9: Pneumatic Systems

Air Conditioning

Pressurization Definitions

Outflow Valve and Control of Cabin Rate-of-Climb

Pressure Ranges and Dump Valve

Negative Pressure Relief Valve

Cabin Differential Pressure

Scuba Diving

Air Conditioning

Modern jet aircraft normally use bleed air from the engines' compressor sections for air conditioning and pressurization. See Figure 9-1. Air from this source is under pressure, already heated from compression, and free from contamination. The stage of engine compressor used as the bleed air source varies from one engine type to another, and often air from more than one stage is used.

Figure 9-1. Typical bleed air schematic (Boeing 727)

The bleed air system not only supplies air for the air conditioning and pressurization system, but also for starting the engines and sometimes for wing thermal anti-ice.

Before air from the bleed air system can be introduced into the cabin, it must be cooled. See Figure 9-2. This is accomplished in an air conditioning pack. The air conditioning pack consists of an air cycle system and its controls. An air cycle system has a source of compressed air, heat exchangers and a turbine. Most aircraft have two or more packs.

The air conditioning packs regulate cabin temperature by mixing hot bleed air with bleed air that has been cooled by the Air Cycle Machine (ACM). The hot and cold bleed air is mixed by the air mix valve. Air flowing through the ACM portion of the pack first goes through a primary heat exchanger where it is cooled by outside air. Next, the air flows to the air cycle machine compressor. The action of the compressor results in higher air pressure and temperature. After the compressor, air goes through a secondary heat exchanger that cools it again. Air then passes through the ACM turbine which results in an even greater heat loss. Finally, the now cold air passes through a water separator that removes condensed moisture.

The ACM turbine has two cooling effects on the air passing though it: First, it extracts the energy required to drive the compressor. Secondly, the ACM turbine expands the air, which reduces the air temperature.

Some aircraft use a vapor cycle system to cool air, rather than an ACM. This system uses a refrigerant to cool the cabin. It operates in much the same way as a household refrigerator.

The refrigerant most commonly used in aircraft is dichlorodifluoromethane. It is usually called "R-12." R-12 is a very good refrigerant and is generally non-toxic, but it does present some hazards. If R-12 is passed over an open flame it will become deadly phosgene gas. If it comes in contact with water it will form hydrochloric acid.

Combustion heaters burn fuel within the heater itself. Outside ventilating air transports the heat produced to locations in the aircraft where it is desired.

Figure 9-2. Typical bleed air conditioning pack schematic (Boeing 727)

Pressurization Definitions

Aircraft flown at high altitudes are pressurized to prevent hypoxia. Air inside the cabin is maintained at a higher pressure than the outside. This does not change the percentage of oxygen available, but it does increase the partial pressure of the oxygen to the point where there is enough to sustain normal life functions.

Pressurizing an aircraft's interior imposes tension stress on the fuselage which must be kept within safe limits. All pressurized aircraft have a maximum differential pressure limit that must never be exceeded.

The following terms apply to pressurization systems:

Cabin Differential Pressure—the ratio between the internal and external air pressures acting on the aircraft. This is usually expressed in PSID (Pounds per Square Inch Differential). The flight engineer's panel has a gauge indicating cabin differential pressure.

Cabin Pressure—the actual pressure in the aircraft cabin. This is not usually indicated on any of the flight engineer's instruments.

Cabin Pressure Altitude—the cabin pressure indicated in terms of altitude; that is, the lower the cabin pressure, the higher the cabin pressure altitude. Cabin pressure altitude is usually indicated by a gauge on the flight engineer's panel.

Cabin Rate of Climb (Descent)—the rate of change in the cabin pressure altitude measured in feet per minute. This is indicated on the flight engineer's panel by a vertical velocity indicator vented to the cabin.

Outflow Valve and Control of Cabin Rate-of-Climb

The air conditioning packs pump a relatively constant volume of air into the aircraft's cabin. The cabin pressure is controlled by regulating the rate at which air escapes. There are vents in the galleys and lavatories (to remove odors), in the electronic bays (for cooling) and the cargo pits (for ventilation). In addition, as an aircraft ages, it develops many small unregulated cabin air leaks. But the greatest volume of air exits through the cabin outflow valve.

The purpose of the outflow valve is to dump all pressure in excess of a preset amount. When the valve is fully open, the cabin pressure is equal to the outside air pressure. As the outflow valve closes, the cabin differential pressure increases. When the outflow valve is closed and then opens, the cabin differential pressure decreases.

