Download - Air Conditioning and Cabin Pressurisation Notes (1)

Issue 1 - 20 March 2001

AIR CONDITIONING AND CABIN PRESSURISATION

IntroductionThe atmosphere above10,000ft is too thin and cold for normal breathing.Passenger carrying aircraft, operating above this height need an air conditioning and pressurisation system. The temperature of the air passing through the passenger cabin, flight deck and other compartments must be strictly controlled, as well as flow rate and level of humidity. Cabin temperature will normally be maintained between 15 and 30 degrees Celsius. Additionally, a controlled amount of pressurisation is necessary, so that the air pressure in the passenger cabin and adjacent areas does not exceed the equivalent of the ambient air pressure at 8000ft. Air conditioning is also essential for un-pressurised aircraft types.

A typical air conditioning and pressurisation system comprises eight principle sub-systems:

Air Supplies (Pneumatics ATA 36) Cooling Heating Temperature Control Humidity Control Mass Flow Control Distribution Pressurisation

Air Supply

The source of fresh air supply and arrangement of essential components will vary between aircraft type and each air conditioning system, but in general one of the following methods described in the following paragraphs will be adopted:

Engine Bleed Air (compression)

This method is the most common and is installed on the majority of modern aircraft types. Very hot air is tapped from the main engine compressor stages and supplied to the cabin, flight deck and other areas. Before the air enters the cabin, it is passed through a temperature control system, which reduces its temperature and pressure. Additionally, a means of flow control is utilised and in some aircraft, humidity control forms part of the system. (See Fig 1)In pressurised aircraft, the discharge of the conditioned air is regulated to maintain the cabin pressure at the selected pressure altitude.

Typical (Compression) Bleed Air SystemFigure 1

Air Compressors or Blowers

This method is used on turbo-prop, piston engine or even turbo-jet aircraft where main engine compressor bleed is unavailable or unsuitable. Normally the compressor or blower will be mechanically driven from the accessory gearbox of the main engine and its air supply routed via a temperature control system, in a similar manner to the engine bleed method.

Auxiliary Power Unit (APU)

The APU is a small gas turbine engine, which can be connected into the main air supply system and provide an independent means of air conditioning and pressurisation, either on the ground or in flight, when the main engines cannot supply. It will utilise the engine bleed air principle outlined above.


Ram Air

This method is normally found as the primary ventilation system on un-pressurised aircraft. A ram air scoop placed directly into the airflow, will provide the means of air supply as the aircraft moves forward. Since the air at altitude will be cold, the temperature control system through which it passes before entering the cabin, will normally be a form of heater. A self-contained combustion type heater will be employed, or the some form of exhaust gas heater. The air conditioning ducting will be routed around the combustion heater casing or around engine exhaust duct to obtain convection heating. On pressurised aircraft, a ram air system can be used as a means of emergency ventilation, following a complete loss of the main system.

Typical Combustion Heater SystemFigure 2

Ground Cart

This will be an independent means of heating or cooling the passenger cabin on the ground. It can be used on aircraft that do not have an APU. The trolley will be connected externally to the aircraft, via a purpose built inlet into the air conditioning system and normally employs a combustion type heater and the means to control the output of the air temperature from a control panel the cart.

Cooling

When bleed air is used as the air supply, the air tapped off the engine compressor can reach a temperature in excess of 300 degrees Celsius. This is obviously far too hot to be fed directly into the air-conditioned areas, so it must first be cooled down to around 20 degrees Celsius. There are two main methods of cooling; Air Cycle and Vapour Cycle cooling systems.

Air Cycle Cooling

Air cycle cooling relies on three basic principles; surface heat exchange, expansion and energy conversion. Surface heat exchange, provides cooling by passing the air tapped from the engine compressor (charge air) across some form of heat exchanger. The charge air is subjected to the effect of a colder cross flow, normally ambient air, scooped by an intake and passed across the heat exchanger as the aircraft moves forward (ram air). Although 90% of heat is given up in this way, the charge air temperature can never be reduced below the ram air temperature by this method alone. Expansion, provides cooling when the pressure of the charge air is reduced by increasing its velocity and expanding it across the turbine of a so-called Air Cycle Machine (ACM) or Cold Air Unit (CAU). In this way, the temperature of the charge air can be rapidly lowered to zero degrees Celsius, irrespective of the ram air temperature

Energy Conversion, cools by making the hot air do work. This is achieved by using the charge air to drive a turbine, which is connected by a shaft to the compressor or fan within the cold air unit, thus converting heat energy into kinetic energy. This method will also help to reduce the charge air to zero degrees Celsius.


