737 pressurisation
 

Say for instance you lose pressurisation and you go to alternate, if you then lost alternate and go to manual would this indicate a problem common to the outflow valve or some other area thats common to all systems?

In a way, yes, if I understand the question correctly!

The only similar incident I have experienced is when some pneumatic duct lagging above the outflow valve opening came loose, hung out of the outflow valve like an elephant's entrails and froze rigid in cloud at altitude, thereby jamming the O V. Complete inability to descend the cabin on descent, without turning off the packs, until below freezing layer when cabin de-pressurized with the usual excitement/mist/bang etc. 3 feet of grey/silver lagging found dangling out of O V on the ground at EDI on arrival!

Is that what you mean?

A common problem with the OV is freezing during the cruise when the heater gasket of the valve has failed. Going to manual AC would normally free it off.

Thanks. I might have a bit of a better peak into the outflow vv next walkround!