Aircraft Pressurisation System Chapter 10 PDF

ME3531 Aircraft Systems Chapter 10: Aircraft Pressurisation System For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Pressure of the Atmosphere The gases of the atmosphere (air), al...

ME3531 Aircraft Systems Chapter 10: Aircraft Pressurisation System For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Pressure of the Atmosphere The gases of the atmosphere (air), although invisible, have weight. A one square inch column of air stretching from sea level into space weighs 14.7 pounds. Therefore, it can be stated that the pressure of the atmosphere, or atmospheric pressure, at sea level is 14.7 psi The weight exerted by a 1 square inch column of air stretching from sea level to the top of the atmosphere is what is measured when it is said that atmospheric pressure is equal to 14.7 pounds per square inch. Page ▪ 2 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Pressure of the Atmosphere Atmospheric pressure is also known as barometric pressure and is measured with a barometer. The expression is in the unit of inches of mercury or millimetres of mercury. The measurements come from observing the height of mercury in a column when air pressure is exerted on a reservoir of mercury into which the column is set. The expression of The column must be evacuated so air inside does not act against the mercury rising. A column of mercury 29.92 inches high weighs the same as a column of air that extends from sea level to the top of the atmosphere and has the same cross-section as the column of mercury. The weight of the atmosphere pushes down on the mercury in the reservoir of a barometer, which causes mercury to rise in the column Page ▪ 3 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Pressure of the Atmosphere Aviators often interchange references to atmospheric pressure between linear displacement (e.g., inches of mercury) and units of force (e.g., psi). Standard atmospheric pressure at sea level is also known as 1 atmosphere, or 1 atm. The following measurements of standard atmospheric pressure are all equal to each other Various equivalent representations of atmospheric pressure at sea level. Page ▪ 4 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Pressure of the Atmosphere The atmospheric pressure decreases with increasing altitude because the column of air that is weighed is shorter. The decrease in pressure is a rapid one and, at 50,000 feet, the atmospheric pressure has dropped to almost one-tenth of the sea level value. How the pressure changes for a given altitude Page ▪ 5 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Temperature and Altitude Temperature variations in the atmosphere are of concern to aviators. Weather systems produce changes in temperature near the earth’s surface. Temperature also changes as altitude is increased. Most civilian aviation takes place in the troposphere (On average, it ranges from the earth’s surface to about 38,000 feet above it.) in which temperature decreases as altitude increases. The rate of change is somewhat constant at about –2 °C or –3.5 °F for every 1,000 feet of increase in altitude. The upper boundary of the troposphere is the tropopause. It is characterized as a zone of relatively constant temperature of –57 °C or –69 °F. The atmospheric layers with temperature changes depicted by the red solid line. Page ▪ 6 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Pressurisation Terms The following terms should be understood for pressurisation and cabin environmental systems are as follows: 1. Cabin altitude: Given the air pressure inside the cabin, the altitude on a standard day that has the same pressure as that in the cabin. Rather than saying the pressure inside the cabin is 10.92 psi, it can be said that the cabin altitude is 8,000 feet (MSL). 2. Cabin differential pressure: The difference between the air pressure inside the cabin and the air pressure outside the cabin. Cabin pressure (psi) – ambient pressure (psi) = cabin differential pressure (psid or Δ psi). 3. Cabin rate of climb: The rate of change of air pressure inside the cabin, expressed in feet per minute (fpm) of cabin altitude change. Page ▪ 7 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Rate of Change of Pressure It is common practice for an aircraft to climb at a rate of 1000 feet per minute or more and engines are designed to maintain their performance with the changing atmospheric conditions during a climb. But the effect of rapid altitude changes on the human body causes physical pain and discomfort. If the cabin pressure decreases violently, the nitrogen and other gases in solution in the blood steam expand rapidly in the form of bubbles. This causes acute pain and injury, but would not normally occur except in the event of explosive decompression. If however, the rates of change are large but not violent, the most common effects are: ▪ Sickness ▪ Expansion of gases in the abdomen (uncomfortable) ▪ Expansion of gases in the ear Page ▪ 8 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Pressurization Issues Pressurizing an aircraft cabin assists in making flight possible in the hostile environment of the upper atmosphere. The degree of pressurization and the operating altitude of any aircraft are limited by critical design factors. A cabin pressurization system must accomplish several functions: To ensure adequate passenger comfort and safety. It must be capable of maintaining a cabin pressure altitude of approximately 8,000 feet or lower regardless of the cruising altitude of the aircraft. This is to ensure that passengers and crew have enough oxygen present at sufficient pressure to facilitate full blood saturation. A pressurization system must also be designed to prevent rapid changes of cabin pressure, which can be