Turbo CompressorFigure 3

HEAT EXCHANGERS

These are components within the air conditioning system that transfer heat from one gas stream to another. Ram air is used as the cooling medium to cool the very hot charge air ducted from the engine compressor or the gearbox mounted air compressor or blower. Depending on where they are placed within the air conditioning system, heat exchangers are often described as;

A ‘Pre-cooler’ or ‘Primary Heat Exchanger’ An ‘Inter-cooler’ or ‘Secondary Heat Exchanger’

The basic construction is a sealed unit containing a series of cooling passages; through which the charge air flows and over which the ram air is directed. Between these passages are thin corrugated strips, that also serve to dissipate heat as the ram air passes over them.

AIR CYCLE MACHINE (ACM) OR COLD AIR UNIT (CAU)

The ACM/CAU is the primary component in an air cycle cooling system. A number of different types can be found including; The turbo-compressor, the brake turbine and the turbo-fan. All three use the charge air to drive the turbine and the major differences between each type, relates to the overall weight for a given mass flow, the size and method of dissipating the power output of the turbine.

Turbo Compressor Cold Air UnitFigure 4

The turbo-compressor type consists of a turbine driving a centrifugal compressor and operating in conjunction with an inter-cooler connected between the compressor and turbine stages.Its basic construction consists of two main casings, the turbine volute and compressor volute casings. The two casings are connected together and enclose a bearing housing with two bearing assemblies, supporting a shaft upon which the turbine and compressor wheels are mounted.


The turbine wheel revolves within a nozzle ring and the compressor wheel rotates within a diffuser ring. The very hot charge air from the engine compressor bleed and routed via the pre-cooler, enters the eye of the ACM/CAU compressor. It becomes compressed on passing through the diffuser ring, increasing its temperature and energy. From the compressor, the hot air is directed across the inter-cooler matrix over which ram air passes and is then directed into the turbine volute nozzle ring, where it drives the turbine. The resultant expansion and energy conversion, rapidly lowers the air pressure and temperature. It is then directed towards the passenger cabin. (See Fig 3)

The ACM/CAU compressor and turbine wheels rotate at extremely high speeds, often in excess of 80,000 rpm, so efficient bearing lubrication is essential to ensure smooth and trouble-free running. Two lubrication methods are used; Integral wet sump arrangements, or pressurised air bearings that need no oil lubrication.The wet sump type normally has a sump containing oil and a means of metering it to the bearings usually by the use of integral ‘wicks’ or with an ‘oil slinger’ that pumps an optimum oil/air mix to the bearings. This ensures the correct amount of oil at the bearings at all times. Oil replenishment is critical however, as too much oil will lead to the charge air being oil contaminated and too little oil, may result in a premature seizure of the rotating shaft.The air bearing type uses a pressurised air supply to support the shaft in a similar manner to the ‘hovercraft principal’. As the rotor ‘floats’ on a thin layer of air, it is essential that this type is kept clean and dry and completely free from oil and grease.

Brake Turbine Cold Air UnitFigure 5

The brake-turbine type of ACM/CAU, has its charge air routed directly from the pre-cooler to drive the turbine. The air expands across the turbine as before, resulting in a large temperature and pressure drop. Since this layout dispenses with the need for an inter-cooler, it results in a greater efficiency due to weight saving. To safeguard against the turbine rotating too fast, it is coupled with a compressor, which rotates in ambient air and consequently acts as a braking medium. Additionally, the slower rotation of the shaft further improves turbine output efficiency. (See Fig 5)

Turbo Fan Cold Air UnitFigure 6

The turbo-fan type is mechanically similar to the brake-turbine arrangement. In this case however, the turbine drives a large centrifugal fan instead of a normal compressor. The fan is draws a large quantity of ambient air over the pre-cooler, which cools the incoming charge air. The major advantage of this type over the other two, is that with the fan-induced airflow over the pre-cooler, it can be used with the aircraft stationary on the ground with the aircraft engines running. It does not need to rely solely on ram air as the cooling medium for the pre-cooler.


