Airframe and Powerplant: Assisted Living

Health care for aging airplanes

April 1, 2008

Only Superman is immune to the ticking of the clock. Mountains aren’t, nor are man-made creations, such as marble statues or the Great Pyramid of Khufu. Airplanes are vulnerable as well. Understanding the effects of aging on general aviation airframes is not yet an exact science, so it’s impossible to say when metal fatigue will compromise the airworthiness of the average GA airplane without an expensive and time-consuming analysis. But, as with the human species, the industry has been able to define many life-shortening events.

All of the upgrades

Each year hundreds, if not thousands, of aircraft powerplants are replaced. Interiors are upgraded, old paint is stripped off, and durable space-age coatings are applied in an owner’s favorite colors and designs. And by now everyone knows that there has been more avionics progress—and more money plowed back into general aviation cockpits—in the last decade than in all the years since solid-state devices were first installed. Yet all those upgrades and improvements have no effect on the original aluminum airframe.

Does the march of time mean the average GA airplane is destined to become unsafe? That’s a tough question. Although a number of airplane structural failures have been captured in chilling detail on film, and are now replayed over and over on video viewing sites such as YouTube, not once has a typical GA airplane been the star of the show. That’s not to say that there haven’t been any GA accidents caused by fatigue—there have been, but only a few. A look at the fleet as it now exists clarifies this issue.

Worked hard

Without exception the airplanes that starred in the airframe-failure videos were working airplanes. This work included air taxi operators hauling passengers and freight, aerobatic or mock aerial combat training, ex-military training aircraft, and ex-military airplanes modified for firefighting.

This slice of the aviation fleet is quite small. When the 205,000 fixed-wing aircraft in the GA fleet are divided into use categories, we see that 71 percent are personal aircraft, 17 percent are business aircraft, six percent are used for instruction; four percent are used by aerial applicators; two percent are used as air taxis, and one percent are something else. The accident airplanes are from the two small niches.

Of the personal use general aviation accidents that have been attributed to fatigue, critical steel parts such as wing struts have failed because of rust.

As reported in this column in the June 2000 issue of AOPA Pilot, the topic of airframe aging is an active topic on the FAA’s action list. FAA Aerospace Engineer Marv Nuss of the Small Airplane Directorate, who oversees this program, presented a seminar at AOPA Expo in Hartford, Connecticut, in October 2007. The key points of his presentation follow.

The airframe life “trust fund”

Most of the GA fleet was certified under a set of airworthiness rules established in Civil Aeronautics Regulation 3 in 1949. Under this set of rules, fatigue life-limits were not required. Federal Aviation Regulation (FAR) 23 superseded CAR 3 in 1965. In 1969, amendments to these certification regulations required that manufacturers establish fatigue life limits for wing and wing carry-through structures. Amendments have expanded the number of structures requiring fatigue life-limits. These limits are cited in each airplane’s type certificate (TC).

Most airplanes in the GA fleet do not have published life-limits. It’s safe to say that each airplane is endowed at birth with a trust fund of airframe flight hours in which the airframe integrity and design conforms to its TC, and therefore is airworthy. There are many factors that increase the rate of withdrawals from an airplane’s trust fund, and a few ways to add hours to it.

For instance, airworthiness directives (AD) 2005-12-12 and 2005-12-13 mandate the installation of a wing spar strap per a Cessna service kit on certain 400-series twin-engine airplanes. The installation of this spar strap establishes a published inspection interval time. In essence, this AD has established a life limit on the reinforced wing. In a few cases, structural reinforcement approved under the supplemental type certificate (STC) process extends the service life of a life-limited structure. The wing life-limit of 11,000 hours on the Piper PA-38-112 Tomahawk was extended following the installation of a spar strap modification. The point here is that the industry has proven its ability to respond to fatigue problems with modifications that extend the lives of existing aircraft.

The limits

Airframes are built to comply with load limits. For Normal category airplanes these are plus 3.8 Gs and minus 1.52 Gs. This means the structure must be strong enough to withstand loads that are 3.8 times greater than the normal loads that are applied to the airframe during non-disturbed flight at gross weight.

As a safety margin, ultimate limits of 150 percent of the load limits are factored into the designs. The ultimate limit number is a “fudge factor” meant to account for deviations that may take place during assembly operations, in the materials, and for loads imposed when the airplane is flown outside of its design envelope.

When the structure, material, and integrity of the design remain the same as the day the airplane rolled off the assembly line, the airframe life trust fund would pay out very slowly. Aging factors can be broken down into two categories. Factors that are consistent to every airplane and can’t be easily controlled are called genetic factors, according to Nuss. To some degree, exposure to acids, salts, and other contaminants from airborne pollution is genetic. These factors hasten the onset of corrosion. Other genetic factors for each airplane include the normal fatigue cycles that are part of every taxiing, takeoff, cruising, and landing operation. All of these deduct from each airplane’s trust fund account.

