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?
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.