Vapour Cycle Cooling

The vapour cycle cooling system can be used as an alternative to the air cycle cooling system. Although not commonly used these days for air conditioning systems, the system may be used as the means to remove heat from electrical and electronic equipment. The system relies on the principle of the ability of a refrigerant to absorb heat when changing from a liquid to a gas, through the process of vaporisation or expansion.For example, if you were to put a drop of a highly volatile liquid such as methylated spirits or petrol on the back of you hand, it will feel cold. This is because the liquid starts to evaporate and draws the heat necessary for evaporation from your hand. Liquids with a low boiling point have a stronger tendency to evaporate at normal temperatures than those with a high boiling point. Furthermore, the amount of pressure acting on a liquid substance will affect its state. A sufficient reduction in pressure will cause any liquid to change state into a vapour or a gas. Conversely, a corresponding increase in pressure will reverse the process.

Schematic Vapour Cycle SystemFigure 7

The major components of a typical system are a liquid receiver, a thermostatic expansion valve, an evaporator, a turbo-compressor, a condenser and a condenser fan. Often these components are mounted close together to form a line-replaceable refrigeration pack or vapour cycle cooling pack.The liquid receiver acts as a reservoir and provides storage for the refrigerant, normally a highly volatile chemical such as Freon. The refrigerant will pass from the liquid receiver to a thermostatic expansion valve where it is metered and released into the evaporator. The very hot charge air from the main engine bleed flows across the evaporator, releases heat that vaporises the liquid refrigerant and passes into the passenger cabin at a much lower temperature. Meanwhile, the now vaporised refrigerant gas is directed towards the turbo-compressor. It is drawn into the compressor wheel, the coupled turbine of which is driven by the main engine bleed air. (Note: In some cases, an independent means instead of a turbo-compressor may be used to compress the refrigerant gas, such as an electric motor, as in a domestic refrigerator). The refrigerant gas leaves the compressor at a high pressure and temperature and passes across the matrix of the condenser. The gas is cooled by the ram air, flowing across the matrix and so condenses back into a liquid once again. It then returns to the liquid receiver to repeat the refrigeration cycle once again. The condenser fan is used to induce air across the condenser matrix when the aircraft is stationary on the ground and no ram air is available.

Typical Vapour Cycle SystemFigure 8


Heating

Un-pressurised aircraft use a ram-air system for ventilation. At altitude, the ram-air passing through the cabin would be very cold, so a heating system is required.Heating systems can be generally divided into two types:

Exhaust heating systems Combustion heating systems

Exhaust Heating Systems

In its simplest form, this type of heating system employs a heater muff that surrounds the exhaust pipes coming from a piston engine, or the jet pipe of a turbo-jet. A ram air scoop at the forward end of the heater muff allows some of the cold air to go to directly to a mixing valve. The remainder, enters the muff and surrounds the exhaust/jet pipes. Heat from the pipes is transferred into the ram air and carried to the mixing valve. The heated air joins the cold air at the mixing valve and the combined flow is directed into the passenger cabin.Some form of control lever, operated from within the aircraft and connected to the mixing valve, allows the proportion of hot and cold air to be modulated in order to suit the cabin heating requirements.To cater for the possibility of the ventilation air becoming contaminated from the exhaust pipes, some aircraft will be fitted with carbon monoxide detectors within the cabin area. These are indicators filled with brightly coloured crystals, which turn black if exposed to dangerous levels of carbon monoxide.

Exhaust System HeaterFigure 9

Combustion Heating Systems

This system uses a purpose built combustion chamber heater assembly to provide the heat source, rather than the previously described exhaust heating method. Fuel is directed from the aircraft fuel system, through a pressure regulating and shut off valve that ensures the fuel is at the correct pressure for atomisation. Other components include a fuel filter, a fuel pump and spray nozzle, where it is atomised and ignited with an igniter plug. The combustion chamber assembly heats up the ram air that passes around it.

Temperature Control

In order to operate the aircraft in an infinite number of climatic and operating conditions, the temperature in the passenger cabin, flight compartment and other areas needs to be regulated for comfort.Temperature regulation for the majority of aircraft that employ the engine bleed air method is usually accomplished by controlling the proportion of hot and cold air coming from the air supply system. An electric motor driving a double butterfly type air mixing valve, regulates the cabin temperature, by allowing a controlled amount of hot air to by-pass the air cycle system. This air is then recombined in proper proportions with the cold air that has been directed through the air cycle system at a down stream mix chamber. The position of the air-mixing valve is determined by signals from the temperature control system.The temperature control system is normally operated automatically or as a manual system, if the automatic temperature controller should fail.


