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Pneumatic System


The use of a compressed air system to operate an aircraft's services usually represents a saving in weight compared to a hydraulic system, since the operation medium is freely available, no return lines are necessary and pipes can be smaller diameter. Systems having operation pressure of up to 24MN/m2 are in use, and provide for the rapid operation of service when this is required. However, compressed air is generally not suitable for the operation of large capacity components, leaks can be difficult to trace, and the results of pipeline or component failure can be very serious. 

Extensive high-pressure pneumatic systems powered by engine-driven compressors are generally fitted on the older types of piston-engined aircraft and are used to operate services such as the landing gear, wing flaps, wheel brakes, radiator shutters and, at reduced pressure, de-icing shoes. There are some modern aircraft which also use a high pressure pneumatic system, however, and there are many aircraft which use pneumatic power for the emergency operation of essential services; the latter type of system is usually designed for ground-charging only.

Low Pressure pneumatic system such as are used on most turbine engined aircrafts for engine starting, de-icing, and cabin pressurisation, are supplied with compressed air trapped from the engine compressor. 

Pneumatic System

Typical System

The system illustrated in Fig contains two separate power circuits, each of which is supplied by a four-stage compressor driven from the gearbox of one main engine, and a common delivery pipe to the high-presser storage bottles and system services. A multi-stage cooler attached to each compressor cools the air between each of the compression stages, and a means is provided for off-loading the compressor when the system is not being used.

Air is drawn through an inlet filter into each compressor, and is discharged through an oil-and-water trap, a chemical dehydrator, a filter and a non-return valve, to the main storage bottle and system.


Overall control of main system pressure is provided by means of a pressure regulator, but pressure relief valves are included to prevent excessive pressures in the system, which may be caused by regulator failure or by an increase in temperature in the pipelines and components.

Pressure reducing valves are used to reduce the pressure supplied to some components. A storage bottle for the emergency system is pressurized through a non-return valve from the main system supply, and maintains an adequate supply of compressed air to enable the landing gear and flaps to be lowered, and the brakes to be applied a sufficient number of times to ensure a safe landing.

Isolation valves are fitted to enable servicing and maintenance to be carried out without the need to release all air

from the system, and pressure gauge are provided to indicate the air pressure in the main and emergency storage


High Pressure Pneumatic System



The types of components used in a high-pressure pneumatic system depends on the type of the aircraft, but

the most common components are as follows



A positive-displacement pump is necessary to raise the air pressure sufficiently for the operation of a pneumatic

system, and a piston-type pump is generally used. Some older types of aircraft are fitted with a single-cylinder piston pump, which provides two stages of compression and raises the working pressure to approximately 3MN/m2  (450 1bf/in2 ). To obtain higher working pressures further compression stages are required.

The compressor has two stepped cylinders, each of which houses a stepped piston; a plunger attached to the head of No. 2 cylinder. The reciprocating motion of the main pistons is provided by individual cranks and connecting rods, the cranks being rotated by a common drive gear, and rotating in the same direction. Air passing between each compression stage is routed through an integral cooler, and lubrication is provided by an oil feed connection from the main engine lubrication system.

Compression depends on the volume of each successive stroke being smaller than the stroke preceding it; the

induction strokes for each cylinder and the four compression strokes are accomplished during each revolution of the cranks. Operation of the compressor is as follows :-

  • On the downward stroke of No.1 piston, air is drawn into the cylinder head through a filter and non-return valve (NRV).

  • On the upward stroke of No. 1 piston, air is compressed in the cylinder, opens a NRV in the cylinder head, and passes to the annular space formed between the steps of the cylinder and piston.

  • The next downward stroke of No. 1 piston compresses air in the annular space in this cylinder and forces it through a NRV into the annular space formed between the steps of No. 2 cylinder and piston. No. 2 piston is approximately 90º in advance of No. 1 piston, and is moving upwards as No. 1 piston approaches the bottom of its stroke.

  • On the downward stroke of No. 2 piston, air is compressed in the annular space at the bottom of the cylinder, and passes through a NRV into the small cylinder formed in No. 2 cylinder head.

  • On the upward stroke of No. 2 piston, the plunger attached to it also moves upwards, further compressing the air in the small cylinder and passing it through a NRV to the system.

A pressure warning transmitter is fitted at the second stage outlet, and third stage pressure is connected to the pressure regulator.

Pressure Regulator

The pressure regulator is fitted to control the maximum pressure in the system and to off-load the compressor when the system is idle. With the regulator system pressure is fed to the top connection and acts on a piston, the lower end of which is in contact with the ball of a spring loaded ball valve.


