At the AOPA Expo, I had a quick primer on what makes out tires go flat, and what to do to keep that from happening, especially at inopportune times.

Goodyear engineer Robert Robson was well into his talk when I managed to catch up, but I noted that he stressed that the main reason tires go flat is... that they're underinflated. An underinflated tire can break down internally in a surprisingly short time, due to the inordinate amount of heat that excess flexing causes. When a tire has too little air in it, its sidewall and tread flex more than they should, and that extra work makes short work of the chemistry that keeps the air inside.

OK -- so let's have a look at some causes of underinflation. As summer becomes a memory, it wouldn't hurt to reflect on the notion that air contracts in cold. In fact, Mr. Robson noted, a good rule of thumb to use is that, for every 5 degrees (Fahrenheit) drop in temperature, you'll lose about 1% of your tire pressure. So, as the weather cools down, remember to check your tires' pressure.

If you don't do anything at all, of course, your tires will still lose pressure. Industry standards say that a tubeless tire is still in spec if it loses 5% of its pressure in 24 hours. That's not typical; and tube-type tires (especially butyl tubes) will lose less -- but check, and you won't be rudely surprised some day when you don't have time to change that flat tire.

Speaking of tubes, don't re-use a tube when you mount a tire. A tube stretches, and it's just too much rubber to fit properly, if you re-use it. Sure, you can chance it in a pinch [sorry], but the added expense and inconvenience of tube-snafus make it worthwhile to use a new tube. Incidentally, if you want to for special conditions, or just because you're old-fashioned, you may use a tube in a tubeless-type tire.

Those colorful dots on new tires actually mean something. The red dot is at the light spot, so line that red dot up with your valve stem. On a more-sophisticated wheel, you may see a yellow stripe; that's the wheel's heavy spot, so line that yellow stripe up with your red dot, and you'll be as close as you can get, without balancing the assembly. (A white dot means the same as a red one, but it's used on a tube-type tire.)

When you put that new tire on, remember that it, too, will stretch, so check that inflation twelve hours after you first bring it up to pressure. Oh -- and if a tire's unloaded, it will typically show pressure about 4% lower than if it's loaded, so remember to check your new tire's pressure on the airplane. That's where you use it; that's where you should check it.

Tires, even those that are so typically underloaded on our little GA airplanes, need their air. In fact, Robson recommends removing the tire, if you note that its pressure is less than 90% of where it should be, and you have run on it. [That's one more reason to check tire pressures on your preflight. If you haven't stressed an underinflated tire, you can usually just pump it up to proper pressure, roll it a way under load to remove any flat spots caused by sitting, and fly it. If you wait until it's been damaged, all you can do is replace it --ed.]

Besides neglect (underinflation), FOD is the next most-common cause of tire failure, so stay out of the nails, safety wire, broken parts, and junk that's often lurking just alongside the taxi areas.

Just as the key to success in retail selling is often "location, location, location," Robert reminded us, the secret to long life and predictable performance for tires is "inflation, inflation, inflation."

FMI: www.goodyear.com