Typical Combustion Heater SystemFigure 10

During automatic operation, the temperature controller continually monitors cabin temperatures and repositions the air mixing valve if necessary to keep the temperature at the selected level.In order to achieve this, the controller receives signals from temperature selector on the flight deck (the temperature requested) and from temperature sensors in the passenger cabin, flight compartment and supply ducts (the actual temperature). If a difference between the requested and actual temperatures occurs, the controller will send an output signal, to re-position the air mixing valve until parity exists once more.During manual operation, the temperature control circuit bypasses the controller and connects the temperature selector on the flight deck, directly to the air-mixing valve. Other sensors in the system transmit compartment temperatures to indicators on the flight deck overhead panel, so that the actual temperatures and the position of the air-mixing valve can be monitored.

Temperature ControlFigure 11

Humidity Control

Humidity control is the means to ensure that the correct amount of water moisture content is in the air conditioning air within the aircraft cabin. This is necessary to ensure occupants do not suffer from low humidity levels that are experienced with high altitude flight.

Humidity control can be achieved two ways; Water Separation Water Infiltration

Water separation is the removal of excessive moisture from the charge air, normally by a water extractor or separator.Water infiltration is the addition of moisture into the conditioned air as it enters the cabin using a water pump and spray nozzle.

Water Separation – Water Extractor

Water can be introduced into the air conditioning system due to the compression and expansion of the air in the ACM/CAU and other areas of the air cycle process.There are three types of water separator in general use; the coalescer/diffuser type, the coalescer/bag type and the swirl vane type.

COALESCER/DIFFUSER TYPE

This type consists of a coalescer constructed from layers of monel metal gauze and glass fibre cloth sandwiched between layers of stainless steel gauze. It is supported by the diffuser cone and held in place by a relief valve housing. As the air leaves the diffuser and passes over the coalescer, moisture in the air is converted into water droplets. The droplets enter the collector shell and are deposited into collector tubes where they drain down to a collector box from where the water is ejected overboard.

COALESCER WATER EXTRACTOR

FIGURE 12


COALESCER/BAG TYPE

A porous bag, supported by a shell is fitted within the extractor to convert moisture into water droplets. A swirl is imparted into the conditioned air and the centrifugal effect forces the droplets to the outlet shell where it collects and drains from the component. A bag visual indicator operated by back pressure, will show when the coalescer bag becomes dirty or blocked. In this case, a relief valve will open to ensure flow is still available.

Bag Type Water ExtractorFigure 13

SWIRL VANE TYPE

This type uses centrifugal force to spin the moisture-laden air outwards against the exit shell. The swirl vane, either fixed or rotating imparts the swirl by rotating the airflow at high speed. The action, separates the heavier water droplets in the moisture and collects them in a sump, to be drained away.

Swirl Vane Type Water SeparatorFigure 14


Water Infiltration

Humidity control can also include the addition of water into the air conditioning system. As an aircraft climbs to high altitude, the moisture level in the air reduces to a much lower amount than at lower levels of altitude. The reduction in moisture may cause discomfort to the aircraft occupants. To counteract this, moisture is added into the conditioned air, by pumping water from a tank to a spray nozzle positioned at the cabin air inlet. Humidity sensors will detect low humidity conditions and automatically turn on the controller water pump to restore the humidity to acceptable levels.

Typical Humidity Control SystemFigure 15

Mass Flow Control

Legislation requires that a minimum amount of fresh air be supplied to passengers and crew. In addition stale air must be removed and odours eliminated. Most pressurisation systems rely on the fact that air is delivered at a constant rate under all conditions of flight in order to function correctly.Mass flow control systems constantly monitor the velocity and density of the air supply by either increasing or decreasing the demand upon the source of supply, or by spilling excess supply air overboard.The mass of air must be controlled at a constant value regardless of aircraft altitude or cabin pressure. It must also adjust for changes in main engine compressor speed in bleed air systems, or changes in rotor speed when a separate air supply from an accessory gearbox driven blower is incorporated.

Mass Flow Controller

This type automatically caters for changes in air density, cabin back pressure and engine compressor supply pressure. At ground level and during take off and the early stages of flight, the pressure available from the main engine compressor outlet is high. As altitude increases or when the engines are set to cruising speeds, the supply pressure drops.The amount of pressure from the engine compressor bleed acting on an altitude-compensated piston valve, determines the position the valve will adopt when opposed by a spring and back pressure from the cabin. The pressure drop across the valve, will vary the size of outlet ports and will thus determine the valve’s degree of opening and closing. This will result in a constant mass flow downstream of the valve at all times.


Distribution Systems

The air distribution system on most aircraft takes cold air from the air conditioning packs and hot air bleed from the engines and mixes the 2 in a mixer unit to the required temperature. The air is then distributed to side wall and overhead cabin vents. On some aircraft the cabin air is then drawn back into the mixing unit by re-circulating fans where it is mixed with new air and then re-distributed.