At the predetermined maximum system pressure, the air pressure on the piston overcomes spring pressure and the ball valve is opened, releasing third-stage compressor pressure to atmosphere and allowing the pump to operate at second-stage pressure only.


If any pneumatic services are operated, or a leak exists in the system, the air pressure trapped in the storage bottle and pipelines will drop, and the ball valve in the pressure regulator will close. The compressor will thus be brought back on line until the maximum system pressure is restored.

Pressure Regulator


Oil-and-water trap

The oil and water trap is designed to remove any oil or water which may be suspended in the air delivered by the

compressor. It consists of a casing with inlet and outlet connections at the top and a drain valve in the bottom. Air

entering the trap does so through a stack pipe, which includes a restriction and a baffle to prevent the air flow stirring up any liquid or sediment in the bottom of the container. Air leaving the trap also passes through a stack pipe, to prevent liquid or sediment entering the system during aircraft manoeuvres.


To protect pneumatic systems from malfunctioning due to moisture freezing in the components and pipelines, the

compressed air may be dehydrated by a substance such as activated alumina, or it may be inhibited by a small quantity of methanol vapour. The handling of methanol presents some difficulties, however and because of its corrosive nature systems must be specially designed for its use : activated alumina is, therefore, more generally used.

Activated alumina is housed in a container through which the compressed air passes after leaving the oil and water

trap, and which generally contains a filter at the outlet end. The charge of alumina in the container will gradually become saturated with moisture, and should be changed at the specified intervals. The number of flying hours at which the alumina charge is changed is normally determined by the aircraft manufacturer through practical experience.

Storage Bottles

In a pneumatic system the storage bottles provide the reservoir of compressed air which operates all services, the

compressors being used to build up system pressure when it falls below the normal level. The volume of the actuators and pipelines determines the size of the bottles required for the normal and emergency operation of the pneumatic services.

Storage bottles are generally made of steel, and may be of wire-wound construction for maximum strength. Bottles

are generally mounted in an upright position, and a fitting screwed into the bottom end contains the supply connection and, usually, a connection to an associated pressure gauge, together with a drain valve by means of which any moisture or sediment may be removed. Stack pipes are provided at the supply and gauge connections in the fitting, to prevent contamination passing to the system or pressure gauge. Pressure testing of high-pressure storage bottles is required at specified periods, and the date of testing is usually stamped on the neck of the bottle.

Pressure Reducing Valves

Some services operate at pressures lower than the pressure available in the air bottle, and are supplied through a

pressure reducing valve. This low pressure is, in some instances, further reduced for the operation of, for

example, the wheel brakes, by the fitting of a second pressure reducing valve.

When pressure in the low-pressure system is below the valve setting, the compression spring extends, and, by the action of the bell-crank mechanism, moves the inlet valve plunger to admit air from the high-pressure system. As pressure in the low-pressure system increases, the bellows compresses the spring and returns the inlet valve plunger to the closed position. The inside of the bellows is vented to atmosphere, and the valve thus maintains a constant difference in pressure between the low pressure system and atmospheric pressure.

Pressure Reducing Valve


Pressure Maintaining Valve

A pressure maintaining valve is designed to conserve air pressure for the operation of essential services (e.g. landing gear extension and wheel brake operation), in the event of the pneumatic system pressure falling

below a predetermined value.

Under normal circumstances air pressure is sufficient to open the valve against spring pressure and allow air to flow to the nonessential services. Should the pressure in the storage bottle fall below a value pre-set by the valve spring, however, the valve will close and prevent air passing to the non-essential services.

Pressure Maintaining Valve


Control Valves

Compressed air stored in the bottle is distributed to the various pneumatic services, and directed to the various types of actuators by means of control valves, which may be manually or electrically operated.

Electrically Operated Control Valve

The electrically-operated control valve for a pneumatic landing gear retraction system is illustrated in Fig. Selection of the landing gear position is made by either of two push-buttons (marked ‘up’ and ‘down’) which are mechanically interconnected to prevent operation of both buttons at the same time. These buttons, when depressed, supply electrical power to the ‘up’ or ‘down’ solenoid as appropriate.


Actuation of this solenoid lifts an attached pilot valve, supplying compressed air to the cylinder at the bottom of the associated valve; the piston moves downwards, and the valve guide attached to it opens the inlet valve, admitting compressed air to the appropriate side of the landing gear actuators. At the same time the beam attached to the extension of this piston transfers movement to the valve guide in the opposite valve, allowing air from the opposite side of the actuators to exhaust to atmosphere.

