Thursday, December 1, 2005
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?
|In just the last few years, a series of T-34s, the military equivalent of a Bonanza, have suffered wing separations. An emergency airworthiness directive (AD) grounded the fleet. Just a couple of months ago, a well-maintained T-6, a World War II trainer, lost a wing doing maneuvers over Florida. With the general-aviation aircraft now averaging just less than 30 years of age, how can you tell if an airplane is safe to fly?|
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.
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