Catching the Cabin on the 737
Per Ardua ad Astraeus
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Telstar - most of the discussion above relates to CPCS (as fitted to the 727-200/200/400/500) You, in a NG have DCPCS, and I believe otherwise the same system as a -400.
Cause is: A rate of descent such that the cabin cannot reach landing altitude by the time the aircraft does. It is a simple equation involving cabin altitude/normal programmed cabin descent rate and time from cruise to landing. If time to landing is LESS than required by the cabin, you 'catch' it, resulting in the neg relief valve opening and the cabin descending at aircraft rate which can be unpleasant for all. EG Prog rate 500fpm. Cabin 'cruise relay' trips at 1500' below cruise alt. Cabin starts at 7500ft, but descent takes 15 minutes = you catch it! I guess your descents are not fast enough if you have not seen it!
With DCPCS the only answer is to increase the cabin diff (=reduce cabin alt) before descent, which is not a good idea. AFAIK this can only be done by decreasing the 'Flt Alt' setting which then schedules a lower cabin altitude (let's leave 'Manual' out of this
).
Best solutionsi are:-
Avoid high rate descents
If unavoidable, reduce R of D when you see the cabin altitude at half your actual altitude (assuming progranmmed 500fpm cabin rate.)
Cause is: A rate of descent such that the cabin cannot reach landing altitude by the time the aircraft does. It is a simple equation involving cabin altitude/normal programmed cabin descent rate and time from cruise to landing. If time to landing is LESS than required by the cabin, you 'catch' it, resulting in the neg relief valve opening and the cabin descending at aircraft rate which can be unpleasant for all. EG Prog rate 500fpm. Cabin 'cruise relay' trips at 1500' below cruise alt. Cabin starts at 7500ft, but descent takes 15 minutes = you catch it! I guess your descents are not fast enough if you have not seen it!
With DCPCS the only answer is to increase the cabin diff (=reduce cabin alt) before descent, which is not a good idea. AFAIK this can only be done by decreasing the 'Flt Alt' setting which then schedules a lower cabin altitude (let's leave 'Manual' out of this
).Best solutionsi are:-
Avoid high rate descents
If unavoidable, reduce R of D when you see the cabin altitude at half your actual altitude (assuming progranmmed 500fpm cabin rate.)
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Thank you BOAC. I have to be honest although the SOPs call for frequent checks of the pressurization, I have never thought to cross check the cabin alt with the actual altitude. Its never been mentioned anywhere so I presume it hasnt been a problem on the 800.
As said before, on the NG you have to work hard to catch up the cabin.
In 3 years time I had it only once during climb and perhaps twice in the descent, just 2 miles before touchdown.
down3gr33ns: thanks for the correction!
In 3 years time I had it only once during climb and perhaps twice in the descent, just 2 miles before touchdown.
down3gr33ns: thanks for the correction!
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b732 last of the romantics
on the 732 always happends at cabin altitude of 330 and more with a lot of headwind , we pass it to stby mode or we reduce speed to 250 after descending through 15000 , that will be ok , those are the two ways you can hadle it , best regards , safe flights
Humus Motor
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As a rough guide when wondering if you'll catch the cabin up, use 10 x diff in nm to go to get the cabin down. This, I stress is an approximation only, but can be a good guide.
The 717, if cruising above FL340 will almost always catch the cabin on a descent to Sea Level when the 250KIAS/10,000ft limit has been removed. I descend at about 20nm short of the FMS descent point using -1000FPM VS until reaching the descent profile. This allows the cabin to start coming down before getting into the descent-proper if doing a high-speed descent.
I use a similar check system to Earthmover, except for 400-500ish ft/min cabin ROD, I find that 11 times the Cabin Altitude should equal the track miles to touch. eg CA at 6000ft, TM should be 66nm or TMx9: 50nm=4500ft.
I use a similar check system to Earthmover, except for 400-500ish ft/min cabin ROD, I find that 11 times the Cabin Altitude should equal the track miles to touch. eg CA at 6000ft, TM should be 66nm or TMx9: 50nm=4500ft.
Last edited by Capn Bloggs; 21st Aug 2006 at 14:56.
ECON cruise, LR cruise...
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Aaarhhhmm....
...why not simply look at cabin ROD and divide it into the cabin altitude? Eg. CA 6000 ft / CROD 500 fpm = 12 min. - if your EET is less than 12 min., you have messed up.
Anyway - with todays typical CI I cannot see it as being a major problem - or are there still some naughty boys out there who like to play fast-jet-jock at 330 KIAS?
Anyway - with todays typical CI I cannot see it as being a major problem - or are there still some naughty boys out there who like to play fast-jet-jock at 330 KIAS?
or are there still some naughty boys out there who like to play fast-jet-jock at 330 KIAS?
). 320KIAS til 15tm. No sweat, with time for a cup of tea after the checklist, which also helps to clear the ears that are now popping profusely because you just caught the cabin! 
