AIRCRAFT PRESSURIZATION SYSTEMS
Learning Objective: Recognize the purpose and function of aircraft pressurization systems to include maintenance and troubleshooting operations.
As aircraft became capable of obtaining altitudes above that at which flight crews could operate efficiently, a need developed for complete environmental systems. Air conditioning could provide the proper temperature and supplemental oxygen could provide sufficient breathable air. The one problem was that not enough atmospheric pressure exists at high altitude to aid in breathing, and even at lower altitudes the body must work harder to absorb sufficient oxygen through the lungs to operate at the same level of efficiency as at sea level. This problem was solved by pressuring the cockpit/ cabin area.
The area of an aircraft to be pressurized must be free from all air leaks. This is accomplished by the use of seals around tubing, ducting, bolts, rivets, and other hardware that pass through or pierce the pressuretight area. All panels and large structural components are assembled with sealing compounds. Access and removable doors and hatches have integral seals. Canopies are constructed with inflatable seals. The pressurizing air is the air from the aircraft ACS. The S-3 aircraft incorporates a cabin pres-surization subsystem. This subsystem regulates the outflow of air from the cabin to control the cabin pressure according to a predetermined schedule. Cabin air is drawn through the internal avionics racks by the cabin exhaust fan and is modulated by the cabin pressure regulator valve. A cabin pressure regulator control provides the pres-surization schedule.
The cabin pressurization subsystem is governed by the pressure regulator control, which provides five modes of operation: unpressurized, isobaric, differential cabin-to-ambient pressure, dump, and repressurization. Below 5,000 feet, the cabin is normally un-pressurized. Between 5,000 and 25,000 feet, the cabin altitude will remain at 5,000 feet. Maximum cabin pressure-to- ambient differential is 6.7 ± 0.1 psi. Table 3-1 displays various cabin pressure differentials and cabin altitudes for different flight levels.
During the unpressurized mode of operation, the pressure regulator control directs low-pressure air to the pressure regulator valve to command it to the full open position. This mode of operation occurs at all altitudes below 4,350 feet. In this mode, cabin pressure is maintained at a near ambient pressure. The pressure is slightly above ambient because of the duct pressure losses, the quantity of air flowing into the cabin, and the pressure across the internal avionics ventilation subsystem. During flight operations between 5,000 and 24,000 feet, the isobaric mode maintains the cabin altitude between 4,350 and 5,000 feet. The pressure regulator control, using the sensed ambient pressure as a low-pressure source and the sensed cabin pressure as the high-pressure source, modulates the pressure regulator open or closed to maintain cabin pressure at the specific altitude. The differential mode of operation overrides the isobaric mode of operation when the aircraft is flying at altitudes in excess of 24,000 feet. As cabin-to-ambient differential pressure reaches 6.7 ± 0.1 psi, a spring-loaded diaphragm in the pressure regulator control positions a poppet valve to supply this differential pressure as a control pressure to the pressure regulator valve. The pressure regulator valve compares this control pressure to cabin pressure, and it positions
Table 3-1.- Cabin Altitude versus Flight Altitude Schedule
Figure 3-20.- Cabin pressurization subsystem schematic.
the butterfly valve to maintain the required differential pressure. The cabin pressurization system also makes provision for dumping cabin pressure in an emergency. By setting the cabin pressure switch on the environmental control panel to the DUMP position, the latching solenoids on both the cabin outflow pressure regulating valve and on the cabin safety valve are actuated to the dump position. In addition, the recirculation air shutoff valve will be actuated to the full open position, provided electrical power is available. A secondary method of achieving cabin depressurization is to turn the air-conditioning switch to the OFF/ RESET position and select the auxiliary vent mode. This selection will cause the cabin outflow pressure regulator valve to open, but it will not actuate the cabin safety valve to the open position. The repressurization mode of operation is used when returning to the normal mode from the dump mode or during a rapid descent in excess of 4,000 feet per minute. In this mode, the pressure regulator control modulates the rate of cabin repressurization with an integral isobaric and differential pressure control system. The pressure regulator control compares the existing cabin pressure to a lagging cabin pressure reference. If the result of this comparison exceeds the calibrated rate, control pressure output from the pressure regulator control is reduced. This causes the pressure regulator valve to sense a relatively higher pressure on the opening side of its actuating diaphragm, thus allowing the diaphragm to open the pressure regulator valve butterfly. This reduces cabin pressure and the rate of repressurization. Precautions for operating the S-3 cabin pres-surization subsystem on the ground, where the elevation is 5,000 feet or higher, are required because the cabin pressurization subsystem does not have provisions for automatic repressuriza-tion. Therefore, the cabin will be pressurized whenever the ground elevation is above 5,000 feet. To ensure adequate cooling of the internal avionics during operations at ground elevations above 5,000 feet, one of the following steps must be used: 1. Keep the cabin pressurized as in flight. 2. Set the cabin pressure switch to DUMP to ensure a full-open pressure regulator valve and a full-open pressure safety valve. 3. If the outside air temperature is below 80° F, turn the auxiliary vent selector to ON and open the cabin entry door.