View Full Version : Catching the Cabin on the 737

2nd Jul 2006, 06:31
In my company we operate, amongst others, the 737-300. On occasion we catch the cabin on descent. This usually occurs when we have been at a high cruise FL ie 360/370, with no speed restriction below 10,000', no level segment other than to slow down and then a visual approach joining via a 3 DME final. It can be a bit of a hassle but usually predicted and negated by initiating an early descent using the Alt Intervene function on the MCP. However if we do catch the cabin it is often followed by grumbling from the captain along the lines that "this never happens on the 747/767/A320" the inference being that the cabin pressure controller (digital) on the 737 is crap. Now I may be just an F/O but it seems to me that on these other aircraft you usually don't find yourself with all the factors listed above combining at once. What are your expereinces with different aircraft and their systems?

2nd Jul 2006, 08:33
I found it was not uncommon on the 737 200/400. Rare on the 747 Classic, and never on the 747-400. I'm interested to find out about the 737 300/NG. It's such a little dart- geting up to altitude so quickly and zipping down again- I think its performance outpaces the cabin controller rather than the cabin controller underperforming.

2nd Jul 2006, 08:41
The 'auto' system descends, IIRC, at about 500fpm. Therefore if you are at FL400, say, (cabin at about 8000ft), you need about 16 mins to get the cabin 'down'. Make that descent in 15 minutes and you 'catch the cabin'. Perhaps the other a/c you mention have either a lower cabin altitude or higher cabin descent rate? The programmed descent does not begin until around ?2000ft? (1500ft on the 'classic' 737) below cruise altitude so it can be a good idea in the circumstances you describe to make that first 2000ft 'drop' early.

Putting the system in 'standby' (where fitted) will also allow an early cabin descent but does produce a 'bump' in pressure when selected.

2nd Jul 2006, 10:41
Happens on the A320 but not the A321.

Capt Chambo
2nd Jul 2006, 10:49
IIRC the B737-300's with the older analogue pressurisation controllers came with a customer option for the cabin rate controller. Either 500fpm or 350fpm in descent.
If you don't know the version of your aeroplane then assume it has the 350fpm.
We used to check the cabin rate of descent on the descent forecast page, if it was greater than 350fpm then we used to begin descent 15-20nm early to trigger the controller (about 1500' below CRZ ALT IIRC), once the cabin descent had begun then we could go back to VNAV PTH. The cabin descent rates of greater than 350fpm usually occurred as you described also where you are battling on into a strong headwind. One cure was to show TAI on for most of the descent!
A cross check is to check the cabin altitude to be lost vs time to go as part of your descent checks. So where your descent check says AIR COND & PRESS....SET, actually compare the cabin altitude to be lost with the PROG PAGE's ETA. If you have a 350fpm controller then it is about 3mins per 1000'cabin alt that needs to be lost. eg checks carried out at FL200, the cabin will be roughly 6000' at 3mins per 1000' to be lost means you want about 20mins to landing.
Sorry getting a bit carried away but to answer your question, the later 737's and the NG's with Digital controllers seem to be better. And the Airbus 320's are all completely automatic and never get caught out.
Standing by for Airbus V Boeing protagonists!

2nd Jul 2006, 11:03
Caught the cabin in 320, 321 and 330, mind you we were doing 9,000 f.p.m. in the 330 before it happened :ok:

2nd Jul 2006, 11:06
Did you still have wings when you landed?

2nd Jul 2006, 11:06
Capt Chambo, ours are very late model -300's with DCPCS. The ROD for the cabin is 500'/min. Also our new Boeing checklists do not have AIR COND/PRESSURISATION as part of the descent checks anymore.
Also, I admit I have caught the cabin going up to cruise FL once or twice now that I think about it.

Thanks for the replies so far....

2nd Jul 2006, 21:05
Yep, we got held high by Miami when the Grand Turk radar head went off line. Got descent in Santa Domingo's airspace with 80 track to POP, entered downwind at 210kt/1500' :E

2nd Jul 2006, 21:17
what will happen if you catch the cabin?
Never seen this in more than 4000 hrs on the 737,but I have to admit we often use Vnav,and speed reduction bellow FL100...

2nd Jul 2006, 22:29
It means that cabin diff exceeds the 'limit' due to insufficient time to lower the cabin altitude from cruise and the cabin rate goes towards aircraft rate.

3rd Jul 2006, 01:18
This is a common problem with the 300/400 series aircraft.

If you look through the engineering manuals you will find that the Cabin ROC indicator is a "ground" calibrated instrument, so you are not seeing an accurate figure in flight.

Why the ground calibration? I have no idea.