Non-genetic aging factors—those that can be controlled—include poor maintenance and high loads and stresses connected with the spectrum of maneuvers encompassed in the operation of the airplane. Life-shortening events include a lack of knowledge that attributes to skipped or cursory airframe inspections during scheduled maintenance, a lack of knowledge about the different types of corrosion and how to treat corrosion, and a lack of knowledge or expertise in regards to damage evaluation and structural repairs.

The spectrum of maneuvers that accelerate the trust fund withdrawal include flight with continual heavy or maximum loads, aerobatic flight, abrupt pull-ups, low-altitude flight, and short cycle times such as those typical with flight-seeing aircraft. A cycle is one takeoff and one landing.

Fatigue-fund withdrawals occur at a faster rate on a given airframe during a series of short duration flights with heavy loads than during a single long flight with a light load, even though the flight time hours may be equal.

Fatigue cracking

Nuss told the Expo audience that fatigue cracking is inevitable in tension-loaded structures. The lower spar cap of a wing is an excellent example of a tension-loaded structure, he said. It’s also the most highly stressed part of an airplane’s wing because the flight loads are concentrated on this part. Eventually, fatigue cracks will occur. It’s important that each aircraft owner realize that cracks will happen as a natural function of aging. Yet by all indications, today’s 40- and 50-year old airplanes that are well maintained still provide the margin of protection designed into the load-limit parameters. This has been borne out during teardown testing of high-time airframes.

Approaches to fatigue cracking

There are two ways to handle fatigue. The first is to react to fatigue-caused problems. This reactive approach requires an event—such as a component failure or the discovery of a crack—to locate a potential problem. The next step is to sift through accident and incident databases for similar occurrences to determine the frequency and severity of the problem. A lack of data limits the effectiveness of this approach, according to Nuss.

The FAA accident and incident database is dependent on the users of the system for the flow and clarity of the data. Scheduled airlines, air taxi operators, and repair stations submit data on service difficulty report (SDR) forms. General aviation and anonymous submissions are filed on malfunction and defect (M&D) report forms. Forms and data base access are available online.

The quantity and quality of SDR data is generally good. Unfortunately, GA M&D data is not up to the same standard. This hampers the industry’s ability to make informed decisions. If it’s determined that the problem requires a fix, it’s detailed in a service publication such as a service bulletin, or through the issuance of an airworthiness directive (AD). In years past, almost all AD notes were issued after the event—in some cases a fatal crash—occurred.

The second method of dealing with fatigue is to address cracking and wear before they compromise strength to the point that the trust fund is empty and a failure takes place. An important part of this proactive approach is specialized, detailed inspections designed to target critical areas for precursors such as tiny cracks. When precursors are discovered, the next step is to go back to the database to discover if they signal a trend toward more serious cracking.

Type clubs—those groups of pilots, technicians, and owners that voluntarily band together in order to get the most out of their airplanes—are also notified by airworthiness concern sheets (ACS), which are part of the FAA’s airworthiness concern process. Type clubs are often the link between individual airplane owners and the FAA. On receipt of an ACS, the type club technical committee studies the concern before formulating a reply to the issues specified. The club may survey its members, conduct actual engineering tests, or endorse the action proposed for dealing with the problem. Type club contact information is available online.

Best practices

In 2003, a booklet titled The Best Practices Guide for Maintaining Aging GA Aircraft, endorsed by AOPA, was distributed to owners of GA aircraft built before 1974. This booklet is invaluable to owners. This 26-page guide can be downloaded as a pdf file from AOPA Online.

The guide charts a two-step approach. The first step is to compile as complete a record of the history of the airplane as possible through relatively inexpensive tools such as the maintenance records (logbooks) and FAA records. The registry branch of the FAA maintains a file that includes all changes in ownership; all major repairs and major alterations on what’s called a Form 337; and all bills of sale. All of the forms are available on AOPA Online. These records are helpful for estimating whether environmental factors and repairs have increased the drawdown on an airplane’s aging trust fund.

If ownership records show that an airplane was tied down on the ramp at an airport near a warm salt-water coastline such as Miami, Florida, or Mobile, Alabama, the owner would know to search for corrosion since the corrosion process is accelerated in a warm, salty atmosphere. If the airplane spent most of its life in a dry climate, the possibility of serious corrosion is lower, but still present. If the 337s show that a major repair had been done after a gear-up landing, this would alert the owner to be especially vigilant for deformation and associated damage when inspecting the repaired fuselage.

The second step suggested by the best practices manual is to consult with type club experts to learn what inspections are unique to your airplane model. The best practices guide contains links to resources and includes an aging aircraft inspection and maintenance baseline checklist. Owners will gain a much clearer understanding of the aging process and how time, maintenance, environmental factors, and operations have affected their airplane after working through the steps in this inspection and maintenance checklist.

As pilots age, their doctors do more testing and conduct more detailed examinations than they did in the past. We should apply the same kind of scrutiny to our aging airplanes.

With proper care, both pilots and airplanes can look forward to long and healthy lives.

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