Question about Cabin Air Pressure
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Question about Cabin Air Pressure
I should like to ask the pilots out there about cabin air pressure. I understand that the norm is eqiv to 8000 feet (except 787).
Is this always the case?
Do you change the pressure during the fllight for any reason?
How does the aircraft pressurize?
Does the cabin pressure change as the plane ascends until 8000 ft and stay there?
What happens during the descent?
Are you aware of any history of passengers fainting becaue of the low pressure or any other sysmptons caused by low cabin pressure?
Thanks for your help
Is this always the case?
Do you change the pressure during the fllight for any reason?
How does the aircraft pressurize?
Does the cabin pressure change as the plane ascends until 8000 ft and stay there?
What happens during the descent?
Are you aware of any history of passengers fainting becaue of the low pressure or any other sysmptons caused by low cabin pressure?
Thanks for your help
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Every aircraft is different on how it pressurizes and un-pressurizes.
For Example, In our aircraft the owner is an old man and has sensitive ears. We set our Cabin Pressure to 4500 ft. Most airlines I believe the standard is like you said 7000, but it is not a hard driven number.
For Example, In our aircraft the owner is an old man and has sensitive ears. We set our Cabin Pressure to 4500 ft. Most airlines I believe the standard is like you said 7000, but it is not a hard driven number.
Dvlcom...
Basically pressurisation is pumping the air press inside the cabin to an equivalent pressure/ altitude.
How...early a/c used engine driven air pumps, nowadays air is tapped off the engine compressors and pumped into the cabin. An outflow valve is used to restrict the exit of the air thereby creating a back pressure. The difference between this press and the ouside air press is referred to as cabin differential press (Diff press).
Each airframe has a maximum press diff that can be achieved.
Most a/c are designed to have a max diff equal to a
cabin alt of around 7-9000 ft at normal cruising altitude.
"Do you change the press during flight"...only if the a/c changes its cruising altitude.
"Does the cabin press change as the a/c ascends"....yes. The cabin is usually climbed/ descended at 400/500 feet per minute (fpm) for pax comfort whereas the a/c may be climbing at up to 3-4000 fpm initially.
During descent much the same happens.
" Pax fainting"....never had any problems over 30 yrs. System designed to avoid that sort of thing.
I always used to think of the system as operating like a piece of string! The lower end was the air pressure in the cabin and the upper end was the air press at the operating altitude of the a/c. That piece of string could only be as long as the max diff of the a/c. As long as the piece of string was slack (below max diff) the cabin rate of climb/descent could be controlled. If the string was tight the cabin rate of change would be the same as the a/c.
Confused?
Basically pressurisation is pumping the air press inside the cabin to an equivalent pressure/ altitude.
How...early a/c used engine driven air pumps, nowadays air is tapped off the engine compressors and pumped into the cabin. An outflow valve is used to restrict the exit of the air thereby creating a back pressure. The difference between this press and the ouside air press is referred to as cabin differential press (Diff press).
Each airframe has a maximum press diff that can be achieved.
Most a/c are designed to have a max diff equal to a
cabin alt of around 7-9000 ft at normal cruising altitude.
"Do you change the press during flight"...only if the a/c changes its cruising altitude.
"Does the cabin press change as the a/c ascends"....yes. The cabin is usually climbed/ descended at 400/500 feet per minute (fpm) for pax comfort whereas the a/c may be climbing at up to 3-4000 fpm initially.
During descent much the same happens.
" Pax fainting"....never had any problems over 30 yrs. System designed to avoid that sort of thing.
I always used to think of the system as operating like a piece of string! The lower end was the air pressure in the cabin and the upper end was the air press at the operating altitude of the a/c. That piece of string could only be as long as the max diff of the a/c. As long as the piece of string was slack (below max diff) the cabin rate of climb/descent could be controlled. If the string was tight the cabin rate of change would be the same as the a/c.
Confused?
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Instead of quoting a cabin pressure of say, that of what is equivalent to 8000 feet. What would that be in terms of "Pounds per Square Inch". Or to put it another way, what pressure (in P.S.I.) do you need to raise the cabin to in order to equate to 8000 ft. And what is the maximum pressure that can be safely applied.
Thanks
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Usually a max differential pressure of about 8.5 psi on an airliner. Increasing the DPI is what lowers the cabin altitude. Corperate jets and the 787 lower their cabin altitude this way.