All major components are usually located together in a designated bay for ease of maintenance. ( Figure 14).

A gasper fan provides cold air to the individual overhead air outlets for the aircrew and passengers. This air can be drawn direct from outside or from the cooling packs. Each passenger or crew can control the amount of air received by controlling the position of the air outlet. This outlet could be a rotary nozzle or a louvre.

Mass Flow ControlFigure 16

Air Conditioning Distribution ManifoldFigure 18

Conditioned air systems dispense temperature controlled air evenly throughout the cabin and crew areas. One duct system supplies the cockpit (Figure 17) while another supplies the cabin. The cabin ducting is then divided into 2 systems, the overhead (Figure 15) and the sidewall systems (Figure 16). The overhead system releases air into the cabin from outlets in ducting running fore and aft in the cabin ceiling. The sidewall duct system takes air through ducting between the sidewall and cabin interior linings and releases it through cove light grills and louvres.


A cockpit controlled selector valve located on the main distribution manifold allows all overhead, side wall or any combination of the two systems to be used and varies the flow between the two.

Overhead PanelFigure 19

Duct sections throughout both the cabin and cockpit are joined together with clamps or clips. Means of equalising the duct pressures and balancing the air flows are designed into each system. The systems are protected from excess pressures by use of a spring loaded pressure relief valve usually located in the main distribution manifold. The main manifold is located immediately downstream from the mixing units in the air conditioning bay.

On large aircraft a cockpit controlled dual selector valves divides the air between cockpit and cabin areas. These butterfly valves are interlinked. When one is fully open the other is fully closed and vice versa.Air is exhausted from the passenger cabin through grills and outflow valves in the sidewalls above the floor. This air can then be directed around the cargo compartment walls where it assists in compartment temperature control. Some air then flows to the cargo heat distribution duct under the compartment floor and is then discharged overboard through the outflow valves.

Sidewall DuctingFigure 20

Below each floor air exhaust outlet is a flotation check valve. This valve is a plastic ball held in a cage. If the cargo compartments become flooded the balls float up the cage and seals off the floor to help prevent water from entering the cabin.

Cockpit Air DistributionFigure 21

Aircraft may be separated into zones each with its own air conditioning system and controls for that zone located in a distribution bay. Some areas may have a remote heat exchanger and fan assembly in the vapour cycle system, to allow cooling to specific areas such as avionics bays, fed from one of the zone packs.


Re-circulation Air System

To improve cabin ventilation and supplement airflow the cabin air is recirculated back to the main distribution manifold where it is mixed with conditioned air form the cooling packs. The use of re-circulated air improves airflow and offloads the air supply system. This off loading of the air conditioning packs is converted into a fuel saving.

The re-circulation fan will draw air from the cabin area, through a check valve and filter assembly to remove any smoke and noxious odours before passing it to the mixer unit for re-distribution. The check valve prevents any reverse flow through the fan and ducting when the fan is not in use.

Pressurisation Systems

As aircraft became capable of obtaining altitudes above that at which flight crews could operate efficiently, a need developed for complete environmental systems to allow these aircraft to carry passengers. Air conditioning could provide the proper temperature and supplemental oxygen could provide sufficient breathable air.

The problem was that not enough atmospheric pressure exists at high altitude to aid breathing in 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 is overcome by pressurising the cockpit/ cabin area. Cabin pressurisation is a means of adding pressure to the cabin of an aircraft to create an artificial atmosphere that when flying at high altitudes it provides gives an environment equivalent to that below 10000 feet. The minimum quantity of fresh air supplied to each person on board must be at least 0.5lb/ minute.

Aircraft are pressurised by sealing off a strengthened portion of the fuselage. This is usually called the pressure vessel and will normally include cabin, cockpit and possibly cargo areas. Air is pumped into this pressure vessel and is controlled by an outflow valve located at the rear of the vessel.

Sealing of the pressure vessel is accomplished by the use of seals around tubing, ducting, bolts, rivets, and other hardware that pass through or pierce the pressure tight area. All panels and large structural components are assembled with sealing compounds. Access and removable doors and hatches have integral seals. Some have inflatable seals.