Electrically Operated Control Valve


Manually Operated Control Valve

The valve illustrated is a simple two-position valve, and may be used as an isolation valve in some systems. The sleeve valve is operated by a cam, and is spring-loaded to the ‘off ‘ position; linkage from the cam spindle connects the valve to an operating lever. When used as an isolation valve the operating lever would normally be wire locked in the ‘on’ position, and would only be used to permit servicing operations to be carried out.

Manually Operated Control Valve


Brake Control Valve

Some older types of aircraft may be fitted with a type of brake control valve (known as a dual-relay valve) by means

of which total brake pressure is applied by the operation of a single hand-control, and distribution to either or both

brakes is affected by means of a mechanical connection to the rudder bar.


The type of brake control valve illustrated in Fig. is used on some modern aircraft and is operated by linkage from brake pedals attached to the rudder bar; separate valves supply compressed air to the brake units on each wheel.


Operation of the valve is as follows :

  • In the ‘off ‘ position the inlet valve is closed and pressure in the brake line is connected to the exhaust port.

  • Pressure applied to the associated brake pedal is transmitted via the brake linkage to the valve sleeve, which moves up to close the exhaust valve.


Further pressure applied through the valve sleeve and lower spring tends to open the inlet valve, and air pressure in the brake line combined with the force exerted by the upper and centre springs tends to close it. This produces a balanced condition in which any increase in the force applied to the valve sleeve results in a higher air pressure in the brake line, and a decrease in the force applied to the valve sleeve result in opening of the exhaust valve and a reduction in the air pressure in the brake line.

Brake Control Valve



The purpose of an actuator is to transform the energy of the compressed air into linear or rotary motion. Actuators

in pneumatic systems are normally of the linear type. Because of the nature of the operating medium, however, actuators in pneumatic systems are often damped to prevent violent operation of the service.

Air Conditioning System

The air conditioning system of an aircraft is designed to maintain selected temperature conditions within flight crew,

passengers and either compartments, and comprises five principle sections:

  • Air Supply

  • Heating

  • Cooling

  • Temperature Control

  • Distribution.

In some aircraft humidity control section also forms part of the air conditioning system.

In pressurised aircraft, the air conditioning and pressure installation systems are intrinsically linked, and it is the

controlled discharge of pressurised and conditioned air, which maintains the elected cabin altitude.

Basically two types of Air conditioning cycles are used.

  • Air cycle system

  • Vapour cycle system.

Air Conditioning

Air Cycle System

 There are two types of Air Cycle System:

  • Ram air system

  • Engine bleed air system

Ram Air System

This method is adopted in certain small types of unpressurised aircraft utilising either combustion heating or engine

exhaust heat exchanger systems. Typical location for ram air intake are at the nose of an aircraft or in a dorsal fairing at the base of the fin or vertical stabiliser. The air after circulating through the cabin, is discharged to atmosphere via a spill vent.



The method of heating the air depends on the type of air supply system and can be either combustion heating or engine exhaust heating.

Combustion heating

This method is normally associated with a direct type of ram air ventilating system, and depends for its operation

on the combustion of a fuel and air mixture within a special cylindrical combustion chamber.


Air for combustion is obtained from a blower and the fuel is metered from the aircraft fuel system by a solenoid operated control valve. A filter and safety valve are also incorporated in the fuel supply line to the combustion chamber. The fuel air mixture is ignited by a spark plug, the burning gases travelling the length of the combustion chamber and passing through transfer passages to an exhaust outlet.


Ventilating air from the ram air intake passes through the heater and is heated by contact with the outer surfaces of the combustion chamber. Blower operation and supply of fuel is normally controlled by a single switch. Regulation of the cabin temperature is carried out by the manual setting of a mechanically controlled switch installed in the ducting downstream of the heater.

Ram Air System


Engine exhaust heating

The method is also associated with ram air ventilating systems, but heating of the air supply is affected in a simpler

and more direct manner. Air enters through an intake connected to a heater muff which surrounds the exhaust pipe of a piston engine exhaust system. After heating, the air passes into the cabin via a chamber through which cold air also flows from an intake situated either in the fuselage or in the wing depending on the installation. Mechanically operated valves are provided to control the mixing of the airflows and so regulate the temperature.


In ram air supply systems the cooling method is of the simplest type whereby the cold air can be directly admitted

to the cabin via adjustable louvres. In the more complex systems cooling may be accomplished by either the air cycle or the vapour cycle method.

