October 1, 2002
Linda D. Pendleton
Turbine airplanes — and some piston aircraft — fly at altitudes that are incompatible with human life, and yet the pilots and passengers are healthy and happy because of pressurization and environmental systems on board.
The regulations require that an aircraft certified under FAR Part 25 be capable of maintaining a cabin altitude no higher than 8,000 feet at the maximum authorized flight altitude. It's the pressurization system that accomplishes this. Interestingly, FAR Part 23 pressurized aircraft have a maximum cabin altitude limit of 15,000 feet. The pressurization system in the Cessna P210, for example, provides a cabin altitude of 12,100 feet at its maximum altitude of 25,000 feet.
Meanwhile, The environmental system makes sure that the cabin is kept at a comfortable temperature while flying in outside air temperatures that can be as low as minus 65 degrees Celsius.
Air pressure drops as altitude increases. At sea level the atmosphere weighs in at 14.7 pounds per square inch (psi). At 18,000 feet the pressure exerted by the atmosphere is down to about 3.5 psi. The pressure of the atmosphere at 8,000 feet is 10.9 psi. So, how do we get that extra 7.4 psi of pressure into the cabin?
Turbine engines are really just large air pumps. The air enters the front of the engine and is compressed on its way to the burner can and turbines. Some of this air is removed before the allocation for combustion and this air — called bleed air since it is "bled" off the compressor — is used for pressurization, among other things. (This bleed air is also called service bleeds or normal bleeds and is taken into account by the fuel control unit when it calculates the amount of air entering the engine.)
The bleed air is routed through heat exchangers and filters and sent to the cabin. For simplicity, the system assumes a constant inflow of air and regulates the amount that is allowed to escape the cabin through outflow valves. Opening the outflow valves causes the pressure in the cabin to decrease and the cabin altitude to climb. Closing the outflow valves has the opposite effect. Overpressure relief valves prevent the cabin from overpressurizing in the event the outflow valves stick in the closed position. (By the way, one of the most common causes of sticky outflow valves is a buildup of tars from cigarette smoke. Once you see a "gunky" outflow valve, it will make you wonder what smokers' lungs must look like. And smoking is OK on bizjets — for some that's the appeal of having your own kero-burner.)
Most systems prevent the aircraft from pressurizing with weight on the wheels unless the power is advanced to the takeoff range. This slight ground prepressurization on takeoff allows the outflow valves to be driven into the pressurization range and provides a smoother transition when the system goes into the air mode upon takeoff. Upon landing, the squat switch will open the outflow valves to depressurize the cabin in the event any residual pressure remains.
During normal operation, the crew sets the pressurization controls to the cruise altitude before takeoff. A cabin rate control allows the pilot to select the rate of change for the cabin (the normal range is between 300 and 800 feet per minute) to account for passengers with sensitive ears. Before descent and landing, the destination airport elevation is entered and the pressurization system schedules the cabin descent to arrive at field elevation before landing. When all works well the system is taken for granted and is unnoticeable to the passengers.
Normal operation of the pressurization system does not include adding any additional oxygen to the air. The air provided to the cabin is outside air compressed in the compressor section of the engine. Ambient air at altitude has the same percentage of oxygen as sea-level air. The pressure of the oxygen, however, is too low to allow it to dissolve in the blood. And because the same volume of air contains fewer oxygen molecules at low pressures, the lungs can't hold enough air to meet the body's oxygen requirements at high altitudes. Increasing the pressure of the air solves this problem. Oxygen is carried aboard pressurized aircraft to allow crew and passengers to maintain consciousness while descending to a lower altitude in a pressurization emergency.
A pressurization emergency is one of the few times it is necessary to act with some speed in a high-flying aircraft. If an explosive or rapid decompression occurs, it is vital that the crew gets oxygen masks on and working, and gets the airplane started down as quickly as possible. This is no time for troubleshooting. The time of useful consciousness shortens rapidly with altitude and the only cure is oxygen and a lower altitude. Slow leaks and pressurization controller malfunctions usually give the crew more time to respond.
The same air used for pressurization is used for environmental control — cabin heating and cooling. You may have noticed that turbine aircraft don't have cabin heaters. That additional equipment is not necessary since the air used for pressurization is taken from the compressor section of the engine at a temperature of 600 to 700 degrees C. It's obvious where cabin heating comes from. What may not be as intuitive is the fact that the same 600-degree air is used for cabin cooling.
To provide for cabin cooling, the hot compressor bleed air is routed through an air cycle machine — called cooling packs in air carrier-type aircraft — which uses heat exchanges and expansion chambers to lower the air temperature. This energy exchange is so powerful that the air temperature can be lowered to below freezing. This can cause any moisture in the air to freeze and block the system, so the temperature is usually regulated to a point above freezing to avoid any problems. After the air is cooled, compressor discharge air can be mixed in with the surrounding air to modify the temperature to exactly that called for by the crew. In smaller aircraft, which typically have lower volumes of bleed air, an auxiliary air conditioner may be used to provide additional cooling.
Ice blockage of the air cycle machine can cause the pressurization system to revert to emergency pressurization. This is usually bleed air routed directly to the cabin and bypassing the air cycle machine. This air is hot and noisy and will certainly cause the crew to troubleshoot and solve the problem expeditiously. Ironically, it is usually a result of the crew operating the environmental system in a manual mode, thereby bypassing the low-temperature limits of the automatic system in an effort to get more cold air into the airplane, that results in this shutdown and introduction of hot bleed air directly to the cabin.
Pressurization and environmental systems are transparent when they are working well, and when they are not, they can provide the crew with some of the most urgent emergencies they will ever face in a turbine aircraft.
Linda D. Pendleton, AOPA 525616, is the curriculum development manager for Eclipse Aviation. She has been flying for 27 years and has more than 10,000 hours.
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