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.

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 primarily an airframe engineering and manufac-turing problem. 2. There must be a means of limiting the maximum pressure differential to which the cabin walls will be subjected. This is provided by the cabin safety valve. 3. The aircraft must have an adequate supply of compressed air. This is provided through the compressor section of the jet engine. A separate compressor or supercharger is used on aircraft having reciprocating engines. On all jet aircraft, the air is taken directly from the compressor section of the jet engine. This is generally referred to as bleed air. 4. There must be a means of cooling the bleed air before it enters the cabin. This is provided by an aircraft refrigeration unit. 5. There must be a means of controlling the cabin pressure. This is provided by the cabin pressure regulator, which regulates the outflow of air from the cabin. In addition to the major components, various valves, controls, and other allied units are necessary to complete an aircraft pressurization and air-conditioning system. The design, construc-tion, and use of these components may vary somewhat with different manufacturers; however, the systems on all jet aircraft operate on the same principles. The system used as an example in this text is in the F-18 aircraft. The environmental control systems of most aircraft include cabin air conditioning and pressurization, equipment cooling, defogging, windshield washing and rain removal, and equip-ment pressurization subsystems. Coverage in this section is limited to air cycle cabin and equipment pressurization and air conditioning.