How Old is Too Old?

When you're talking about the age of airplanes, you're actually discussing a couple of factors that are intertwined---hours and condition---and each seriously affects the other. There's yet another subset of factors that should be considered, however---the material the airplane uses in its construction: aluminum, steel, wood or composite.

Let's talk about the different materials first. Aluminum is, hands-down, the most common airframe material with which most of us will interface, although in the more sport-oriented fields, we'll run into lots of steel and a little wood. Composites are something of a wild card because they're so new that we haven't accumulated enough hours or years to make definitive statements about aging, other than they don't rust.

Aluminum
There are a couple of basic facts about aluminum that can't be avoided. Because of its metallurgy, it's nearly impossible to design an aluminum structure that, if used enough, won't eventually develop fatigue cracks. This tendency is driven strictly by the hours flown (with a fudge factor for the type of flying tossed in). Given enough load cycles, aluminum will crack. But the structure can be designed so that the number of cycles required for it to crack may be so enormous that we're talking about flight hours measured in lifetimes. Fatigue always is a concern with aluminum, however.

To make matters worse, aluminum easily can be compromised by condition problems that are as simple as scratches. A scratch causes a stress riser, which can promote fatigue cracks. That's why during a preflight, you're cautioned to always run a hand down a prop's leading edge to look for nicks and scratches. At the bottom of a V-shaped nick or scratch, the stresses can be as much as 15 times higher than on either side of it. Nicks and scratches are most harmful on highly stressed parts of the airplane, such as propellers, wing and strut fittings, and landing gear legs.

Corrosion is another form of stress riser that's both hard to find and insidious. Corrosion does more than just act as a stress riser; it actively removes metal, thus reducing the structure's ability to resist the loads it was designed for. So, besides the problem of reducing the structure's long-term fatigue life, corrosion actually can lead to local failure just because metal is missing.

The good news about aluminum and corrosion, though, is that aluminum will only corrode in certain environments. The sheets from which aircraft are constructed usually are "alclad," meaning they have a thin layer of pure aluminum on both sides that will oxidize and create a barrier to further corrosion (which is really nothing more than oxidation gone wild). The primary cause of aluminum corrosion is the presence of salt air or chemicals in the environment (acid rain). If none of these are present, an aluminum airplane can theoretically sit out forever, and only the steel parts will deteriorate. Witness the Arizona boneyards, for example.

Steel
Steel components (such as fuselages, fittings and bolts) easily can be designed to have an infinite fatigue life because one of steel's characteristics is that as long as the loads are kept within design limits (under its S-N curve), no fatigue is seen in the material. Just as with aluminum, the fatigue life of steel can be greatly reduced by nicks and scratches, but it's not nearly as critical as with aluminum. Steel is extremely susceptible to garden-variety rust, however. There's no magic coating, like alclad on aluminum, that can make a fuselage last forever, although some space-age techniques come close. Bolts and fittings can be cadmium-plated, but even there, the protection won't necessarily last forever, and you can't easily plate a fuselage or landing gear. The new epoxy primers and paints are super-long-lasting, as is powder coating, but no one claims they're good forever. All it takes is a screwdriver scratch to penetrate the coating, and moisture has an instant path to start rusting. Even worse, steel doesn't need a salty environment for rust to start. Any source of moisture will do---dew, internal condensation because of temperature changes and so forth. Steel can't sit around exposed to the elements indefinitely without eventually being reduced to dust. Properly treated, however, it will last longer than any of us reading this.

One additional note: Both aluminum and steel have to worry about critters. A nice little mouse nest will heavily damage both metals in a matter of weeks unless you housebreak your mice, of course.

Wood
It's unlikely you'll be encountering wood on a regular basis unless you fly older classic airplanes or Bellanca low-wing aircraft. The spars in many 1940s and 1950s classics are wood, and Bellancas have all-wood wings. Wood, if properly protected, will last for many decades, but the operative words are "properly protected." Wood hates moisture. It hates extremely high heat and extremely low humidity. It won't fatigue, but it will certainly rot, dry rot, support fungus, develop drying cracks and feed termites. In other words, like steel, unless wood receives occasional care and the proper protection, it definitely can't live forever.

