It operates in the same manner as the two engine pressure regulation and shutoff valves with the following exceptions. The valve is normally spring-loaded open and regulates at a set pressure of 110 ± 5 psi, thus acting as a safety regulator in the event one or both engine regulators fail and allow pressure to build up in excess of system design. Primary Bleed-Air Overpressure Switch This switch is located downstream of the bleed-air check valves (fig. 3-l). This switch activates at 250 psi and provides a signal to the Digital Display Indicator (DDI). The DDI is located in the nosewheel well of the aircraft and stores failed systems/component code numbers. These code numbers are used in troubleshooting the aircraft after flight, and aids in pinpointing malfunctions rapidly. Secondary Bleed-Air Overpressure Switch The pressure switch is mounted in the ducting downstream of the secondary bleed-air regulator. If bleed-air pressure at this point exceeds 150±10 psi, the overpressure switch provides a signal to close the three pressure regulator and shutoff valves as well as store a failed system code number in the DDI. Air Isolation Valve The sir isolation valve serves two purposes. First, it is used to cross start engines. After starting one engine on the auxiliary power unit (APU) or ground air, with the APU switch in the off position, the engine crank switch will automatically open the air isolation valve when starting the other engine. Bleed air from the engine running is routed through the air isolation valve to the engine starter control valve (fig. 3-1) of the engine to be started. As the engine accelerates to a self-sustaining speed, the switch automatically returns to the off position. The air isolation valve is then closed by spring pressure. The air isolation valve can also be used to route APU air to augment the bleed-air supplv to the air-conditioning system at times when engine output is low. This could be when waiting to launch, with engines at idle power and air temperatures high and humid. BLEED-AIR LEAK DETECTION The bleed-air leak detection system warns the pilot of a leak in the bleed-air distribution lines or shuts down the system, as necessary. The leak detection system consists of a control unit and nine detectors. When one of the detectors senses an overheat condition, it sends a signal through the control unit. The control unit signals the respective bleed-air pressure regulator to close and lights a warning light on the advisory panel in the cockpit, giving the location of the detector sensing the overheat condition. AIR CYCLE AIR-CONDITIONING SYSTEMS Learning Objective: Recognize the operating principles and components of air cycle air-conditioning systems (ACS). Most naval aircraft are designed with an air cycle ACS because it is efficient for the weight and space required and is relatively trouble free. The name air cycle or air-to-air comes from the principle of cooling the air without the use of refrigerants by compression and expansion of hot bleed air. The F-18 air cycle ACS is an example of this type system (fig. 3-2, a foldout at the end of this chapter). SYSTEM OPERATION The air cycle ACS was designed to operate by passing hot engine bleed air through the primary heat exchanger where ram air, forced across the heat exchanger by the aircrafts forward motion, absorbs heat from the bleed air, reducing the air temperature. On the ground and during low-speed operation, ram air is pulled across the heat exchangers by hot air ejected into the heat exchanger exit ducts by the primary and secondary heat exchanger ejectors. The cooled bleed air then passes to the flow modulating system pressure regulator valve where, controlled by electrical and pneumatic sensors, the downstream pressure is maintained by regulator modulation. The air enters the compressor end of the refrigeration turbine/compressor assembly where it is com- pressed to approximately twice its inlet temper- ature. The compressed air enters the secondary heat exchanger where ram air absorbs the heat acquired through compression. From the secondary heat exchanger, air enters the refrigera- tion cycle. The cycle is made up of a reheater heat 3-4