021 06-00-00 Pneumatic - Pressurisation and Air Cond Amend0

8/9/2019 021 06-00-00 Pneumatic - Pressurisation and Air Cond Amend0

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TEXTBOOK

PNEUMATIC PRESSURISATION AND AIR CONDITIONING SYSTEMS

020 00 00 00 AIRCRAFT GENERAL KNOWLEDGE

021 06 00 00 PNEUMATIC PRESSURISATION AND AIR CONDITIONING

SYSTEMS

HP

LP

NO. 2ENGINE

TO DEICINGSYSTEM

TO NO. 2AIR-

CONDITIONINGPACK

TO NO. 1AIR-

CONDITIONINGPACK

FROM NO. 1 ENGINEBLEED-AIR SYSTEM

(SIMILAR TO NO. 2 SYSTEM)

AIR CONDITIONING

TEMP

CONTROL

OFF

RECIRC

CABIN

OFF

RECIRC

F/C FAN1 BLEED 2

COOL WARM COOL WARM

NORM

AUTO

MAN

MIN MAX

BLEED

0

2040

60

80

100

C

DUCT TEMP

OFF

PACKS

F/A

CABIN FLT COMP

CABIN

GAUGE

CABDUCT

F/CDUCT

F/C FAN

OFF


2/29

Pneumatic / Pressurisation / Air Conditioning

Infowerk Page 2

Table of Contents:

Ai r driven systems _____________________________________________________ 3

Pneumatic systems ____________________________________________________ 4

Ai r condi tioning system _______________________________________________ 14

Pressurisation _______________________________________________________ 25


3/29


Infowerk Page 3

Ai r driven systems

"Pneumatic, Vacuum systems" Pneumatic, or air driven systems, can be used to

power gyroscopic flight instruments, provide pressurization and air conditioning,

operate de-icing systems and power landing gear and brake systems in lieu of

hydraulics.

Alternate

Landing Gear

Control System

Alternate

Wheelbrake

System

Wheelbrake

SystemLanding Gear

Control System

Nose Wheel

Steering

F - 27 Pneumatic Landing Gear / Brake System

L.H. Nacelle R.H. Nacelle

Alternate

Storage Bottle

Main

Storage Bottle

Sensing Line

IsolatingValves

Isolating Valves

ROD

Air Filter

Pneumatic Panel

Heading

indicator

Attitude

indicator

Suction relief

valve

Turn and slip

indicator

Needle valve

Vane-type

vacuum pump

Central

air filter

Suction

gage

Oil

Air

Engine Oil

Oil

seperator

luprication

and cooling


4/29


Infowerk Page 4

Pneumatic systems

"Gyro pneumatic systems"

Venturi suction systems are simple systems, where a venturi tube mounted on

the outside of the fuselage is directed into the slipstream of the propeller.

Air flowing through the venturi produces a low pressure inside the instruments.

N 330030

L R2 M i n . T u rn

L R

2MIN TURN

Venturi

Suction

regulator

Pressure - reducing

needle valve

Heading

indicator

Attitude

indicator

Turn und Slip

indicator

Propeller Wash

Heading

Indicator

Attitude

Indicator

Turn and slip

Indicator

Needle valve

Venturi suction

Air filterAir flows into the instrument

cases through built-in filters to

spin the gyros.

The likelihood of ice buildup

and venturi blockage during

IFR conditions limits the use of

venturi systems to light VFR

aircraft.


5/29


Infowerk Page 5

Modern aircraft equipped with pneumatic gyros use engine driven vane type

pumps.

Two types are in use - wet and dry pumps.

Wet vacuum pumps are lubricated by engine oil. The oil lubricates and cools the

pump, but has to be routed through an air-oil separator and drained back into the

tank before the air can be directed overboard.

