Originally Posted by ImbracableCrunk
So, I think the computer looks at your FLT ALT and then says, "Ok this is the target PSID I will give them when they are there." (eg. 8.35PSID) Once within the 0.25PSID of FLT ALT, it goes into a "alt hold" type of mode where it now varies the PSID to keep a constant cabin alt.
No. The CPC calculates a target cabin pressure, or cabin altitude if you will. If will then decrease the cabin pressure until the target pressure is reached and then maintains it whenever that is. The 0.25 PSI has to do with cancelling the off schedule descent function.
The CPC does not control differential pressure, it is a mere by-product. The CPC does however monitor differential pressure as if it is too high it may indicate a controller failure.
The FCOM is poorly worded as it gives the impression that differential pressure is an active control parameter, when it actually is a theoretical parameter used in determining the target cabin pressure.