Temperature Control

Control of air temperature conditions in passenger cabins, flight crew and other compartments, is accomplished by

modulating the valves installed in the air ducting of heating and cooling sections of the air conditioning system. The

methods of control vary and depend on the type of aircraft and the air conditioning system employed. In general, two principal methods are adopted, mechanical and electromechanical.

Mechanical control

One mechanical method, which, for example, is employed in aircraft utilising an engine exhaust heating system, and consists of valves which can be manually positioned to regulate the temperature by varying the proportions of hot and cold air passing through a mixing box before delivering it to the cabin. In some installations, hot and cold air enters the cabin through separate valves and ducting.

Electromagnetic control

The electromechanical method of temperature control used in some types of combustion heating system, is also

used in all air conditioning systems which utilise the compression method of heating, and air cycle or vapour cycle

methods of cooling.


In a combustion heating system, the electrical power supply to the solenoid valve is automatically controlled by the duct thermostat. When the temperature of the air flowing from the heater exceeds the thermostat setting, the thermostat de-energises the solenoid valve to isolate the fuel supply to the heater. As the heater cools, the thermostat opens the valve to restore the fuel flow and combustion process. By cycling on and off, the heater

maintains an even temperature in the cabin.

Engine Bleed Air System ('Bootstrap Type')

This method is adopted in certain types of turbojet aircraft, in which hot air, readily available from main engine

compressors is tapped off and supplied to the cabin. Before the air enters the cabin it is passed through appropriate control valves and a temperature control system to reduce its pressure and temperature.

Air Supply

Auxiliary power unit (APU)

The auxiliary power unit, where fitted, is an independent source of pressurised air. Operation of the APU is , however, subject to certain limitations.

Typical bleed air system


Compressors or blowers

This method is utilised in some types of turbojet, turbo propeller and piston-engine aircraft, the compressors or

blowers being driven by the engines via accessory drives, gear boxes or bleed air.

Air is drawn in through a ram air intake located in a wing leading edge or an engine nacelle fairing. A filter unit may

be provided to protect the blower rotors from foreign matter and to ensure a clean air supply. In order a reduce the level of noise emanating from the blower, silencers are incorporated in the main supply ducting.


Engine bleed air system employs compression heating. This system of heating relies on the principle whereby the

air temperature is increased by compression and forms the basis of the heating method employed air supply system utilising engine driven compressors or engine bleed air.



The operation of an air cycle cooling system is based on the principal of dissipating heat by converting its energy

into work. The principle components of a typical system are the primary and secondary air-to- air heat exchangers, turbo-compressor cold air unit and a water separator. The interconnection of these components in a ‘bootstrap’

arrangement, is illustrated in Fig above.

Heated air is directed through air passages of a matrix assembly within the primary heat exchanger and is pre-cooled by air entering a ram air intake and passing across the matrix. The pre-cooled air then enters the cold air unit via the axial inlet of the compressor and is compressed by the action of the compressor impeller and diffuser assembly.

The air leaves the compressor outlet and passes through a matrix assembly of the secondary heat exchanger which dissipates a large proportion of heat produced by compression. From the secondary heat exchanger the air enters the turbine of the cold air unit. The air expands through the turbine and in causing the latter to drive the compressor, sufficient pressure drop across the turbine is achieved to cause further cooling of the air.

The water separator (coalescer) is installed downstream of the cold air unit to extract a percentage of free moisture

from the air which subsequently ventilates and pressurises the cabin. Air from the cold air unit turbine enters the

separator and passes through an assembly in which the moisture in the air coalesces into large water droplets. The

droplets are then carried by the air to a separator assembly which extracts the water. The water is then drained away through a drain line to an overboard vent, or into the heat exchanger ram air supply to provided additional cooling. To ensure that the flow of air to the cabin is maintained in the event of the water separator assembly becoming obstructed by ice, a safety valve is normally provided. In some systems the water separator is combined with in airflow silencer unit.

Temperature Control

Electromagnetic control

In systems utilising compression heating, air cycle, or vapour cycle methods of cooling, the electromechanical

temperature control system is designed to automatically modulate actuator motors which control particular valves.


A typical system comprises a duct temperature sensing element, a temperature selector, cabin temperature sensing element and automatic control unit. These components are electrically interconnected to form a resistance bridge circuit which is only in balance when the cabin air temperature is at the selected valve.

If the bridge circuit is placed out of balance by a resistance change in either of the sensing elements due to temperature variation, or by varying the selector switch setting, an error signal is produced which is fed to an amplifier stage of the control unit. The amplified signal is then fed to the appropriate actuator motors which position their respective valves to adjust the air flows and so correct the temperature change until the bridge circuit is restored to a balanced condition.