Vane Type Air Pump

Inlet Outlet

Case

Rotor

Vane

Shaft

Heading

indicator

Attitude

indicator

Suction relief

valve

Turn and slip

indicator

Needle valve

Vane-type

vacuum pump

Central

air filter

Suctiongage

Oil

Air

Engine Oil

Oil

seperator

lupricationadn cooling


6/29


Infowerk Page 6

Dry air pumps are lighter in weight and require no lubrication. They can drive

gyroscopic instruments with the suction they produce, as seen in this schematic.

Dry air pumps can also be used to produce positive air pressure.

Pressure systems are necessary on aircraft which fly at high altitudes where

there is not enough ambient air pressure to drive the gyros.

Twin Engine Vacuum System

Filter

Heading

indicator

Vacuum

regulator

Air

Pump

Suction

gage

Attitude

indicator

Vacuum

regulator

Air

Pump

Manifold check valve

Dry Vane-type

vacuum pumpRotor and vanes are

made from carbon compounds.

Wear generates a microscopic

carbon deposit which acts as

lubricant

Twin Engine Pressure System

Pressure

regulator

Pressure

regulatorInline

filter

Inline

filterManifold check valve

Inlet

filter

Inlet

filter

Air PumpAir Pump

Gyro

pressure

gage

Pilots

gyros

Copilots

gyros


7/29


Infowerk Page 7

Bleed air"

Turbine engine powered aircraft usually use bleed air from the engine

compressor.

This air is free from contamination and can be safely used for cabin

pressurization and air conditioning.


8/29


Infowerk Page 8

Some aircraft use independent cabin compressors driven by bleed air to increase

the volume of air taken into the cabin.

Another design uses a jet pump flow multiplier to increase the volume of air.

The jet pump is essentially a venturi inside a line from the outside of the aircraft.A nozzle blows a stream of high-velocity compressor bleed air into the throat of

the venturi which produces a low pressure that draws air in from the outside.

This outside air is mixed with the compressor bleed air and carried into the

aircraft cabin.

Compressor Bleed Air

Ambient Air

To Cabin

Jet pump

Flush air inlet Outside skin

Pressure

vessel

Outflow

valve

Turboprop

engine

Bleed air

Flush air inlet Outside skin

Pressure

vessel

Outflow

valve

Turboprop

engine

Bleed air

Compressor turbine

Compressor


9/29


Infowerk Page 9

"APU bleed air"

The Auxillary Power Unit installed on most larger airplanes is usually capable of

providing bleed air to operate air cycle-machine-based air conditioning systems

on the ground.

Some APUs are designed to

operate in flight and can, if

required, provide bleed air for

pressurization and air-

conditioning as a backup inemergencies.

Pre-cooler

Pressure

regulatorand

relief valve

Water

seperator

AIRFOIL

DEICING

ECS

PACK

LH

APU

ECS

PACK

RH


10/29


Infowerk Page 10

"Ground Service Cart"

To pre-condition the airplane on the ground predominantly diesel powered

ground servicing carts can provide pre-heated or pre-cooled air at high volumes.

Ground service connections on the outside of the fuselage allow the supplied air

to be ducted into the aircraft air condition ducts.

Ground service connectors usually house a check valve to avoid loss of ship-

supplied air once the cart is disconnected.

To prevent the aircraft from becoming pressurized on ground at least one door

should be left open if an air cart is connected and operating.

Air conditioning

ground coupling

CABIN AIR

FLT COMPT AIR

FLT COMPT CABINBAGGAGE

COMPARTMENT

Ground air

service connector

CONDENCER

MIXING

BOX

EXPANSION

TURBINE

AIR CYCLE

MACHINE (ACM)

COMPRESSOR


11/29


Infowerk Page 11

"Indications and warnings"

As an example of bleed air related indication and warnings in a modern glass

cockpit, look at the system of the Embraer 145.

There are 5 types of indication for the air conditioning system.

