> Quite a bit, apparently. There are automagic systems, but those can
> be manually overridden as required. Pilots have control over cabin
> altitude (and, by implication, pressure differential). In both
> classic and NG types, there is a cabin altitude guage and a pressure
> differential guage, controls to set flight and landing cabin
> altitudes (note the placarded max pressure differential of .125PSI).

> Direct manual control of the outflow valve is alo available in
> both types.
>
> See <http://www.b737.org.uk/pressurisation.htm> for pictures and
> descriptions.