uncomfortable or injurious to passengers and crew. A pressurization system should circulate air from inside the cabin to the outside at a rate that quickly eliminates odors and to remove stale air. Cabin environmental systems establish conditions quite different Cabin air must also be heated or cooled on pressurized aircraft. Typically, from these found outside the these functions are incorporated into the pressurization source. aircraft. Page ▪ 9 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Pressurization Issues (Refresh, FYI) It must be capable of maintaining a cabin pressure altitude of approximately 8,000 feet or lower regardless of the cruising altitude of the aircraft. This is to ensure that passengers and crew have enough oxygen present at sufficient pressure to facilitate full blood saturation. Page ▪ 10 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Structure Consideration Positive Differential To pressurize, a portion of the aircraft designed to contain air at a pressure higher than outside atmospheric pressure must be sealed. Compressible seals around doors combine with various other seals, grommets, and sealants to essentially establish an air tight pressure vessel. This usually includes the cabin, flight compartment, and the baggage compartments. Air is then pumped into this area at a constant rate sufficient to raise the pressure slightly above that which is needed. Control is maintained by adjusting the rate at which the air is allowed to flow out of the aircraft. Controlling the cabin to ground level values provides maximum comfort; however, if the cabin altitude was held at sea level whilst the aircraft was climbing, the resultant differential pressure would be much higher than is really necessary. Thus an important point arising when deciding on cabin altitude and rate of change values are the loss payload and increased fuel consumption due to increased structural weight required to give the necessary strength to allow very high differential pressure to be used if low cabin altitudes are to be established at very great heights. Page ▪ 11 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Structure Consideration Negative Differential In order to save weight, the aircraft designer only considers the inside pressure being greater than the outside. In the design the fuselage does not cater for a reverse or negative differential pressure and yet, such an instance can occur. In the case of perfectly sealed fuselage being flown with a greater pressure inside than outside and then suddenly being dived to a low altitude where the outside atmospheric pressure would be greater than inside. To cater for such an emergency would complicate the structural design so an inward relief valve is fitted, set to open generally at a negative differential pressure of 0.5 psi. Page ▪ 12 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Structure Consideration Pressure Relief Setting for Different Aircraft It will be seen from the table that if the normal equipment fails to control normal positive differential, there is a secondary safeguard to ensure that the differential pressure does not reach the maximum proof test figure. This secondary safeguard is known as safety valve and is set so that normally it does not come into operation. Differential figures for B707 aircraft. Page ▪ 13 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Structure Consideration A key factor in pressurization is the ability of the fuselage to withstand the forces associated with the increase in pressure inside the structure versus the ambient pressure outside. This differential pressure can range from 3.5 psi for a single engine reciprocating aircraft, to approximately 9 psi on high performance jet aircraft. Differential pressure (psid) is calculated by subtracting the ambient air pressure from the cabin air pressure. Page ▪ 14 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Sources of Pressurised Air The source of air to pressurize an aircraft varies mainly with engine type. Reciprocating aircraft have pressurization sources different from those of turbine-powered aircraft. Note that the compression of air raises its temperature. A means for keeping pressurization air cool enough is built into most pressurization systems. It may be in the form of a heat exchanger, using cold ambient air to modify the temperature of the air from the pressurization source. Reciprocating Engine Aircraft There are three typical sources of air used to pressurize reciprocating aircraft: supercharger, turbocharger, and engine driven compressor. Turbine Engine Aircraft The main principle of operation of a turbine engine involves the compression of large amounts of air to be mixed with fuel and burned. Bleed air from the compressor section of the engine is relatively free of contaminants. As such, it is a great source of air for cabin pressurization. However, the volume of air for engine power production is reduced. The amount of air bled off for pressurization compared to the overall amount of air compressed for combustion is relatively small but should be minimized. Page ▪ 15 For Training Purpose Only.  