Catching the Cabin on the 737
 

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?

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.

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.

Happens on the A320 but not the A321.

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!

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:

Did you still have wings when you landed?

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....

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

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...

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.

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.

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)

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.

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.

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.

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.:)

Agree with Charles.

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:

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.

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.