The life of a tire, whether mounted or unmounted, is directly affected by storage conditions. Tires should always be stored indoors in a dark, cool, dry room. It is necessary to protect them from light, especially sunlight. Light causes ultraviolet (UV) damage by breaking down the rubber compounds. The elements, such as wind, rain, and temperature changes, also break down the rubber compounds. Damage from the elements is visible in the form of surface cracking or weather checking. UV damage may not be visible. Tires can be protected from light by painting the storeroom windows. Tires must not be allowed to come in contact with oils, greases, solvents, or other petroleum products that cause rubber to soften or deteriorate. The storeroom should not contain fluorescent lights or sparking electrical equipment that could produce ozone.

Tires should be stored vertically in racks and according to size. See figure 11-15. The edges of the racks must be smooth so the tire tread does not rest on a sharp edge. Tires must never be stacked in horizontal piles. The issue of tires from the storeroom should be based on age from the date of manufacture so the older tires will be used first. This procedure helps to prevent the chance of deterioration of the older tires in stock.

There are two types of inspections conducted on tires. One is conducted with the tire mounted on the wheel. The other inspection is conducted with the tire dismounted.

During each daily or special inspection, tires must be inspected for correct pressure, tire slippage on the wheel (tube tires), cuts, wear, and general condition. Tires must also be inspected before each flight for obvious damage that may have been caused during or after the previous flight.

Maintaining the correct inflation pressure in an aircraft tire is essential to safety and to obtain its maximum service life. Military aircraft inner tubes and tubeless tire liners are made of natural rubber to satisfy extreme low-temperature performance requirements. Natural rubber is a relatively poor air retainer. This accounts for the daily inflation pressure loss and the need for frequent pressure checks. If this check discloses more than a normal loss of pressure, you should check the valve core for leakage by

Figure 11-16.Tire slippage mark.

putting a small amount of suitable leak detection solution (Leaktec) or soapy water on the end of the valve and watch for bubbles. Replace the valve core if it is leaking. If no bubbles appear, it is an indication that the inner tube (or tire) has a leak. When the tire and wheel assembly shows repeated pressure loss exceeding 5 percent of the correct operating inflation pressure, it should be removed from the aircraft and sent to the AIMD or IMA.

Overinflation or underinflation can cause catastrophic failure of aircraft tire and wheel assemblies. This could result in injury, death, and/or damage to aircraft or other equipment.

After making a pressure check, you should always replace the valve cap. Be sure that it is screwed on fingertight. The cap prevents moisture, salt, oil, and dirt from entering the valve stem and damaging the valve core. It also acts as a secondary seal if a leak develops in the valve core.

Tires that are equipped with inner tubes, and operate with less than 150 psi, and all helicopter tube tires must use tire slippage marks. The slippage mark is a red paint strip 1 inch wide and 2 inches long. It extends equally across the tire sidewall and the wheel rim, as shown in figure 11-16. Tires should be inspected for slippage on the rim after each flight. If the markings do not align within one-fourth of an inch, the wheel assembly should be replaced and the defective assembly forwarded to the AIMD or IMA for repair. Failure to correct tire slippage may cause the valve stem to be ripped from the tube.

Tire treads should be inspected to determine the extent of wear. The maximum allowable thread wear for tires without wear depth indicators is when the tread pattern is worn to the bottom of the tread groove at any spot on the tire. The maximum allowable tread wear for tires with tread wear indicators is when the tread pattern is worn either to the bottom of the wear depth indicator or the bottom of the tread groove. These limits apply regardless of whether the wear is the result of skidding or normal use.

The tread and sidewall should be examined for cuts and embedded foreign objects. Figure 11-17 shows the method for measuring the depth of cuts, cracks, and holes. Glass, stones, metal, and other materials embedded in the tread should be removed to prevent cut growth and eventual carcass damage. A blunt awl or screwdriver maybe used for this purpose. You should be careful to avoid enlarging the hole or damaging the cord body fabric.

When you are probing for foreign objects, be sure you keep the probe from penetrating deeper into the tire. Objects being pried from the tire frequently are ejected suddenly and with considerable force. To avoid eye injury, safety glasses or a face shield should be worn. A gloved hand over the object may be used to deflect it.

Aircraft should not be parked in areas where the tires may stand in spilled hydraulic fluids, lubricating oils, fuel, or organic solvents. If any of these materials is accidentally spilled on a tire, it should be immediately wiped with a clean, absorbent cloth. The tires should then be washed with soap and thoroughly rinsed with water.

Extra care should be taken when you inspect mounted helicopter tires. Because of the long intervals between tire changes, helicopter tires are subject to weather and UV damage.