Now Let's Talk Condition
Other than the cosmetic stuff, like faded paint, dings and other ugly stuff, condition is a difficult thing to judge. Although bad cosmetics definitely say something about the owner's attitude toward his or her airplane, they don't necessarily say the airplane is unsafe. The factors that really count don't show at all.

Hours flown monthly: An engine that isn't flown regularly is prone to corrosion forming inside the engine, especially in the bores. Plus, Lycomings love to rust the rear cam lobe. This is a serious problem that makes airplanes that have accumulated a low number of hours over a large number of years very suspect. A sleeping airplane also attracts dust, and dust attracts moisture. An airplane that doesn't move attracts critters, and all of its joints (bearings and such) get stiff.

Storage: Airplanes live longer if they spend most of their lives indoors. However, that doesn't guarantee they're safe to fly. All it means is that their airframe is likely to last longer. An airplane that sits outside in super-dry Arizona, for instance, but is flown 400 hours a year and is well-maintained, is probably safer than the hangar queen in often-humid New Jersey that flies 20 hours a year, regardless of how much better the paint and Plexiglas may look.

Average local humidity: High humidity aggravates engine problems and makes everything that is mentioned above worse.

Total time: In theory, any airplane can be flown so much that it's worn out, but that's seldom the case. Most airplanes die or develop problems from lack of use, not overuse. Unless the airplane is worked commercially, it isn't likely to see much over 100 to 200 hours a year, and the vast majority of airplanes see less than 50 hours a year. Even though we have several airplanes over 50 years old (all the classics), few, if any, are worn out. They die because, at some point in their life, they're treated as derelicts and not maintained.

Even though fatigue life of light airplanes can't be accurately predicted or even properly estimated, we're seeing a small percentage of light aircraft approaching 7,000 to 9,000 hours, yet we're seeing very few serious fatigue failures. When an airplane gets that much time on it, an owner gets wary and maintenance people know to look for fatigue-related problems so they're caught before they become catastrophic.

Maintenance: Logbooks that show a regular maintenance cycle means the owner spent money on the airplane, which means problems were fixed as they popped up, rather than being allowed to accumulate. It also means someone was poking around inside the airplane on a regular basis, so rust and corrosion were stopped before they got a good foothold.

So, How Old Is Too Old?
There's a great line in an Indiana Jones movie when he's explaining why he's so tired, "It's not the years. It's the miles that count," and that's exactly the way it is with airplanes. Years are nothing but a number. The question is how many miles of bad road did this particular airplane live through to get to this point? How many nights did it sit out in the rain? How many thousands of hours did it hug the treetops, its pilot staring at pipelines while getting his or her brains beat out in turbulence? How many years did it sit in the weeds ignored? How many students have hammered it onto the runway? How many operators said, "Nah, just fix what's absolutely necessary"? It's not how long it has been alive, but how well it lived life that counts.

When trying to decide whether an airplane is too old to fly safely, you can't think only in terms of years. Condition is everything. The only easy guide is to try to get a handle on the owner's attitude by the way the important items look. Even if the paint is faded and it has a dent here and there, but the engine compartment is clean and fresh, the belly shows no stains or wrinkles, and the flight deck is worn, the airplane is probably perfectly flyable.

If the airplane is quite new, but there's oil-soaked dust on top of the engine and mags, the ignition leads are frayed and the belly is streaked with soot and oil, start worrying. If, as you're being checked out, the owner says, "That doesn't work, but we really don't need it," find a good reason to not take that flight. When it comes to airframe reliability and safety, the owner's attitude is everything.

Don't be fooled by a shiny coat of paint and a youthful appearance. That's not what keeps you in the air.