Are you in the market for new tires for your airplane? Ever wonder what those numbers stood for?
As an engineer, I like things to be logical. Unfortunately, the basic system for tire specifications is not.
Let’s start with a “Type I” tire. As you might expect, it was an early attempt at a specification. It only defined the outer diameter of the tire—nothing about the hub size. Tires were listed by aircraft application. You want a tire for your Waco? Here it is!
Next logical specification is a Type II. But I can’t find any information on them.
Fortunately, Type III should be familiar to nearly everyone—it’s the classic two-part designation, such as “7.00×5.” The “7.00” is the maximum width of the tire; the “5” is the size of the wheel it’s designed to be mounted to.
Notice one critical lack: There’s no specification for the overall diameter of the tire! Doesn’t matter that much for fixed-gear aircraft, but those with retractable undercarts need to know if the tire will fit in the wheel well.
Time for a Type IV? Not hardly. The next step is referred to (I kid you not) as a “Three-Part” designation. A typical designation is “15×6.0-8,” where “15” is the outside diameter, “6.0” is the maximum width, and “8” is the wheel size. Oh, and don’t take the diameter and width specifications literally. The actual size is a range, which may fall short or exceed the indicated numbers. For example, one manufacturer’s “15-inch” diameter may run from 14.55 inches to 15.2. It’ll fit an 8-inch wheel, though.
Type III and Three-Part specifications should handle most aircraft. I don’t know where Types IV through VI went, but there is a Type VII. It’s mostly tires intended for higher-performance aircraft, with more inflation pressure and rated for faster takeoffs and landings.
Confused for which to buy? Not really a problem, for the most part. For fixed-gear aircraft, the wheel size is the critical dimension. And in any case, the best approach if you need new tires is to lean down and read the size of the old ones.
