NG cabin ROC fluctuating

[Shumway]

There's been quite a few reports during cruise cabin ROC fluctuates, about 500ft/min. Duct pressure is stable.
Ground pressurisation check found satisfactory within leak rate limits. CPC bite shows low in flow / hi leakage.

Any idea what could cause this?

[Dunny]

vacuum toilet use can cause a brief change in rate of climb

[Shumway]

Definitely not toilet usage, it happens throughout, even without toilet being used...

[Dunny]

I would have thought +/- 500ft cabin alt ROC would have been able to be felt in the ear.

Are the pilots reporting an actaul cabin alt change or just the needle on the gauge fluctuating ?

[Shumway]

This defect has occurred on several aircraft, definitely not indication, most pilots don't really complain if it's 500fpm , but there was one which went up to almost 1000fpm & everyone (that means pax too) complained of ear discomfort.
Can't be CPC either cause changed to alternate as still the same.

[motortek]

just check temperature control valves.

[Dunny]

How about the pack flow control shut off valve or the Trim air PRSOV, but of these have a pressure regulating function.

[yayoo]

suggest to check the cargo door seal per FIM 21-31 Task 823 step B (6).

[Stenovic]

As per FIM last manufactured Airplanes got this problem due to TCV. They still modulating and causing ROC fluctulation.