LEM,ALEXBAN & Cirrus01,
Gents,
There are two different pressurisation config`s available for 737Classic depending upon build dates.
The latest uses dual digital cabin pressure controllers(evident by the cabin pressuization panel mode selector having "Auto"."ALT" and "MAN" selection available").The dual digital system is almost identical to the 737NG.
The NG System`s outflow valve`s two main electric actuators on the valve has a fail-safe aneroid switch which causes the outflow valve to fully close if the cabin pressure reaches 14,500ft.
This function overrides normal automatic control only.
It does not override manual mode of the valve.
Reference Boeing B737NG AMM part1 Chapter 21-30-00p40.
I know that this system is fitted to the AIR NEWZEALAND B737-300`s as they were some of the last of the Classic`s built.
This is yet another good reminder of how there can be drastic differences between aircraft models and series depending upon customer option`s and build dates.
Regards DDG