Before we start pressurization I want you to know these definitions.
so :-
Differential pressure is the difference between the pressure outside the aircraft and inside the
aircraft .
Vertical speed is the rate of climb or descend, the cabin is being maintaining .so what is
happening is ,if your aircraft is climbing at a certain rate,that is not the same rate at which the cabin is
climb at. So both are different things.
Cabin attitude is altitude at which the cabin is being maintained at. so this is also different from
your aircraft altitude. for example if your aircraft is at 35000 feet ,yor cabin altitude might be
somewhere around 7000-8000 .ok so it's important to differentiate between the actual aircraft altitude
and the cabin altitude and cabin vertical speed .
Pressurization System
The A320 pressurization system is to adjust cabin altitude and rate of change of cabin altitude.you just
keep in mind the word pressurization in the system is actually not pressurizing the aircraft , it is the
system that is maintaining cabin altitude of rate of change of cabin altitude.Pressurization is already
coming from the air conditioning bay by the packs.So it is important to know what the system is doing.
It works in three modes :
1.fully automatic
2.semi automatic (Pilot only setting up the Landing Elevation)
3. manual (Outflow valve movement is under full control of pilot)
you will understand that minute differences in our study :
Its components are as follows :
1.It is controlled by 2 CPC(Cabin Pressure Controllers )
2. One Outflow valve with actuator (which is kind of flap that opens in the right side of the aircraft
….and is controlled by the CPC via two motors for automatic operation and one motor for manual
operation. It is used to regulate the amount of air allowed to escape the aircraft and is located at the
right side of the aircraft) .
3.One overhead control panel
4.Two safety valves used to avoid excessive differential pressure .so if the aircraft is being pressurized
over excessive value the safety was open and the pressure will be released so that you don't stress the
structure of the aircraft.
looking at the pressurization flight profile ,we will be able to understand automatic operation of the
pressurization system so when in the automatic or semi automatic mode either of the CPC alternatively
controls the pressurization of the aircraft. so what is happening ,if on one sector CPC 1 is active while
returning on that same sector your CPC 2 will be active. so it automatically alternatively take rolls.So, if 1
CPC fails the other one takes over completely and the indication to be shown on the ECAM.
CPC obtained various data from FMGC,LGCIU, EIU,ADIRS
FMGC is flight management and guidance system where it will get its landing elevation information .It
is a navigational database updated in every 28 days.
LGCIU is landing gear control interface unit ,it will know the position of the landing gear i.e whether
you are on ground or you are in-flight…… from that it will retrieve the data .
EIU is in engine interface unit ,it will knows the thrust level in climb mode, cruising mode or take of
trust or whatever.
ADIRS is air data inertial reference system so from this it obtains pressure of the outside air,
temperatures and all sorts of a air data related stuff with which it can calculate the required
pressurization flight profile.
so let's begin I will come back to the image .
Automatic Mode Of Operation
Before Flight:
So before flight was happening you are parked at the bay and that time :
Outflow valve is fully open,
There is no pressure differential so that means whatever is the pressure of the field that is the
same pressure inside the aircraft
There is no cabin vertical speed because you’re not anyway climbing or descending.
Cabin altitude will be equal to the field elevation because outflow valve is fully open.
During takeoff:
Pre pressurization takes place to avoid pressure bump or surge at rotation for passenger
comfort .when you rotate the aircraft it actually climbs out pretty fast because of jet take off and
this sudden climb out really can cause trouble to the passengers.
Outflow valve is slightly closed.
It descends the cabin altitude to 400 ft per minute until the differential pressure reaches
1.1 psid.See the pre-pressurization section of the above graph.
During Cruise:
The outflow valve is controlled by CPC as per Flight profile i.e CPC see the flight profile,calculate
all data and modulates the outflow valve accordingly.
The CPC does not allow the differential pressure to exceed 8.06 psid (i.e this may be perhaps the
structural limit,which the airbus may be designed for aircraft) and cabin altitude to exceed 8000
feet……………………See the pressurization box is empty in cruise mode,which means that it may be
adjusted as per flight profile requirement.
During Approach/Landing :
Approach is loaded in the database of FMGC,it will calculate at the top of descent and shares the
information to CPC,the outflow valve is then controlled by CPC just before the landing.CPC will decide…
what decent rate of pressurization is to be maintaned so that shortly before landing the aircraft will be
de-pressurized.
The maximum cabin rate of descend (ROD),it will maintain is 750 fpm.(beyond this pax . feels
dis-comfort)
After landing if there is any residual cabin pressure. It is released at 500 fpm till the diffrence is
0.1 psi….55 seconds after landing.
Now compare the aircraft altitude profile and cabin altitude profile….look at the top of descent,both
aircraft descent(say around 35000 ft) and cabin descent (say at around 7000 ft) starts together with
very slow de-pressurization rate