- the indication on the engine indicating and crew alerting system display,

- the indication on the environmental page on the multi function display,

- the engine indicating and crew alerting system caution messages,

- the engine indicating and crew alerting system advisory massages and

- the central maintenance computer massages on the maintenance page of the

captains multi function display.

0

AA

KG KG

CRZ

88.288.2

OFF

4000

7.4

81818181

1540 1540

94.3 94.3

800800

790 790

UP UPUP

100 1

KGH KGH

88.288.2

RTN T/0 FUEL HYD ELECM/PRNG

ECSA/I

FMSBSNB3.5NM1 MIN

-38-12446

SATTATTAS

CABIN TEMP

ECS BLEED

TEMP

C

C+28

+28

CKPT TEMP

OXY

PRESS

1800 PSI

N

TGTTX

10

BSNB25 25

155

LUMEL

PACK 1 OVLDPACK 2 OVLDPACK 1 OVHT

PACK 2 OVHTRAM AIR VLV FAILPACK 1 VLV FAIL

PACK 2 VLV FAILPACK 1 VLV CLSDPACK 2 VLV CLSD

MAINTENANCE MESSAGES 1/03

AIR COND 1 LEAKAGE10/03 20:50 OCCUR:01

AIR COND 2 LEAKAGE10/03 20:50 OCCUR:01

DIG TEMP CONTROL 1 FAIL10/03 20:50 OCCUR:01

DIG TEMP CONTROL 2 FAIL10/03 20:50 OCCUR:01

DUCT TEMP SENSOR 1 FAIL10/03 20:50 OCCUR:01

DUCT TEMP SENSOR 2 FAIL

10/03 20:50 OCCUR:01


12/29


Infowerk Page 12

"Interface with other systems"

Hot engine bleed air, mainly used for pressurization and air conditioning, can also

be used for anti-ice and de-ice purposes such as engine intake and wing leading

edge heating and de-ice boot inflation.

HP

LP

NO. 2ENGINE

TO DEICINGSYSTEM

TO NO. 2

AIR-CONDITIONING

PACK

TO NO. 1AIR-

CONDITIONINGPACK



AIR CONDITIONING

TEMP

CONTROL

OFF

RECIRC

CABIN

OFF

RECIRC

F/C FAN1 BLEED 2

COOL WARM COOL WARM

NORM

AUTO

MAN

MIN MAX

BLEED

0

2040

60

80

100

C

DUCT TEMP

OFF

PACKS

F/A

CABIN FLT COMP

CABIN

GAUGE

CAB

DUCT

F/C

DUCT

F/C FAN

OFF


13/29


Infowerk Page 13

Bleed air is used to inflate door seals, power pneumatic autopilot servos and with

the use of a venturi to generate suction, to control cabin outflow valves or drive

pneumatic instrument gyros.

Inflatable ( pneumatic) door seal

Bleed air pressure reduced to 18 psi

is used to inflate the door seal

Quilted Fabric

1.00" Min Clearance

Door Seal

Side view of

Quilted Fabric and Retainers


14/29


Infowerk Page 14

Ai r condi tioning system

"Conditioning the air in an airplane" used to just mean turning on the heat to

warm the cockpit and cabin as airplanes generally fly in low temperature at high

altitudes.

Now with people accustomed to more creature comforts, cooling systems are

used to make the cabins more comfortable when the aircraft is on the ground.

Airplane pressurization and air conditioning requires outside or "ambient" air to

be forced into the aircraft cabin.

The easiest way to achieve this is by using an air-scoop that extends into the

slipstream. This ram air flow might be sufficient for low flying single or small multi-

engine aircraft.

Airplanes that cruise at altitudes of 10,000 ft or more require air supplied at a

higher pressure to maintain an air pressure in the cabin which is comfortable and

safe for crew and passengers.

The source for this air depends largely on the type of propulsion used on a

particular airplane.