Official (Closed), Non-Sensitive Aircraft Pressurisation System Reciprocating Engine Aircraft Supercharger A supercharger is mechanically driven by the engine. Despite engine performance increases due to higher induction system pressure, some of the engine output is utilized by the supercharger. Furthermore, superchargers have limited capability to increase engine performance. If supplying both the intake and the cabin with air, the engine performance ceiling is lower than if the aircraft were not pressurized. Superchargers must be located upstream of the fuel delivery to be A reciprocating engine supercharger used for pressurization. Turbocharger Turbochargers are driven by engine exhaust gases. The turbocharger impeller shaft extends through the bearing housing to support a compression impeller in a separate housing. By using some of the turbocharger compressed air for cabin pressurization, less is available for the intake charge, resulting in lower overall engine performance. A turbocharger used for pressurizing cabin Nonetheless, the otherwise wasted exhaust gases are put to work in air and engine intake air in a reciprocating the turbocharger compressor, enabling high altitude flight with the engine benefits of low drag and weather avoidance in relative comfort and without the use of supplemental oxygen. Page ▪ 16 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Turbine Engine Aircraft The pressurizing an aircraft using turbine engine compressor bleed air is to have the bleed air drive a separate compressor that has an ambient air intake. A turbine turned by bleed air rotates a compressor impellor mounted on the same shaft. Outside air is drawn in and compressed. It is mixed with the bleed air outflow from the turbine and is sent to the pressure vessel. Turboprop aircraft A turbo compressor used to pressurize cabins often use this device, known as a turbocompressor. mostly in turboprop aircraft. The most common method of pressurizing turbine-powered aircraft is with an air cycle air conditioning and pressurization system. Bleed air is used, and through an elaborate system including heat exchangers, a compressor, and an expansion turbine, cabin pressurization and the temperature of the pressurizing air are precisely controlled. The air cycle air conditioning system on a Boeing 737 Page ▪ 17 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Control of Cabin Pressure Aircraft cabin pressurization can be controlled via two different modes of operation. The first is the isobaric mode, which works to maintain cabin altitude at a single pressure despite the changing altitude of the aircraft. For example, the flight crew may select to maintain a cabin altitude of 8,000 feet (10.92 psi). In the isobaric mode, the cabin pressure is established at the 8,000-foot level and remains at this level, even as the altitude of the aircraft fluctuates. Isobaric Range Page ▪ 18 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Control of Cabin Pressure The second mode of pressurization control is the constant differential mode, which controls cabin pressure to maintain a constant pressure difference between the air pressure inside the cabin and the ambient air pressure, regardless of aircraft altitude changes. The constant differential mode pressure differential is lower than the maximum differential pressure for which the airframe is designed, keeping the integrity of the pressure vessel intact. When in isobaric mode, the pressurization system maintains the cabin altitude selected by the crew. This is the condition for normal operations. But when the aircraft climbs beyond a certain altitude, maintaining the selected cabin altitude may result in a differential pressure above that for which the airframe was designed. In this case, the mode of pressurization Isobaric/differential system automatically switches from isobaric to constant differential mode. This (F14 aircraft cabin pressure schedule) occurs before the cabin’s max differential pressure limit is reached. A constant differential pressure is then maintained, regardless of the selected cabin altitude. In addition to the modes of operation described above, the rate of change of the cabin pressure, also known as the cabin rate of climb or descent, is also controlled. This can be done automatically or manually by the flight crew. Typical rates of change for cabin pressure are 300 to 500 fpm. Also, note that modes of pressurization may also refer to automatic versus standby versus manual operation of the pressurization system Page ▪ 19 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Cabin Pressure Controller The cabin pressure controller is the device used to control the cabin air pressure. Selections for the desired cabin altitude, rate of cabin altitude change, and barometric pressure setting are all made directly to the pressure controller from pressurization panel in the cockpit. Adjustments and settings on the pressure controller are the control input parameters for the cabin pressure regulator. The regulator controls the position of the outflow valve(s) normally located at the rear of the aircraft pressure vessel. Valve position determines the pressure level in the cabin. A pressure controller for an all pneumatic cabin pressure control system. Page ▪ 20 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Cabin Pressure Controller Modern aircraft often combine pneumatic, electric, and electronic control of pressurization. Cabin altitude, cabin rate of change, and barometric setting are made on the cabin pressure selector of the pressurization panel in the cockpit. Electric signals are sent from the selector to the cabin pressure controller, which functions as the pressure regulator. It is remotely located out of sight near the cockpit but inside the pressurized portion of the aircraft. Modern pressurization control is fully automatic once variable selections are made on the pressurization control panel. This pressurization panel from an 800 series All pressurization systems contain a manual mode that can override Boeing 737 has input selections of flight automatic control. This can be used in flight or on the ground during altitude and landing altitude. maintenance. The operator selects the manual mode on the pressurization control panel. A separate switch is used to position the outflow valve open or closed to control cabin pressure Page ▪ 21 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Cabin Air Pressure