the surface may burst, causing hemorrhages in the ears, eyes, and breathing passages. The outside air temperature also changes with altitude. For example, at approximately 18,000 feet the outside air temperature will be – 4°F (– 20°C), and at approximately 37,000 feet the outside air temperature will be – 67°F ( – 55°C). Above 37,000 feet the air continues to thin, but the air temperature will remain constant for several miles and then begin to rise slowly. Thus, the lowest outside air temperature to be encountered by an aircraft would occur at a height of about 7 miles. NOTE: The conversion formula for con- verting Fahrenheit to Celsius (centigrade) is ~ (F-32). For example, – 4°F is converted as Conversion of a Celsius temperature to a Fahrenheit reading is accomplished using the following formula: For example, – 55°C is converted as Remember not to drop the + and – signs when converting. These variations in outside air temperature and atmospheric pressure are considered by the air- craft manufacturer when designing the aircraft. ATMOSPHERIC CONSIDERATIONS Pressurization and air conditioning of aircraft are necessary at high altitudes. With operational ceilings now in excess of 50,000 feet, flight personnel, and in some cases aircraft components, are supplied with an artificial means of maintain- ing a reasonable pressure around the entire body and/or equipment. This is done be sealing off the entire cabin/cockpit and any equipment area that may require pressurization and maintaining an in- side air pressure equivalent to that at substantially lower altitudes. This is known as the pressurized cabin, cockpit, or compartment, as applicable. 3-2 In addition to pressurizing them, the cabin, cockpit, and some compartments are also air- conditioned if the aircraft is to fly at high speeds. This requirement is partly due to the difference in temperatures at various altitudes and also aerodynamic heating. For example, an aircraft flying at supersonic speeds at an altitude of 35,000 feet may generate a temperature on its skin of 200°F, and twice that temperature at altitudes near sea level. In addition to aerodynamic heating, other factors affecting cabin/cockpit temperatures are engine heat, heat from the sun (solar heat), heat from the electrical units, and heat from the body. Through research and tests, it was determined that the average total temperature of these five heat sources will raise the cabin/cockpit temperature to approximately 190°F (88°C). Through ex- periments it was determined that the maximum temperature that a person can withstand and maintain efficiency for extended periods is 80°F (27°C); therefore, air conditioning of the cabin/cockpit area is just as essential as pressurization. Under low-speed operating con- ditions at low temperature, cabin/cockpit heating may be required. The proper operation of much of todays air- craft electronic equipment is also dependent on maintaining a reasonable operating temperature that will prolong the life of various components. In most cases equipment cooling is provided by teeing off with ducting from the cabin/cockpit system. On other aircraft a separate cooling system may be used primarily for equipment cooling. ENVIRONMENTAL CONTROL SYSTEMS Learning Objective: Recognize the need for environmental control systems. The combined pressurization and air con- ditioning of the cabin is the function of the air- craft pressurization and air-conditioning system; a system now in all naval aircraft. The inspection and maintenance of this system is one of the important duties of the AME. There are five requirements necessary for the successful functioning of a pressurization and air- conditioning system. 1. The cabin must be designed to withstand the necessary pressure differential. This is