2000 ft

4000 ft6000 ft

8000 ft

10.000 ft

12.000 ft

14.000 ft

16.000 ft

18.000 ft

20.000 ft

22.000 ft

Non pressurized

Pressurization or

supplemental Oxygen required

Sea Level


15/29


Infowerk Page 15

"Cooling systems". Vapor cycle cooling systems use a compressor to pump a

refrigerant through a closed system achieving a cooling effect once the gas

expands into a heat exchanger - the so-called evaporator.

Vapor Cycle System

Re-ciculated cabin air

Blower

Cold air

Inlet Outlet

Compressor

Condenser

Hot exhaust air

Ambient air

Receiver

dryer

Thermostatic

expansion valve

Heat exchanger - evaprator

Ovbd

PrimaryHeat

Exchanger

Secondary

Heat

Exchanger

Ram air inelt

Emerg

ram

Groundconnection

Reheater

Condenser

Environment control systempackAIRCOND / PNEUMATIC

CKPT CABIN

PAXRECIRC

C H C H

ATTND

PACK 1 PACK 2

XBLEED

BLEED 1 APU BLEED BLEED 2

WING 1

START 1

WING 2

START 2GND

CONN

START 2

LEAK LEAK

From engine To cabinT

T

T

T

T

T2C T1

CollectorWater

Fan

Air cycle cooling systems are

the true air conditioning

systems since they are

capable of controlling the air

temperature over a wide range

from maximum hot to

maximum cold depending on

the environment.


16/29


Infowerk Page 16

"Heating systems"

Most small aircraft use one of two types of heating systems:

The exhaust system heaters where air flows around the exhaust components and

picks up heat before it is carried into the cabin.

Or combustion heaters that use fuel from the aircraft fuel tanks to operate an

independent heater.

Exhaust System Heater

From engine cylindersOverboard

To cabin

Cabin heat

box

Exhaust

overboard

Heater muff

Ram air

Combustion air scoop

Fuel FilterPump

Heated

air

ExhaustDrain

Fuel

filter

Ignition unit

Spray nozzle

Combustion chamber

Fuel tank

Exhaust shroud

DrainDrainVentilating

air scoop

Sealed fuel control assembly

COMBUSTION HEATER


17/29


Infowerk Page 17

"Sources of air supply"

Airplane pressurization and air conditioning requires outside or "ambient" air to

be forced into the aircraft cabin.

The easiest way to achieve this is by using an air-scoop that extends into the

slipstream.

This ram air flow might be sufficient for low flying single or small multi-engine

aircraft.

Air-scoop


18/29


Infowerk Page 18

Airplanes that cruise at altitudes of 10,000 ft or more require air supplied at a

higher pressure to maintain an air pressure in the cabin which is comfortable and

safe for crew and passengers.

The source for this air depends largely on the type of propulsion used on a

particular airplane.

"Air cycle machines"

Gas turbine powered transport aircraft use compressor bleed air for pressurizing

the cabins with temperature controlled air.

AIR SUPPLY:

Turbo - Chargers

Blowers or Compressors

Gas -Turbine Bleed Air

For airconditioning and pressurization


19/29


Infowerk Page 19

Our schematic shows the air conditioning system for a twin-engine jet transport

aircraft with the engines mounted on the aft fuselage.

This airplane has two independent air conditioning systems that supply the cabin

with heated and cooled air that is mixed to produce pressurized air at the right

temperature.

Hot compressed bleed air is taken from the engines and from the auxiliary power

unit. It passes through pressure regulating and shutoff valves, flow limiters and

flow control valves to the air cycle machine where it is cooled.

Some of the hot air is tapped off before it goes through the cooler and is mixed

with the cold air by a temperature control valve to achieve the correct

temperature.

Gas turbine powered transport aircraft use compressor bleed air for pressurizing

the cabins with temperature controlled air.

