PLY RATING - The term "ply rating" is used to indicate an index to the load rating of the tire. Years ago when tires were made from cotton cords, "ply rating" did indicate the actual number of plies in the carcass. With the development of new higher-strength fibers such as nylon, fewer plies are needed to give an equivalent strength. Therefore the definition of the term "ply rating" (actual number of cotton plies) has been replaced to mean an index of carcass strength or a load carrying capacity.

LOAD RATING - This is the maximum allowable load that the tire can carry at a specific inflation pressure.

RATED PRESSURE - Rated pressure is the maximum inflation pressure to match the load rating. Aircraft tire pressures are given for an unloaded tire; i.e., a tire not on an airplane. When the rated load is applied to the tire, the pressure increases by 4%.

OUTSIDE DIAMETER - This measurement is taken at the circumferential center line of an inflated tire.

SECTION WIDTH - This measurement is taken at the maximum cross sectional width of an inflated tire. rim diameter - This is the nominal diameter of wheel/rim on which the tire is mounted.

SECTION HEIGHT - This measurement can be calculated by using the following formula:

Section Height = Outside Diameter - Rim Diameter
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                                2

ASPECT RATIO - Aspect ratio is a measure of the tire's cross section shape. This can be calculated by the following formula:

Aspect ratio = Section Height 
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               Section Width

FLANGE HEIGHT - This is the height of the wheel rim flange

FLANGE DIAMETER - This is the measurement taken at the top of the wheel rim flange.

FREE HEIGHT - This measurement can be calculated by using the following formula:

Free Height = Outside Diameter - Flange Diameter
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                             2

STATIC LOADED RADIUS - This is the measurement from the center of the axle to the runway for a loaded tire.

LOADED FREE HEIGHT - This measurement can be calculated by using the following formula:

Loaded Free Height = Static Loaded Radius - Flange Diameter
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                                       2

TIRE DEFLECTION - A common term used when talking about aircraft tires is the amount of deflection it sees when rolling under load. The term % Deflection is a calculation made using the following formula:

% Deflection = Free Height - Loaded Free Height
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                         Free Height

Aircraft tires are designed to operate at 32% deflection, with some at 35%. As a comparison, cars and trucks operate in the 17% range.

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Aircraft Tire vs. Other Tire Applications

Many people believe that all tires are alike. This chart shows a comparison of an aircraft tire versus a passenger tire. The tires may be similar in size, but that is where similarities end.

Comparing, in particular, the LOAD and SPEED ratings of these two tires, the aircraft tire carries 13,000 lbs., which is approximately sixteen times the passenger tire load of 835 lbs. It is also traveling almost three times as fast.

Also, notice that the operating pressure of the aircraft tire is almost 10 times that of the passenger tire; and that the aircraft tire is operating at a deflection of 32%, as compared to 17% for the passenger tire.

The HEAVY LOAD coupled with the HIGH SPEED of aircraft tires makes for extremely SEVERE OPERATING CONDITIONS. Several of the following charts are centered around these two major factors. The purpose of these charts is to present items that minimize and maximize these adverse effects. The ultimate goal is to not only understand what needs to be done, but why.

This chart shows the SPEED versus LOAD operating ranges of Passenger, Truck, Race, Industrial, Off The Road, and Aircraft tires. Only Aircraft tires have the worst of both loads and speeds. This means that maintenance practices and operating techniques that work fine for passenger tires are not acceptable for aircraft tires. Because of the severe conditions under which aircraft tires operate, any deviation from proper techniques and practices will have immediate and severe consequences.

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Courtesy of The Goodyear Tire & Rubber Company