Bomber Harris
3rd Jul 2006, 07:01
Alexban, good question.
the only protection you have from this situation is the 'negative pressure relief valve'. Thats the flapper door that opens inwards. This is simply a spring loaded door; once the pressure outside the aircraft is higher than inside then the door opens against (relatively) light spring pressure. Now the cabin will in theory descend at the same rate as the aircraft, however this is limited by the size of the opening of the relief valve, so there will be a "slight" negative pressure diff on the aircraft if the crew continues to descend at a high rate.

Although I haven't seen it written anywhere, if you do catch the cabin it is prudent to reduce rate of descent to less than 1000fpm for pax comfort and to avoid exposing the aircraft to negative pressure diff's (which the structure does not like very much)

2nd Aug 2006, 05:50
For the DCPCS on 737-300/400/500 and NG's, the Cabin Altitude Rate Limits are pin programmable. Many operators get the defaults: 600fpm in climb and 350fpm in descent, but 750fpm in climb and 500fpm or 750fpm in descent are available options. If your typical flight profiles result in occasionally flying through the cabin during descent, going to a higher descent rate limit may help.

Regarding the rate indicator, it is calibrated in "sea level feet per minute" because it is a very simple calibrated-leakage type gauge, pretty much unchanged for 40 years, that actually reacts to changes in pressure. A 200 ft. change in cabin altitude at sea level is a pressure change of about 7.3mBar, whilst the same 200 ft. change from an 8000 ft. cabin in cruise is only about 5.8mBar of pressure change. Looking at it another way, a 7.3mBar pressure change is 200 ft. at sea level, but corresponds to about 253 ft. change from an 8000 ft. cabin altitude. That is why the rate indicator shows a higher rate at higher cabin altitudes.

We can forego any Airbus vs Boeing issues here because the 737 DCPCS is made by the same company that provides cabin pressure control systems for all the Airbus models.

ray cosmic
2nd Aug 2006, 07:23
A trick I've seen being performed by the oldtimers was to dial in another cruise altitude (f.e. the level you'd fly on the return) in the cabin altitude window, in order to get the cabin descending right away after leaving cruising level.
Don't know if it can have a negative effect once, but it seemed to work.
In other cases you'd typically catch up the cabin on short final after performing an unrestricted, high speed, idle descent.

2nd Aug 2006, 13:33
sort of agree with ray cosmic - dialing in a higher cruise altitude into the FLT ALT window (not the CAB ALT window) about 2 mins before descent tricks the descent relay into thinking you've begun descent and starts pushing the cabin down. Otherwise use the CAPTURE prompt on the descent page around 10-15 nm before the FMC calculated TOD and that, too, will overcome cabin catching problems.

Charles Darwin
2nd Aug 2006, 14:42
What I used to do on the 737 3&400 was to initially set the rate manually (in sby) at around 800fpm. At high cabin altitude this rate can hardly be felt (unless the pax has severe cold). Then later on, in denser air, the rate can be set to normal.
Worked every time.:)

Paolo de Angelis
4th Aug 2006, 11:34
Agree with Charles.

Putting the system in 'standby' (where fitted) will also allow an early cabin descent but does produce a 'bump' in pressure when selected.

Before doing so, it's always a good idea to select the smaller rate, 50 fpm, in order to avoid that bump.
Then, reincrease to 800 or whatever... :ok:

4th Aug 2006, 12:28
Unfortunately, Paolo, the 'bump' has nothing to do with selected rate, but occurs from the 'kick' changing to 'Standby' applies to the outflow valve - at least that is my experience.. Try it with 'Zero' rate.

5th Aug 2006, 12:44
Feeling a little embarrased here chaps! :O

I fly the -800 and never even knew or thought of this problem. What are the indications of catching the cabin. My airline flys to some very quite airports so we often use high speed, high descent rate appraoches. This problem has never been brought to my attention?

From your discussions of the Air Cond/Press configuration I am presuming that the setup is a little more similar to the 200 of which I have a passing knowledge.

5th Aug 2006, 13:56
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:eek: ).

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.)

5th Aug 2006, 14:00
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.

ray cosmic
7th Aug 2006, 09:15
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!

17th Aug 2006, 17:24
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

19th Aug 2006, 21:42
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.

Capn Bloggs
21st Aug 2006, 15:46
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.

Empty Cruise
21st Aug 2006, 16:51
...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? :=

Capn Bloggs
22nd Aug 2006, 02:28
or are there still some naughty boys out there who like to play fast-jet-jock at 330 KIAS?

Yes there are, especially when ATC tell you to do so (which they do here regularly :ok: ). 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! :}