Manual controls are provided to permit overriding of the automatic circuit, low temperature and high temperature limit control devices are also provided and respectively they prevent icing in the water separator, and ensure that upper limits of supply air temperature are not exceeded.

Vapour Cycle Systems

The principle of vapour cycle cooling is based upon the ability of a refrigerant to absorb heat through a heat exchanger in the process of changing from a liquid into a vapour. The major components of a typical system and their interconnection with each other is diagrammatically illustrated in Fig.


The components are generally mounted together to form a refrigeration pack, and comprise the following:

  • A liquid receiver: To provide a storage area for the liquid refrigerant.

  • A thermostatic expansion valve: To control and meter the liquid refrigerant into the evaporator.

  • An evaporator: Which is a form of heat exchanger designed to extract heat from the main air supply prior to distribution into the aircraft.

  • A compressor: To provide the motive force for refrigerant re-circulation, and in conjunction with the thermostatic expansion valve, maintain a pressure differential between the condenser and evaporator. The effect of this differential improves both vaporisation and condensation of the refrigerant as follows.

The compressor in drawing vapour from the evaporator assembly, decrease the effective pressure acting upon it, the consequence of which reduces the boiling point of the refrigerant. Conversely, on the discharge side of the compressor, vapour pressure is increased. This has the effect of increasing the boiling point and condensation point of the refrigerant, which returns to a liquid state when the latent heat is removed in the condenser.

NOTE: The coupled turbine of the compressor may be driven by an independent air supply (e.g. a tapping from a wing deicing system), the main air supply, or electrically

  • A condenser: Which is a form of heat exchanger designed to extract heat from the vaporised refrigerant.

  • A condenser fan: Which provides (in the absence of ram air), cooling air for the condenser.

  • The refrigerant: Which is a low boiling point volatile liquid such as; ammonia, sulphur dioxide, or dichlorodifluoromethane generally referred to by the trade name of ‘Freon’.

Vapour cycle cooling system


The Vapour Cycle is as follows:

  • Liquid refrigerant passes from the liquid receiver to the thermostatic expansion valve for controlled release into the matrix of the evaporator.

  • Heated air from the main air supply system (prior to entry into the cabin distribution system) passes through the evaporator matrix and by induction releases heat into the liquid refrigerant.

NOTE: The main air supply entering the distribution system is now at a reduced temperature.

  • As a consequence, the liquid refrigerant boils to a vapour.

  • The vaporised refrigerant is then drawn into the compressor, compressed to a high pressure and temperature to enter the condenser.

  • The condenser; cooled by ram air, reduces the temperature of the vaporised refrigerant, and as a consequence returns the vapour back to a liquid form which then flows back to the liquid receiver to repeat the cycle.


The air used for conditioning purposes is distributed by a ducting system the layout of which depends on the type

of aircraft and its air conditioning system. In a basic system, such as that employing ram air supply and combustion heating the ducting is generally in two distinct sections and provides for separate flows of cold and heated air.

The outlets for cold air are normally of the adjustable louvre type and are installed so that air flows from such points as below hat racks, cockpit and cabin sidewalls. Heated air is distributed through outlet grilles situated at floor level, the degree of heat being regulated by mechanical valves directly controlled at the outlets, or by control knobs in the flight compartment. The heated air duct also has a branch duct which directs heated air to the wind shield panels for demisting purposes.

In larger aircraft the air conditioning equipment is normally grouped together in its own compartment or bay. The

conditioned air is distributed to passenger cabins through underfloor and hat rack ducting, the latter containing outlet grilles and the requisite number of individual adjustable cold air louvres which are supplied from a cold air source. The distribution of air to flight crew compartments may, in some cases, be through separate ducting or it may be through ducting tapped into the passenger cabin ducting. Typical locations for the air outlets are at floor and roof levels and in sidewalls.

Tapping are taken form the cabin and flight crew compartment ducting systems for supplying warm air to cabin

windows and wind shields for demisting purposes. After circulation the air is exhausted to atmosphere through the

discharge or outflow valves in the pressurisation system.

Air Conditioning System Distribution



Materials used in the manufacture of typical ducting systems are light alloy, plastic, fibre glass reinforced plastic

and stainless steel, the latter being normally used for the hot air sections of engine bleed air supply systems. There

are various methods of joining the duct sections together and to components. In those most commonly used the joints are made by flanges and ring clamps of V-section, by rubber sleeves fitted over the ends of duct sections and secured either by adjustable clamps or by a rubber adhesive, and by bolted flanges.

