The Pressure Principle
Wednesday, September 1, 2004
Very few pilots realize how important they really are
|When we were student pilots, we were told to check the tires for condition and inflation before each takeoff. But as we progressed in our flying careers, some of us have taken tires for granted. Sure, we’re careful to check the “important” stuff—engine oil, fuel, headset batteries and radios—but we keep tires on a second-class status, merely glancing at them to make sure that they’re all accounted for and aren’t flat. |
And as far as obtaining the right pressure goes, Robson advises, “Go with the recommended pressure, cold.” He also claims to never run your tires underinflated because, compared to overinflation, underinflation is the worse of the two evils.
Here’s what happens. An underinflated tire can’t maintain its shape and becomes flat while in contact with the ground. This excess deflection causes a traction wave that becomes more severe with speed, putting not only unnecessary stress on the tire, but also creating a dangerous situation on the runway. So, check your tires to make sure that they’re not lying too flat on the ground. When in doubt, it doesn’t hurt to err on the safe side and add air or nitrogen to your tires.
Robson advises to watch out for overinflation, however. And even though tires are sometimes better able to cope with overinflation when it comes to service pressure than the rims do, this fact shouldn’t allow you to run overinflated.
According to Robson, “There are some drawbacks to running overinflated. Although tires have to withstand four times the rated pressure—that’s straight from the FAA—I still wouldn’t run them overinflated on purpose. I know of at least one time when the wheel bolts failed [with severe overinflation]. The tire didn’t fail. It was the wheel that failed. And it cut the poor guy in half.”
So, Robson strongly recommends, “Don’t go over the rated maximum pressure. Those maximum pressures are rim-and-tire defined.”
Although the force of a tire’s explosion depends on its size, with smaller tires generating less force when overinflated compared to large aircraft tires, experts advise that this shouldn’t give you reason to overlook the problem. For example, a 50-pound-rated tire, which carries a 35-pound standard inflation recommendation, inflated to 80 psi, probably won’t cause any major problems, but it will quickly wear out the tire and unduly stress other components. The overinflation problem, some predict, often comes from using a high-pressure air or nitrogen source without a regulator or without monitoring your actions.
Experts also suggest to pay attention to the color spots and dots on your tires. They can help guide you in determining your tires’ light and heavy areas. The red dot signifies that it’s at the light spot, so line up the red dot with your valve stem. On a more-sophisticated tube, you may see a yellow stripe. That’s the tube’s heavy spot. So align that yellow stripe with your tire’s red dot, and you’ll be as close as you can get without balancing the assembly. The white dots represent the vents on a tube-type tire, and the green dots are the vents on a tubeless tire.
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