B737-200ADV Pressurization probs

[capster]

Hi folks.

One aircraft in our fleet has a problem. at TOC , when closing the thrust levers for descent the cabin starts climbing, and only starts to descend if some thrust is left on. This occurs throughout the descent and the cabin VSI needs to be monitored and the thrust modulated to achieve a cabin ROD.

Also, on the take off roll as thrust is applied, the cabin goes to about 3000 fpm ROD.

Any one have an idea of what causes this??

Cheers thanks again

[IFixPlanes]

Just a guess:
The leakrate of the fusulgage could be too high.
You could pressurize the cabin to approx. 4psi ∆p and check all doorseals and fusulage-drainvalves for abnormal leaks.

[capster]

thanks,

If thats the case , why would the cabin descend on applying takeoff thrust?

cheers

[IFixPlanes]

If thats the case , why would the cabin descend on applying takeoff thrust?
You wrote "3000 fpm ROD". In a descending cabin the pressure is rising.
With take off thrust you get more pressure for the packs.
That may equalize a high leakrate.
I don´t have license on the 737-Jurassic, but i think they have also a pre-T/O pressurize.
With engines in idle the controller may not get his wanted 200ft (?) below airportlevel.
At T/O he gets enough air and try to get the 200ft as soon as possible (3000 fpm ROD)

[slip and turn]

I have seen two streams of condensing air spewing from the ceiling abeam the front main exit in one RYR 737-200 before they got rid. I think that was just after I noticed throttles retarded for descent. Is that symptomatic of anything similar? Certainly cabin pressurisation in the 200s was much more of a rough ride for ears than in most NGs.

[capster]

thanks for the help gents, at least i have a direction to point the engineers in now :ok:

[LME (GOD)]

could also be a sticky hi stage controller..used to give lots of problems, but easy to change

[Terraplaneblues]

Long time ago, but seem to remember a flex hose that used to fail under the nacelle slipper fairing, it connects the hi stage regulator to the pack valve and any leak usually means no muscle air for the pack valve to open at idle. However when you open the throttles the pack can open (T/O symptom) and at TOD when throttles close, so does the pack valve.
When you look at the hose (12" long) it looks fine (braided) but a simple leak test finds it has turned into a colander.
But I could be wrong!