed from typical GA airplane operations that it's hard to see the connection between metal fatigue in a Boeing 737 and a Cessna 172. But GA airplanes are aging — at least one-quarter of the fleet is more than 40 years old. That's important because the mechanisms that contribute to metal fatigue and corrosion are the same for every metal airplane. (Time will tell the story for all-composite components). Cessna Aircraft Company is so concerned with the loss of airframe airworthiness resulting from fatigue and corrosion-induced cracks that it has developed structural modifications (service kits) to reinforce the wings of its 300- and 400-series piston twin airplanes.

One way the FAA is learning about how corrosion and fatigue can affect a light twin is by taking apart — rivet by rivet and wire by wire — two commuter airplanes. This study is headed by Dale Cope at the Aging Aircraft Laboratory at Wichita State University's National Institute of Aviation Research (NIAR). Guidelines for the inspections mandated in the upcoming aging-aircraft rule, called supplemental inspection documents (SIDs), also are being established by the NIAR study.

Two Cessna 402s are being disassembled. The first, a 1969 402A that was used to haul tourists from Las Vegas to the Grand Canyon, has 19,800 hours on the airframe. This airplane was manufactured under simple (some say primitive) regulations established by the Civil Aeronautics Board (1940 through 1958), predecessor to the FAA, in Civil Aeronautics Manual 3 (CAM 3). CAM 3 barely acknowledged metal fatigue. According to a recent report published by Cessna, the only regulatory requirement related to metal fatigue in CAM 3 was to avoid points of stress concentration where variable stresses above the fatigue limit were likely to occur in normal service. When CAM 3 was written, no one could foresee a GA airplane with 20,000 airframe hours.

The NIAR study, which has received initial funding from the FAA, has two objectives. The first — a two-year plan — is to determine if continuing airworthiness of general aviation aircraft is related to aging. The long-term objective — a five-year goal — is to establish guidance to ensure that current maintenance programs for GA airplanes provide acceptable levels of continued airworthiness. The study is focusing on the metal airframe components and aircraft wiring.

So far there have been only a few accidents directly traceable to metal fatigue, but no one thought airliners would blow out fuselage sections until it happened. Big strides have been made in understanding widespread metal fatigue (WFD). WFD is the critical loss of structural strength caused by a network of tiny cracks. Big cracks have always been a cause for concern — only recently has the strength loss caused by tiny cracks been realized. Since corrosion is also a factor in metal airplanes, its effects on aging must also be accounted for. That's easier said than done.

It's long been known that cracks originate because of the stresses that take place during airplane operations. Repeated stresses are normal — wings flex up and down during everything from taxiing to turbulence. Corrosion contributes to crack formation because corrosion pits, even tiny ones, concentrate these stresses. These concentration points are called stress risers.

Stress analysis engineers can accurately predict when critical wing components, such as a wing lower spar cap, could start to crack (measured in number of hours or cycles). They can even predict where the crack will originate and how fast it will grow.

These predictions get skewed when corrosion pits occur, partly because the location of corrosion-caused stress risers can't be foreseen, and because the effects of corrosion on metal structures are extremely variable. These predictions are further muddied because, as Cope points out, "we don't have an established methodology for corrosion growth." In other words, no one yet knows why the upper spar cap of the left wing of the study aircraft corroded while the one on the right wing didn't, when the corrosion occurred, or how fast it grew.

The Australian Civil Aviation Safety Authority (CASA) has taken a very conservative approach to the crack-projection studies conducted by airframe manufacturers, including Cessna, Raytheon, and The New Piper. For instance, CASA has mandated (in AD Cessna 400/40) that the left and right wing lower spar caps, and the left and right wing attachment fittings of the Cessna 402, 402A, and 402B, be replaced or removed from service at 8,200 hours time in service or 12,600 flights. The same components of a 402C's wings must be retired or replaced at 7,700 hours or 13,200 flights. CASA has established life limits for critical components for many GA airplanes. See the Web site ( www.casa.gov.au/avreg/aircraft/ad/) to see if CASA has established life limits for components of your airplane.

CASA believes that wings built under CAM 3 guidelines incorporate features that make it "extremely difficult" (CASA's words) to develop a reliable and safe inspection program. In lieu of a foolproof inspection scheme, CASA has chosen to establish rigid life limits on critical parts in what's called a safe-life approach to fatigue. At the safe-life limit, the component (e.g., the wing) must be replaced or rebuilt with new parts. These are expensive solutions and few general aviation maintenance shops are equipped to carry out this type of major rebuild.

