Originally Posted by
Oakape
During one descent in a B737NG, I was surprised to find the cabin climbing at around 500fpm when I checked the system at around 30,000’. After watching for a short period of time to see if it was a temporary condition, it was obvious we had a problem as the ROC was constant. I directed the F/O to disconnect the A/T & push the power up to around 60% N1. The cabin then went from a 500fpm climb to a 600-700fpm descent. The system stayed in the normal mode, so hadn’t detected a fault. We used speed brake against the power & with a lap of the pattern on the way in (ATC requirement, not ours) we had plenty of track miles to get the cabin down before landing.
The engineers checked it out & found a sticky outflow valve was the issue. Too much foreign material had built up in the mechanism. They couldn’t tell me why the system hadn’t detected the fault & swithed to ALTN. They did concur that it wouldn’t have made any difference anyway, given the nature of the problem.
In addition, the 737NG is very prone to the vacuum toilets being flushed, which can give rates of up to 800 fpm momentarily.
In addition, the 737NG cannot cope well with selecting a higher FL (a new selection above FL370 while previous selection was at or below), this can cause up to -1000 fpm until stabilised.