Compressor

Expansion turbine

Air cycle machine

Ground air

conditioner

connection

Mix chamber

Temperature

control valve

Anti-icethermostat

Aft

pressure

bulkhead

Water seperator

Water seperator temperature control valve

Manual

crossfeed

valve Flow conrol valve

Pressure regulating

& shutoff valve

8th stage

bleed check

valve

Anti-icing

pressure

regulating

shutoff valve

13th stage

augmentation valve

Ground pneumatic

connection

Primary heat

exchanger

Secondary heat

exchanger

APU load control

valve

Cooling air

selector valve

Ground

cooling fan

From L.H. engine

From R.H. engine

Sonic venturi

Ram air valve

Ram

air


20/29


Infowerk Page 20

"ACM cooling"

The cold air for cooling the airplane is produced by removing heat energy from

the hot compressor bleed air.

The hot bleed air from the engines and APU flows into a primary heat exchanger

where it gives up some of its heat to ram air that flows through ducts.

After leaving the heat exchanger it flows through the air cycle machine where it is

further compressed by the centifugal compressor.

The temperature rise caused by this compression allows more heat energy to be

removed as the air flows through the secondary heat exchanger.

After leaving this heat exchanger the air gives up much of its energy as it spins

the expansion turbine which drives the air-cycle machine compressor.

Still more energy is extracted in the last stage of cooling as the air expands upon

leaving the turbine. When it leaves the expansion turbine the air is cold.

Compressor

Expansion turbine

Air cycle machine

Primary heatexchanger

Cooling air

selector valve RamSecondary heat

exchanger

Cold air to temperaturecontrol valve


21/29


Infowerk Page 21

"Flow control valves"

Most flow control valves use butterfly valves to reduce or increase the flow of air

in a duct.

The position of the valve can be changed electrically through an electric motor or

pneumatically, through a bleed air actuated piston and cylinder assembly.

Flow rates can be a fixed setting for optimum performance of the air conditioningsystem or a variable setting allowing the crew to control the air flow within pre-set

limits.

Actuator housingSolenoid

Electricalconnector

Actuator cover

Access tofilter

Flow controlvalve

Butterfly Valve

High pressure shut off valve

TO NO. 1AIR-

CONDITIONINGPACK

TO NO. 2AIR-

CONDITIONINGPACK



TO DEICINGSYSTEM

FROM CABINSUPPLY DUCT

NO. 2ENGINE

HP

LP

AIR CONDITIONING

TEMP

CONTROL

OFF

RECIRC

CABIN

OFF

RECIRC

F/C FAN1 BLEED 2

COOL WARM COOL WARM

NORM

AUTO

MAN

MIN MAX

BLEED

0

2040

60

80

100

C

DUCT TEMP

OFF

PACKS

F/A

CABIN FLT COMP

CABIN

GAUGE

CAB

DUCT

F/C

DUCT

F/C FAN

OFF


22/29


Infowerk Page 22

"Temperature control"

Temperature is controlled within the air conditioning system by mixing hot engine

bleed air with cold ACM discharge air.

Electrically-operated butterfly valves control the amount of hot air added to the

cold ACM discharge air.

Hot bleed air

Expansion turbine

Compressor

Air cycle machine

Cooling air

Primary heat

exchanger

Cold ACM air

Butterfly valve

ACM butterfly valvePack bypass

butterfly valve


23/29


Infowerk Page 23

Dual valve systems can vary the ratio of airflow between the ACM and uncooled

air. The result is a temperature-controlled air conditioning pack.

Each pack consists of ACM, heat exchangers, ducting and control valves.

Each pack feeds into a common mixing chamber from where ducts route the

conditioned air into the cabin and cockpit air supplies.

The temperature of individual ducts may be controlled by adding or restricting

additional hot air using trim valves.

Each pack consists of ACM, heat exchangers, ducting and control valves.

Temperature control panels are typically located in the cockpit with sub panels at

flight attendant stations.