Pressurisation systems do not have to move large volume of air. Their function is to raise the pressure inside the vessel. Small reciprocating engine powered aircraft receive their pressurisation air from the compressor of a coupled turbocharger. Larger reciprocating engine powered aircraft receive air from engine driven compressors and turbine powered aircraft use compressor bleed air

Small Reciprocating Engine Powered Aircraft

Turbochargers are driven by the engine exhaust gases flowing through a turbine. A centrifugal compressor is coupled to the turbine. The compressors output is fed to the engine inlet manifold to increase manifold pressure which allows the engine to develop its power at altitude. Part of this compressed air is tapped off after the compressor and is used to pressurise the cabin. The air passes through a flow limiter (or sonic venturi) and then through an inter-cooler before being fed into the cabin. A typical system is shown at Figure 22.

Sonic Venturi

A sonic venturi is fitted in line between the engine and the pressurisation system. When the air flowing across the venturi reaches the speed of sound a shock wave is formed which limits the flow of air to the pressurisation system

Small Reciprocating Engine Aircraft Pressurisation SystemFigure 22

Large Reciprocating Engine Powered Aircraft


These aircraft use engine driven compressors driven through an accessory drive or by an electric or hydraulic motor. Multi engine aircraft have more than one air compressor. These are interconnected through ducting but each have a check valve or isolation valve to prevent pressure loss when one system is out of action.

Turbine Powered Aircraft

The air supplied from a gas turbine engine compressor is contamination free and can be suitably used for cabin pressurisation (Figure 23). Some aircraft use an independent compressor driven by the engine bleed air. The bleed air drives the coupled compressor which pressurises the air and feeds it into the cabin

Turbo CompressorFigure 23

Some aircraft use a jet pump to increase the amount of air taken into the cabin (Figure 24). The jet pump is a venturi nozzle located in the flush air intake ducting. High velocity air from the engine flows through this nozzle. This produces a low pressure area around the venturi which sucks in outside air. This outside air is mixed with the high velocity air and is then passed into the cabin

Jet PumpFigure 24

Control And Indication

There are 3 modes of pressurisation, un-pressurised, the isobaric mode and the constant–differential pressure mode. In the un-pressurised mode the cabin altitude remains the same as the flight altitude. In the isobaric mode the cabin altitude remains constant as the flight altitude changes and in the constant-differential pressure mode, the cabin pressure is maintained at a constant amount above the outside ambient air pressure.

The amount of differential pressure is determined by the structural strength of the aircraft. The stronger the aircraft structure the higher the differential pressure and the higher is the aircrafts operating ceiling.

The Un-Pressurised Mode

In this mode the outflow valve remains open and the cabin pressure is the same as the outside ambient air pressure. This mode is usually from sea level up to 5000` but does vary from aircraft to aircraft.


The Isobaric Mode

In this mode the cabin pressure is maintained at a specific cabin altitude as flight altitude changes. The cabin pressure controller begins to close the outflow valve as the aircraft climbs to a chosen cabin altitude. The outflow valve then opens or closes (modulates) to maintain the selected cabin altitude as the flight altitude changes up or down. The controller will then maintain the selected cabin altitude up to the flight altitude that produces the maximum differential pressure for which the aircraft structure is rated. At this point the constant differential mode takes control.

The Constant-Differential Pressure Mode

Cabin pressurisation puts the aircraft structure under a tensile stress as the cabin pressure expands the pressure vessel. The cabin differential pressure is the ratio between the internal and external air pressures. At maximum constant-differential pressure as the aircraft increases in altitude the cabin altitude will increase but the internal/external pressure ratio will be maintained. There will be a maximum cabin altitude allowed and this will determine the ceiling at which the aircraft can operate.

Cabin Pressure Indication

Most pressurisation systems have three basic cockpit indicators cabin altitude, cabin rate of climb and the pressure differential indicator. The cabin altitude gauge measures the actual cabin altitude.

Cabin Altitude GaugeFigure 32

The cabin rate of climb indicator tells the pilot the rate that the cabin is either climbing or descending. (I.e. the rate at which the cabin loses or gains pressure) A typical maximum climb rate is 500ft per minute and the maximum descent rate is 300ft per minute. The control can be automatic or manual depending on aircraft type.

Cabin Rate of ClimbFigure 33

The differential pressure gauge (Figure 34) reads the difference between the cabin and the outside air pressures. This differential pressure is normally controlled and maintained to a structural limitation around 7psid. This depends on the aircraft type and the operating ceiling of the aircraft. The differential pressure gauge may be combined with the cabin altitude (Figure 35).

Differential Pressure Gauge Dual Gauge Figure 34 Figure 35


Download - Air Conditioning and Cabin Pressurisation Notes (1)

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