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