COMPRESSOR

EXPANSION TURBINE

AIR CYCLE MACHINE

GROUND AIR

CONDITIONER

CONNECTION

MIX CHAMBER

TEMPERATURE

CONTROL VALVE

ANTI-ICE

THERMOSTAT

AFT PRESSURE

BULKHEAD

WATER SEPERATOR

WATER SEPERATOR TEMPERATURE

CONTROL VALVE

MANUAL

CROSSFEED

VALVE FLOW CONTROL VALVE

PRESSURE REGULATION

& SHUTOFF VALVE

8TH STAGE BLEED

CHECK VALVE

ANTI-ICING

PRESSURE

REGULATION

VALVE

13TH STAGE

AUGMENTATION VALVE

GROUND PNEUMATIC

CONNECTION

PRIMARY HEAT

EXCHANGER

SECONDARY HEAT

EXCHANGER

APU LOAD

CONTROL VALVE

COOLING AIR

SELECTOR VALVE

GROUND

COOLING FAN

FROM R.H. ENGINE

SONIC VENTURI

RAM AIR VALVE

RAM

AIR

FROM L.H. ENGINE

Ducting and

control valves

Ducting and

control valves

Heat exchanger

Air cycle machine

Heat exchanger

Air cycle machinePack 1

Pack 2

AIR CONDITIONING

TEMP

CONTROL

OFF

RECIRC

CABIN

OFF

RECIRC

F/C FAN1 BLEED 2

COOL WARM COOL WARM

NORM

AUTO

MAN

MIN MAX

BLEED

0

2040

60

80

100

C

DUCT TEMP

OFF

PACKS

F/ACABIN FLT COMP

CABIN

GAUGE

CAB

DUCT

F/C

DUCT

F/CFAN

OFF

CABIN TEMP LIGHTING NVS SYSTEM

CABIN

OVERHD

PSU

TEST

O N/ OF F P AU SE

CABIN

SIDEWALL

PSU

ON/OFF

LAVATORY

WARDROBE AIRSTAIR

DOOR

BUFFET

OVERHD

OFF PAUSE

FAULT DEGRADED

Cockpit

Flight attendant


24/29


Infowerk Page 24

"Humidity control"

If a given amount of air is cooled the relative humidity increases in relation to the

temperature decrease.

In an air conditioning system this would lead to condensation in the packs and

ducts.

To reduce the humidity in the system condensers can be incorporated in the pack

downstream of the heat exchangers.

DUCT TEMP

SENSORDUCT O TEMP

SWITCH

TO FLIGHT COMPARTMENTCOMPRESSORDISCHARGEOVERTEMPERATURESWITCH

DUCT TEMPSENSING BULB

BLEED AIR

SILENCERS

TEMPERATURETRIM VALVES

PACK TEMPERATURECONTROL VALVES

RAM AIROVERBOARD

HEATEXCHANGERFILTER

WATER

NOZZLE

RAM AIRSILENCER

TC

WATER

TRAPRECIRCULATIONAIR FAN

GROUND AIR

SERVICE

CONNECTOR

(S.O.O. 8069)

AIR CYCLE

MACHINE (ACM)

MIXING

BOX

CONDENSER

RAM AIR

BAFFLE

REFER TO TEMPERATURE CONTROLDESCRIPTION AND OPERATION

COMP

TURB

PLENUM

FAN

EXHAUST

PACK

BYPASS VALVE

TURBINE

BYPASS VALVE

BLEED AIR

FROM No.1

AND No.2

NACELLES

RAMSCOOP

LEFT WING REFRIGERATION PACK

CHECK

VALVE

WATER SEPARATOR

Alternately water

separators may be

installed as stand-alone

units downstream of the

ACM.


25/29


Infowerk Page 25

Pressurisation

"Aircraft pressurization systems"

High altitude is a hostile environment in which the human body cannot survive

without a great deal of help.

However, it is the ideal environment for long distance flight.

