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ATPL Systems pressurization question?!

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ATPL Systems pressurization question?!

Old 24th Mar 2010, 05:12
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ATPL Systems pressurization question?!

Could someone please confirm that this is right. If you are maintaing fl330 you've reached max diff pressure and cabin alttitude is 8000ft , if you climb to fl370 :

the max diff will remain the same and the cabin alttitude pressure will climb untill 10000ft or less then you can't climb anymore.

Thanks in advance.
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Old 24th Mar 2010, 05:20
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8000 + 2000 = 10 000

33 000 + 2000 = 35 000
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Old 24th Mar 2010, 06:05
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Now, now Spikey; the lad was just asking a simple question.

Yes the cabin will have to climb (if you are to keep your windows and other pressure hull fittings in place) as dictated to by the outflow/safety valves. How much it climbs though, will depend on what the max. press. diff. of the airframe is. With a high diff. you might climb the 4000' and find the cabin has only gone up by 1800'.
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Old 24th Mar 2010, 06:07
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Yeah, the maximum cabin pressure differential will be maintained as the cabin altitude rises.

However, I got the following question in the exam so be sure not to mix the 2 up...

A pressurised aircraft is in cruise at FL330 with the cabin altitude at 8000ft. A climb is then made to FL350. Regarding the operation of the pressurisation system in an ISOBARIC mode:
Answer: The differential pressure will increase to maintain a constant cabin altitude.

Good luck with the exam..it's a fun one
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Old 24th Mar 2010, 08:57
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From red max original question
If you are maintaing fl330 you've reached max diff pressure
From Pilotette
Answer: The differential pressure will increase to maintain a constant cabin altitude.
How, its already at the limit
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Old 24th Mar 2010, 09:53
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They are two different questions...merely giving a pointer about what to look out for in the exam. With regards to Max's question, as I said, the max presure differential will be maintained as the cabin altitude increases.
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Old 24th Mar 2010, 19:39
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Originally Posted by Spikey21 View Post
How, its already at the limit
The question doesn't say that it's at max diff - just that it is holding 8000'. If it's not at max diff, the differential will increase as the aircraft climbs. When max diff is reached, the cabin will climb at the same rate as the aircraft.

Redmax is asking a similar question twice, with slightly different parameters
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Old 24th Mar 2010, 23:24
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I agree with your result.

I'm assuming you are expected to find the max differential which you have interpolated as 6.7 therefore:

ambient pressure = fl370, max diff pressure = 6.7psi
implies cabin pressure must rise to 10,000
(based on graph in Bristol gs notes)
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Old 25th Mar 2010, 02:49
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It's an ASL exam - don't assume anything. Just RTFQ
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Old 25th Mar 2010, 03:18
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the cabin will climb at the same rate as the aircraft

.. doesn't follow as the altimeter deltas are linear while the pressure deltas are not.

Unless you want to do the sums to get the actual figures, it might be better to say things like -

(a) with a given pressurisation setting commanded, the system will follow the system schedule for cabin altitude with increasing/decreasing aircraft altitude so long as the diff is less than max.

(b) at max diff an increasing/decreasing aircraft altitude will result in an increasing/decreasing cabin altitude
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Old 30th Mar 2010, 10:10
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Thanks all, I got a similar question in the actual exam, they didn't ask for the specific psi but what would happen if you climb with max diff , answer cabin will climb max will stay the same , thanks again .
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