Fibreglass, formed into blanket sections by a covering of synthetic material e.g. nylon, is used for lagging of duct

sections. To permit longitudinal movement of ducting as it expands and contracts, expansion bellows, sliding clamps and gimbal mountings are provided in some the larger aircraft systems

Humidity Control

In some aircraft operating for long periods at high altitudes, it is necessary to increase the moisture content of the

air used for conditioning and pressurising the cabin in order to overcome physical discomfort arising from low relative humidity. Various humidity control methods may be adopted but a typical system consists of humidifier unit supplied with water (from and individual at tank or galley water system) and also with air under pressure. The water and air supplies; which are controlled by electromagnetic valves, pass through a jet nozzle system within the humidifier in such a manner that the water is atomises and enters the distribution ducting in the form of a fine spray.

At the other extreme, operation of aircraft at low altitude and on the ground in regions of high relative humidity,

necessitates a reduction of the moisture content of the air supply. In addition to the passenger comfort aspect, it is

necessary to decrease the humidity in order to reduce condensation and its effect.

Air Conditioning Panel

The control of air conditioning panel is done by the air conditioning panel, the type of panel depends on the type of aircraft and the system.

It consist of the control for the source of bleed air, pack on and off switches, Pack flow control, temperature control for each zones and control for bleed transfer switch.

A320 Air Conditioning Panel


Ground Air Conditioning 

In some aircraft provision is made for the conditioning of cabin air while an aircraft is on the ground. The methods

adopted depend on the type of aircraft and the associated air conditioning system.

In aircraft employing combustion heating systems, cabin heating is normally obtained by switching on the heater

and a ventilating fan located in the main air supply ducting. On the ground, limited cooling of the cabin air can be obtained by switching on the ventilating fan.

For heating the cabin air in aircraft equipped with an engine exhaust heating system it is necessary for the engines

to be running, and for the mechanical air flow control valves to be appropriately adjusted to provide the desired


Aircraft using more complex methods of air conditioning are often provided, with special external connection to

which ground service equipment can be coupled. These units supply either preconditioned air into the main cabin air distribution system or pressurised air into the air conditioning packs. In some cases ground service equipment may be used when carrying out ground test procedures.

In addition to the ground connections, some aircraft are equipped with an auxiliary power unit for use in the absence of ground conditioning units. Electrically operated blowers may also be fitted for use either as simple cool air ventilators, or in conjunction with a ‘bootstrap’ air conditioning system, to provide a flow of cooling air to the heat exchangers.

Ground Air Conditioning Connection



These systems are designed to automatically maintain a selected altitude relationship between cabin and aircraft

by controlling the pressure of the air normally derived from an associated air conditioning system.

Pressurisation System

Pressurisation System

In order to protect the occupants of an aircraft from the discomfort and dangers arising from the effects of reduced

atmospheric pressure encountered at altitude, it is necessary to pressurise the cabin. To overcome the problems

associated with these effects, the actual pressure in the cabin is controlled by regulating the rate at which the air supplied from the air conditioning system is discharged overboard. In general this is achieved by a pressure controller passing a signal to one or more discharge valves (which impose a restriction on the discharged air) to establish, and then subsequently to maintain, the required cabin pressure.

Cabin Altitude

With an increase in altitude there is a decrease in atmospheric pressure. From sea level to 7000 ft the oxygen content and pressure of the atmosphere is so sufficient as to maintain all mental and physical functions. At approximately 10,000 ft above sea level, oxygen saturation of the blood is lowered to approximately 90%, and any prolonged exposure to this level of cabin altitude could result in the occupants suffering headaches and fatigue. If the cabin altitude is allowed to rise further to approximately 15,000 ft, disorientation, impaired vision and physical changes may occur. Therefore, the purpose of the pressurisation system is to artificially create a lower altitude within the cabin (cabin altitude) relative to the aircraft altitude using pressurised air.


Design of the pressurisation system will, however, require certain devices to ensure the comfort and safety of the passengers and the structural integrity of the aircraft.

The cabin pressure (relative altitude) is controlled by regulating the rate at which air, normally supplied by the air

conditioning system, is discharged to atmosphere by one or more discharge valves. In general this is achieved by the pressure controller passing a pneumatic or electrical command signal to the discharge valves (outflow valves) which respond by increasing or decreasing restriction to the flow of air from the cabin to atmosphere.

In addition to the basic units which control cabin pressure, pressure limiting valves, inward relief valves, ground

depressurisation valves, (of either automatic or manual control), and associated warning systems are provided as a part of the pressurisation system to safeguard the occupants and airframe, in the event of a system or component failure.