Regulator and Outflow Valve Controlling cabin pressurisation is accomplished through regulating the amount of air that flows out of the cabin. A cabin outflow valve opens, closes, or modulates to establish the amount of air pressure maintained in the cabin. Some outflow valves contain the pressure regulating and the valve mechanism in a single unit. They operate pneumatically in response to the settings on the cockpit pressurization panel that influence the balance between cabin and ambient air pressure. An all-pneumatic cabin pressure regulator and outflow valve The pressure regulating mechanism can also be found as a separate unit. Many air transport category aircraft have an outflow valve that operates electrically, using signals sent from a remotely located cabin air pressure controller that acts as the pressure regulator. The controller positions the valve(s) to achieve the settings on the cockpit pressurisation panel selectors according to predetermined pressurisation schedules. Outflow valves installed in a corporate jet. Page ▪ 22 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Cabin Air Pressure Safety Valve Operation Aircraft pressurization systems incorporate various features to limit human and structural damage should the system malfunction or become inoperative. A means for preventing overpressurisation is incorporated to ensure the structural integrity of the aircraft if control of the pressurization system is lost. A cabin air safety valve is a pressure relief valve set to open at a predetermined pressure differential. It allows air to flow from the cabin to prevent internal pressure from exceeding design limitations. The cabin air pressure safety valves on a large transport category aircraft. On most aircraft, safety valves are set to open between 8 and 10 psid. They open at a preset differential pressure and allow air to flow out of the cabin. Page ▪ 23 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Cabin Air Pressure Safety Valve Operation A negative pressure relief valve is included on pressurised aircraft to ensure that air pressure outside the aircraft does not exceed cabin air pressure. The spring-loaded relief valve opens inward to allow ambient air to enter the cabin when this situation arises. Too much negative pressure can cause difficulty when opening the cabin door. If high enough, it could cause structural damage since the pressure vessel is designed for cabin pressure to be greater than ambient. It opens to equalize the cabin and ambient pressure if the ambient pressure exceeds cabin pressure by more than about 0.3psi Some aircraft are equipped with pressurization dump valves. These essentially are safety valves that are operated automatically or manually by a switch in the cockpit. They are used to quickly remove air (smoke or other contamination) and air pressure from the cabin, usually in an abnormal, maintenance, or emergency situation. Page ▪ 24 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Pressurisation Gauges While all pressurisation systems differ slightly, usually three cockpit indications, in concert with various warning lights and alerts, advise the crew of pressurization variables. They are the cabin altimeter, the cabin rate of climb or vertical speed indicator, and the cabin differential pressure indicator. This cabin pressurisation gauge is a triple combination gauge. The long pointer operates identically to a vertical speed indicator with the same familiar scale on the left side of the gauge. It indicates the rate of change of cabin pressure. The orange PSI pointer indicates the differential pressure on the right side scale. Note: The pilot will be informed via a warning indication Pressurisation Gauges and warning horn, when the cabin pressure altitude exceeds 10,000 feet. Page ▪ 25 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Pressurisation Operations The normal mode of operation for most pressurization control systems is the automatic mode. A standby mode can also be selected. This also provides automatic control of pressurization, usually with different inputs, a standby controller, or standby outflow valve operation. A manual mode is available should the automatic and standby modes fail. This allows the crew to directly position the outflow valve through pneumatic or electric control, depending on the system. Cabin Pressure Control System (CPCS) installed in a 737-200C Page ▪ 26 For Training Purpose Only  Official (Closed), Non-Sensitive Aircraft Pressurisation System Pressurisation Operations During ground operations and prior to takeoff, the weight-on-wheels (WOW) switch typically controls the position of the pressurization safety valve, which is held in the open position until the aircraft takes off. Throttle position switches can be used to cause a smooth transition from an unpressurized cabin to a pressurized cabin. A partial closing of the outflow valve(s) when the WOW switch is closed (on the ground) and the throttles are advanced gradually initiates pressurization during rollout. At takeoff, the rate of climb and the pressurization schedule require the outflow valve(s) to fully close. Passengers do not experience a harsh sensation from the fully closed valves because the cabin has already begun to pressurize slightly. Once in flight, the pressurization controller automatically controls the sequence of operation of the pressurization components until the aircraft lands. When the WOW switch closes again at landing, it opens the safety valve(s) and, in some aircraft, the outflow valve(s) makes pressurizing impossible on the ground in the automatic pressurization mode. Maintenance testing of the system is done in manual mode. This allows the technician to control the position of all valves from the cockpit panel. Page ▪ 27 For Training Purpose Only