The static air pressure, in contrast, provides the reference for the airspeed indicator, vertical speed indicator, and altimeter. The static port is usually located on the fuselage of the aircraft. If it becomes blocked, an alternate static source may be available, often located inside the cabin. In an unpressurized aircraft, the cabin pressure is usually close to the ambient static pressure, although a small deviation may occur.
I like to practise handling these systems using the failure simulation system, which allows us to simulate a system failure and practise managing it by switching to the alternate system.