Flight deck instrumentation systems and controls include indications of: cabin altitude, differential pressure, and

cabin altitude rate of change. These indications can be of either, analogue or digital form, using instruments or cathode ray tube (CRT) displays.

Normally, visual warning systems also include additional simultaneous audible alarms to alert the crew members of

any significant changes taking place, which may require immediate crew action.

Pressurised Air

The source of pressurised air is normally dependent upon the aircraft and engine type. Piston engine aircraft in

general, use superchargers or turbochargers which may be part of the induction system or specifically incorporated

for the purpose of pressurisation.

Where aircraft are powered by a turbo-jet engine(s), bleed(s) air from the compressor section of the main engine core is utilised. However, on smaller aircraft, a system of ram air, supplemented by a small amount of high temperature bleed air for temperature control, may be adopted for air conditioning and pressurisation purposes.

Where fitted, the auxiliary power unit (APU) is another alternative source of pressurised air for the purpose of


Pressurisation Control System


The principal requirements of the pressurisation control system are:-

  • To control, maintain and monitor the cabin altitude relative to the aircraft altitude within the specified parameters.

  • To safeguard the occupants and airframe from 'pressure bumps' when ascending to or descending from altitude, by providing a controlled rate of altitude change.

  • To provide safeguards against total system failure.

Pressure Controllers

These units control the cabin differential pressure, i.e. the difference between the pressure in the cabin and external pressure, to selected values and they operate in conjunction with discharge or outflow valves. The maximum differential pressure values vary between types of aircraft but, in general, they are such that the cabin pressure does not fall below that equivalent to an altitude of 8000 feet.


The rate of pressure or cabin altitude change can also be controlled and within a small pre-determined range, the values which are selected on typical pressure controllers for normal operation are between 300 and 500 feet per minute.

Pressure controllers vary in their construction, but basically they comprise pressure sensing capsules and

diaphragms which are subjected to both cabin and external pressures, metering valves, and controls for selecting the required cabin altitude and rate of pressure change. When the controls are preset, the capsules, diaphragms and metering valves are adjusted to datum positions which ultimately establish the appropriate cabin differential pressure.


As the cabin pressure changes, the controller automatically senses the change relative to the external pressure and transmits a pressure signal via a pressure sensing line connected to the discharge valves. The pressure signal then positions the valve to regulate the release of air from the cabin a  the preselected rate of change thus stabilising the required maximum differential pressure. In some type of controller the datum positions and resultant pressure signals are converted to electrical signals which, after amplification, position discharge valves by means of electric actuators.

Pneumatic Pressure Controllers

Pneumatic pressure control systems comprise; pressure-sensing capsules and diaphragms which are subjected to

both cabin and external pressures, metering valves, and controls for selecting the required cabin altitude and rate of change. With the controls preset prior to flight, the capsules, diaphragms and metering valves assume a datum position which will ultimately establish and maintain the appropriate cabin differential pressure. As the cabin pressure changes, the controller automatically senses the change relative to the external ambient pressure and transmits a pressure signal via a pressure sensing line connected to the discharge valves (outflow valves). The transmitted pressure signal will then open or close the valve to regulate the release of air from the cabin at the pre-selected rate of change, subject to the maximum differential pressure. The following paragraphs describe the function of a typical pressurisation controller and discharge valve of pneumatic operation.

Electronic Pressurisation Control

The operational parameters and requirements of the electronic pressure controller are identical to those of the

Pneumatic Controller.


The basic differences between, pneumatic and electronic pressurisation control, are as follows:-

  • The automatic cabin altitude controllers are duplicated (Auto 1 and Auto 2), with additional inputs from the landing gear (air/ground signal) and thrust lever positions.

  • The signal between the automatic cabin altitude controller and the outflow valve (discharge valve) is electrical as opposed to a pneumatic signal.

  • The cabin altitude control panel is remote from the cabin altitude controller (normally located within the avionics equipment bay) and not an integral part of the controller.

  • The outflow valve (discharge valve in this description) can be actuated by: either of the two A.C. motors, or for manual or emergency control the D.C. motor.

The electronic pressurisation controller is, in basic terms, a shaping and summing network. With information derived from the air data computer, cabin altitude control and various systems within the aircraft, a reference signal is produced by the controller. This reference signal will then be compared by the cabin altitude controller, to the signal produced by the cabin altitude monitor. If a disagreement exists between the two signals, a correcting error signal is produced, which when applied, modulates the outflow valve (s).

