How Old is Too Old?
A number of recent airworthiness directives for the general aviation fleet seem to be directly related to the aircraft’s age and flight time. So when is it safe to fly an aging plane?
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.
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.