Theory question

[neil_1821]

Wondering if anyone could help on a quick question as i dont fully understand it.

The maximum cabin differential pressure of a pressuries aeroplane operating at FL370 is approx?:

answer is 9.0psi

Anyone know how they arrive at this answer?

[Rainboe]

There is no mysterious theory attached to it- it is simply the strength (hence limitation) that the fuselage is designed to achieve. At 37,000', with a cabin differential of 9psi, the effective cabin altitude in the cabin will be about 8,000'. If the aeroplane climbed to 41,000', the cabin differential pressure would remain at its maximum of 9psi, but the cabin altitude would climb to say 9,000'. If you made a stronger fuselage that could achieve for example 12psi, you would probably have a cabin altitude of about 3,000' (rough guess!). The 9psi is simply a design limitation. It boils down to economics of design, manufacture and regular operation.

As the aeroplane descends, the cabin altitude descends automatically to match landing elevation for landing. What would happen if the aeroplane descended very rapidly and the aeroplane altitude passed the cabin altitude and started getting lower (ie, the pressure inside the aeroplane started getting lower than outside? It's called negative differential, and aeroplanes aren't built to withstand that (submarines are). There are special vents which will open to relieve this, but it causes rapidly increasing pressure inside which will have the passengers complaining on pprune!

Interestingly, think about this. Imagine an aeroplane flying to somewhere where the landing altitude is higher than the cruise cabin altitude- for example, Mexico City, Addis Ababa, Bogota. The cabin altitude may be 7,000', but the landing will be at 8,500'! It does happen. The cabin altitude has to be raised to the landing elevation for landing.

[neil_1821]

ok i understand it now thanks, it's actually a figure to learn not a formula or any theory.:)

[antic81]

Hi there

Not sure of the exact figures, but I know that Concorde had a cabin Alt of around 5000 ft?
And that was in the cruise at over 50 000 ft.
I am open to correction, but i am sure it was close to that.
Also the smaller jets ie Gulfstreams and Lear's and the like also have a relatively low cabin alt.

Also as an airframe ages so its cab Alt gets increased over the years, to cope with the wear of the constant pressure cycles it has to endure, this is to prolong its life, I believe and again stand ready for any corrections!

Cheers

Ant