tire be scrapped. Inner liner condition on tubeless tires is also  critical. Replacing the tube in a tube-type tire is performed  by the technician as are mounting and balancing all types  of aircraft tires.  Aircraft tires are very expensive. They are also extremely  durable. The effective cost of a tire over its life can be  reduced by having the tread replaced while the carcass is  still sound, and injuries are within repairable limits. Federal  Aviation Administration (FAA) certified tire retread repair  stations, often the original equipment manufacturer (OEM),  do this work. The technician inspects a tire to pre-qualify  it for retread so that the cost of shipping it to the retread  repair facility is not incurred if there is no chance to retread  the tire. The tire retreader inspects and tests every tire to a  level beyond the capability of the hangar or line technician.  Shearography, an optical nondestructive testing method that  provides detailed information about the internal integrity of  the tire, is used by tire retread repair facilities to ensure a tire  carcass is suitable for continued service. Tires that are retreaded are marked as such. They are not  compromised in strength and give the performance of a new  tire. No limits are established for the number of times a tire  can be retreaded. This is based on the structural integrity of  the tire carcass. A well maintained main gear tire may be able  to be retreaded a handful of times before fatigue renders the  carcass un-airworthy. Some nose tires can be retread nearly  a dozen times. An aircraft tire can be damaged if stored improperly. A  tire should always be stored vertically so that it is resting  on its treaded surface. Horizontal stacking of tires is not  recommended. Storage of tires on a tire rack with a minimum  3–4-inch flat resting surface for the tread is ideal and avoids  tire distortion. If horizontal stacking of tires is necessary, it should only be  done for a short time. The weight of the upper tires on the  lower tires cause distortion possibly making it difficult for  the bead to seat when mounting tubeless tires. A bulging  tread also stresses rib grooves and opens the rubber to ozone  attack in this area.  [Figure 13-151]  Never stack aircraft tires  horizontally for more than 6 months. Stack no higher than  four tires if the tire is less than 40-inches in diameter and no  higher than three tires if greater than 40-inches in diameter. The environment in which an aircraft tire is stored is critical.  The ideal location in which to store an aircraft tire is cool,  dry, and dark, free from air currents and dirt.  An aircraft tire contains natural rubber compounds that are  prone to degradation from chemicals and sunlight. Ozone  13-88 Many aircraft tires accept a tube inside to contain the inflation  air. Tube-type tires are handled and stored in similar fashion  as tubeless tires. A number of issues concerning the tubes  themselves must be addressed. Tires are made to operate in a wide range of temperatures.  However, storage should be at cool temperatures to minimize  degradation. A general range for safe aircraft tire storage  is between 32 °F and 104 °F. Temperatures below this are  acceptable but higher temperatures must be avoided.  Common hydrocarbon chemicals, such as fuels, oils, and  solvents, should not contact a tire. Avoid rolling tires through  spills on the hangar or shop floor and be sure to clean any  tire immediately if contaminated. Dry the tire and store all  tires in a dry place away from any moisture, which has a  deteriorating effect on the rubber compounds. Moisture with  foreign elements may further damage the rubber and fabric  of a tire. Dirty areas must be avoided. (O$_{3}$) and oxygen (O$_{2}$) cause  degradation of tire compounds.  Tires should be stored away  from strong air currents that  continually present a supply of one or both of these gases.  Fluorescent lights, mercury  vapor lights, electric motors,  battery chargers, electric welding equipment, electric  generators, and similar shop equipment produce ozone and  should not be operated near  aircraft tires. Mounted inflated  tires can be stored with up to  25 percent less pressure than  operating pressure to reduce vulnerability from ozone attacks.  Sodium vapor lighting is acceptable. Storage of an aircraft  tire in the dark is preferred to minimize degradation from  ultraviolet (UV) light. If this is not possible, wrap the tire in  dark polyethylene or paper to form an ozone barrier and to  minimize exposure to UV light.  Figure 13-151.  Ozone cracking in a tire tread groove is facilitated  by horizontal stacking. Tire Storage  Aircraft Tubes