As a safeguard against the cabin altitude or rate of change exceeding defined limits within the pressurisation

schedule, override circuits constantly monitor the performance of the system in control. If any deviation from the

schedule exists, the override circuits will either: automatically transfer control to the standby system, (Auto 1 to Auto 2) or de-activate the system completely with the appropriate flight deck indications and aural warnings. 

Outflow Valves (Discharge Valves)

The primary function of discharge or outflow valves is to regulate the discharge of cabin air in response to the

pressure signals received from the controller. They also vary in design and construction but, in general, they are of

two main types.


In one type the valves are operated by diaphragms and in the other by electric actuators. The size and number of valves required for a particular type of aircraft is governed by the amount of air necessary for pressurizing, heating and cooling purposes. In some types of discharge valve, safety valves and inward relief valves are incorporated.

A means of locking the valve to the closed position in the event of a forced descent on water (ditching) is also a feature of some discharge valves

Out Flow Valve


Safety Valves and Inward Relief Valves

Safety Valves

Safety valves are provided to relieve excess cabin pressure in the event of a failure of the pressure controller and/

or discharge valves. In ward relief valves are provided to limit any possible negative differential pressure to a safe value.


Depending on the system adopted for a particular type of aircraft, the valves installed may either be in the form of separate units, single integrated units, or they may be combined with the discharge valves. The valves vary in construction and operation but those most commonly used are either of the type utilizing diaphragm control similar to a discharge valve, or of the spring-loaded hinged flap type.

Inward Relief Valves

Inward relief valves are provided to limit any possible negative differential pressure to a safe value. Depending on

the system adopted for the particular aircraft type, the valves installed may either be in the form of separate units, single integrated units, or combined with the discharge valves. The valves vary in construction and operation but those most commonly used are of the type utilising diaphragm control (similar to a discharge valve).

Ground Automatic Relief Valves

The ground automatic relief valve is a form of discharge valve used on some aircraft types which is additional to

those discharge valves which form part of the pressurisation system.


The valve as its title implies, is effectual whilst the aircraft is in a ground mode (i.e. ground/air signal) with the following primary functions:-

  • To maintain a free flow of ventilating air within the aircraft when parked.

  • To prevent cabin pressurisation and pressure surges whilst the aircraft is taxiing.

  • To transmit ground/air signals derived from the landing gear and engine thrust levers, to the cabin altitude controller for controlled pre pressurisation and depressurisation of the cabin when respectively the aircraft is taking off or landing.

Instrument and Indicator

The presentation of instrumentation forming part of the pressurisation control system, is largely dependent upon

the method of pressurisation control. The instruments form a part of the cabin altitude control panel.


The principal instruments in respect of pressurisation control are, Cabin altitude, Differential pressure, and Rate of

Change (vertical speed). The instruments maybe either digital or analogue (dial and pointer) form in the main instrument panel. Instruments may also be provided to indicate the position of certain valves, e.g. discharge valves.


Crew compartment indications, are in general, presented in visual and audio form, i.e. warning light or CRT image

with an accompanying sound (bell, chime or horn etc). The factors which will activate those indications, in respect of the cabin altitude control are:-

  • Excessive cabin altitude.

  • Discharge valve failure or disagreement.

  • Inward relief valve operation.

  • An automatic system changeover.

  • Excessive differential pressure.

  • Positive pressure relief valve operation.

Cabin Pressurisation Instruments and Con


Emergency Controls

In addition to the normal devices which control pressure to the required values, provision is made for the normal

operating cabin pressure to be reduced rapidly in the event of, emergency landings, clearing the cabin of smoke or other contaminations, and the rapid reduction of cabin pressure. In all such cases cabin pressure is reduced by the ‘dumping’ of air. This may be achieved in a number of ways and the methods most commonly adopted include, separate manually operated dump valves, manual override control of a discharge valve or a safety valve, and in some cases manual control of a pressure controller.

Filters and Driers

Filters are connected in the cabin air pressure sensing lines to the pressure controllers and discharge valves and

normally consist of a casing housing a replaceable filter cartridge and fitted with appropriate inlet and outlet

connections. In some aircraft installations, air driers are provided to eliminate the possibility sensing lines to discharge valves, safety valves and inward relief valves.


Two types of driers are in common use; one utilizing the properties of a silica gel drying agent, and the other consisting of a baffle box mounted on the inside of the fuselage skin and utilizing the skin temperature to condense any water vapour present in the cabin air. The moisture deposited in the box eventually drains away through an outlet in the box and aircraft skin.

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