The FAA is developing a more focused approach that's termed the damage tolerance based safety method. Instead of using manufacturers' projections as the final word, as CASA has done, the FAA's study is in the process of gathering many pieces of data that may relate to aging. Crash studies, field reports, data from the Airworthiness Assurance Nondestructive Inspection (NDI) validation center at Sandia National Laboratories, and manufacturers' engineering studies are just some of the data being used to establish inspection intervals and methods to validate the manufacturers' crack analysis predictions. The point of a damage-tolerance based method is to inspect each airplane individually since variables such as maintenance, environment, and service history have a direct bearing on when cracks start to develop. The NIAR study is part of this data gathering process.

The first step in the NIAR study was to complete an inspection in accordance with all pertinent Cessna maintenance manuals and inspection guidelines in the form of service letters, instructions, and bulletins. This resulted in 110 separate inspections. The crew at NIAR also carried out 40 additional inspections developed by Cessna to determine if proposed inspection programs (which will be used to develop the SIDs) are good enough to find hidden cracks and corrosion.

Then the airplane disassembly began. Wing and fuselage skins were removed. Wing attachment fittings and landing gear components were removed. Built-up structures such as wing spars were disassembled. Painted parts were stripped and etched for a visual inspection using a 50-power to 300-power digital optical stereoscope. Some parts will be examined using a scanning electron microscope for fault confirmation. The lab can see faults down to the molecular level.

The first airplane is almost unbuilt. "The aircraft is in excellent condition. I'd love to have this airplane as a private owner so it could keep flying," said Cope. Now the FAA wants to know what else contributes to fatigue and corrosion damage — is maintenance a factor? How much of a factor is environment? How big a factor is calendar time? Is time in service or the number of cycles more important? There are a lot of pieces to the puzzle.

Just as a corrosion pit can focus the strain in critical airplane components, so can the inevitable scratches, tool marks, and rough handling that occur during maintenance. Since the capabilities of GA maintenance facilities vary, this factor is hard to quantify. What's really needed to guarantee the structural integrity of aging aircraft is something akin to Superman's X-ray vision — an MRI for aluminum aircraft structures, if you will. The current "X-ray vision" is eddy current testing, which was commercially developed during World War II. This method can detect surface and subsurface cracks, pits, and corrosion on inner surfaces — it's better at detecting suspected cracks in localized areas than scanning large areas for randomly oriented cracks. Aircraft manufacturers' engineering departments apply fracture-analysis science to their airframes to predict crack development. A trained and certified eddy current operator can detect the cracks. Once a crack is found, what next? That's where the term damage-tolerance based comes from.

A damage-tolerance based inspection system is centered on two factors — using engineering studies to predict the location of stress-produced cracks and scheduling inspection intervals so that the critical location will be inspected twice during the time it takes the crack to grow from detectable to critical size. The FAA's goal is to detect cracks as short as 0.05 inch with 90-percent probability. This isn't easy to achieve. Cracks this size can hide under the head of a common rivet or fastener. As cracks grow, the odds of finding them increase.

The FAA's decision to implement a damage-tolerance based solution has been reinforced by the fact that this type of program has worked well for large Transport category airplanes.

Cessna has already published a SID for the 402. CASA has suspended the AD mandating wing and/or wing spar cap retirement for operators that have completed the SID program. According to Cessna, cracks or corrosion have been found on every airplane that has been inspected. Cessna has developed service kits for the installation of spar straps to reinforce the critical wing lower spar cap for airplanes that traditionally make up the commercial Cessna fleet — the 400-series twins — and for the lighter-weight 300-series airplanes. As this article goes to press, the FAA is asking owners for comments on the proposal to mandate spar strap installations on 401-, 402-, and 411-model Cessnas.

The SID inspections and spar strap kits are expensive — the FAA estimates it will cost $43,000 to install the kit on 414A and 402C models — with installation being required after 8,500 or 14,500 hours, depending on the model and serial number. One Australian company took 525 hours to complete a prototype SID inspection on a 402. The numbers will probably come down as the process is streamlined. On the plus side, expensive repetitive inspections will be suspended and the kits will guarantee airworthiness for many years.

Airplanes that were built in the 1940s, 1950s, and 1960s are still in service and some of them have been used hard for many years. The typical owner-flown light twin doesn't accumulate hours at the same rate as a commercial airplane, nor is it subjected to such fatigue-inducing factors as day-in-and-day-out turbulence, heavy loading, or the inevitable less-than-smooth landings that are part of all-weather operations. That doesn't mean owner-pilots should do nothing.

Pilots who own single- and twin-engine airplanes should take steps to slow the aging process. Since corrosion contributes to crack formation, every aluminum airplane should be on a corrosion-control program. Make it a point to keep your airplane clean. Exhaust trails and breaks in the paint give corrosion a launching pad, and once this insidious blight gets started it is almost impossible to stop.


E-mail the author at steve.ells@aopa.org.