Turbine engines operate efficiently and the lower density of the air decreases

drag. The humidity at high altitudes is low, so weather conditions are excellent

most of the time.

Trial flights flown in the 1930's with pilots wearing pressurized suits proved the

existence of strong high altitude winds today called jet streams.The first pressurized airplane flew in 1936. A special version of the Lockheed

Model 10 Electra with turbocharged engines had a fully pressurized cabin and

was able to make flights to an altitude of 25,000ft maintaining a cabin altitude of

10,000 ft or less.


26/29


Infowerk Page 26

"Principles of pressurization"

Aircraft are pressurized by sealing off a strengthened portion of the fuselage

called the pressure vessel and pumping air into it.

The cabin pressure is controlled by one or more outflow valves, usually located at

the rear of the pressure vessel.

The opening of these valves is controlled by the cabin pressure controller whichregulates the amount of air allowed to leave the cabin in order to achieve and

maintain a desired differential pressure or cabin altitude.

-1

0

1

2

34 5

6

7

8

9

101000 ft

CAB ALT

CABINE ALTITUDE

NORM

AUTO

DUMP

CABSETBARALT

M

A

N

INCR

RATE

Control panel

Outflow

valve

Outflow

valve

Cabin pressure

controller

Air conditioning pack

CABIN AIR

FLT COMPT AIR

FLT COMPT CabinBaggage

compartment


27/29


Infowerk Page 27

"Modes of pressurization"

There are three modes of pressurization:

- the unpressurized mode,

- the isobaric mode and

- the constant differential mode.

"Unpressurized mode"

In the unpressurized mode the cabin altitude is always the same as the flight

altitude. The outflow valve remains fully open and the cabin pressure is the same

as the ambient air pressure.

100020003000400050006000

10000

15000

20000

Sea level

02

ALT

4

6

81012

14

FT x1000

20

302

1

0

RATEFPM x1000

2

1UP

DOWN

01

2

34

5

6

DIFF PSI

CABIN

tail section

(unpressurized)Cabin

(Pressure Vessel)

Outflow Valve

fully open


28/29


Infowerk Page 28

"Isobaric mode"

In the isobaric mode the cabin altitude always remains constant.

The cabin pressure is maintained at a specific cabin altitude as the flight altitude

changes.

The cabin pressure controller begins to close the outflow valve at a selected

cabin altitude.

The outflow valve closes and opens or modulates to maintain the selected cabin

altitude up to the flight altitude that produces the maximum differential pressure

for which the aircraft structure is rated.

100020003000400050006000

10000

15000

20000

Isobaric Mode

Sea level

02

ALT

4

6

81012

14

FT x1000

20

302

1

0

RATEFPM x1000

2

1UP

DOWN

01

2

34

5

6

DIFF PSI

CABIN

tail section

(unpressurized)Cabin

(Pressure Vessel)

Outflow Valve

closed

Max Differential Pressure


29/29


"Constant-differential mode"

In the constant-differential mode the cabin pressure is maintained a constant

amount above that of the outside air pressure.

Cabin pressurization puts the structure of an aircraft fuselage under a tensile

stress as the pressure inside the pressure vessel tries to expand it.

The cabin differential pressure expressed in "psid" is the difference between the

internal and external air pressure and is a measure of the stress on the fuselage.

When the cabin differential pressure reaches the maximum for which the aircraft

structure is designed the cabin pressure controller automatically shifts to the

constant-differential mode and allows the cabin altitude to increase, but maintains

the maximum allowable pressure differential.

100020003000400050006000

10000

15000

20000

Sea level

02

ALT

4

6

81012

14

FT x1000

20

302

1

0

RATEFPM x1000

2

1UP

DOWN

01

2

34

5

6

DIFF PSI

CABIN

Diff Pressure Constant

Diff Pressure Build Up

Documents

021 06-00-00 Pneumatic - Pressurisation and Air Cond Amend0