The normal flight profile is to take off with the cabin altitude at or near the field elevation (differential pressure equal to zero). As the aircraft climbs, the outflow valve slowly closes, increasing the differential pressure. However, there are almost no circumstances in aircraft requiring a flight engineer in which the cabin altitude is kept equal to the departure field elevation. Usually the cabin altitude is slowly raised so that the maximum allowable differential pressure will not be reached before the aircraft reaches its cruise altitude. The cabin rate-of-climb is set considerably less than the aircraft's rate-of-climb, usually 300-500 feet per minute.

When the aircraft descends, the outflow valve slowly opens to decrease the differential pressure. Properly controlled, the cabin should reach the destination field altitude before the aircraft does. If in a climb, the cabin altitude is increasing too rapidly, the flight engineer should adjust the pressurization controls to cause the outflow valve to close faster. If the cabin outflow valve is adjusted to close too fast during a climb, the cabin rate-of-climb will be slower than desired and the cabin will reach its cruise altitude after the aircraft levels off, which could cause the aircraft to reach the maximum differential.

Pressure Ranges and Dump Valve

The outflow valve works in one of three modes—differential, isobaric or auxiliary (ambient) ventilation.

In the differential mode, a differential metering valve keeps the cabin differential pressure from exceeding its maximum allowable value.

In the isobaric mode, the outflow valve will cause the cabin altitude to climb (or descend) at a preset rate to the desired cabin altitude. Once that cabin altitude is attained, operation of the regulator bellows will maintain the cabin there. If the maximum cabin differential pressure is reached, the outflow valve reverts to the differential mode.

The auxiliary (ambient) ventilation mode will cause the outflow valve to open and the cabin to de-pressurize. If ambient ventilation is selected in pressurized flight, a rapid decompression can result.

All aircraft have either a separate dump valve or a dump mode of the outflow valve. The purpose of this valve is to relieve all positive pressure from the cabin. When the dump valve is open on the ground, the aircraft cannot be pressurized.

Negative Pressure Relief Valve

In a very rapid descent, it is possible for the aircraft's altitude to get lower than the cabin altitude, resulting in a negative differential pressure. The aircraft is not designed for that type of stress and a negative pressure relief valve must be installed. The function of the negative pressure relief valve is to prevent atmospheric pressure from exceeding the cabin pressure.

Cabin Differential Pressure

The flight engineer must ensure that the maximum differential pressure limits are not exceeded. FAA Figure 6 lists the air pressure in pounds per square inch (PSI) at various altitudes. When the differential pressure and cruise altitude are known, the cabin altitude can be easily calculated.

For example, an aircraft is cruising at FL340 with a cabin differential pressure of 8.6 PSI. What is the cabin pressure altitude? The outside air pressure at FL340 is 3.63. The cabin pressure is the outside air pressure plus the differential (3.63 + 8.6 = 12.23). This puts the cabin altitude between 4,000 and 6,000 feet.

The pressure for 4,000 feet is 12.69 and the pressure at 6,000 feet is 11.78, a difference of .91 PSI (12.69 - 11.78 = .91). The pressure for the unknown cabin altitude is .46 PSI less than the pressure at 4,000 feet. This means that the cabin altitude is 51% of the difference between 4,000 and 6,000 feet (.46 ÷ .91 = 51%). This equals 1,020 feet (51% of 2,000 feet = 1,020 feet). The cabin altitude is 1,020 feet above 4,000 (4,000 + 1,020 = 5,020).

Another variation of this problem is the situation in which a maximum flight altitude must be calculated when the cabin altitude and maximum differential are known. For example, what is the maximum flight altitude when the desired cabin altitude is 8,000 feet and the maximum differential pressure is 5.0 PSI? When a cabin is pressurized to 8,000 feet, the cabin pressure is 10.92 PSI. A maximum differential of 5.0 PSI means that the minimum outside air pressure is 5.92 PSI (10.92 - 5.0 = 5.92). The table indicates that is just a bit above 22,000 feet. As a practical matter, the aircraft could not cruise above FL220 and stay within its pressurization limits.

Some problems on the test use inches of Hg instead of PSI. The same calculations apply.

Scuba Diving

When scuba diving, a person breathes air under greater-than-normal pressures. This forces some gases into solution in the bloodstream. If the diver returns to the surface too quickly from too great a depth, the evolved gas may cause decompression sickness (the bends). To avoid this situation, divers use decompression tables to calculate a safe rate of ascent after a dive. But these tables do not anticipate high altitude flight shortly after diving, and extra time should be allowed after a dive before going flying. Allow 24 hours to pass before flight, if planning a flight above 8,000 cabin pressure altitude.