Definition
A backup source of static air pressure used by the pitot-static instruments (altimeter, airspeed indicator, and vertical speed indicator) when the primary static port becomes blocked. In unpressurized aircraft, the alternate static source typically draws air from inside the cabin; in pressurized aircraft, it draws from a second external port. Selecting it usually causes small, predictable indication errors that the pilot must account for.
Plain English
A backup air source the pilot can switch to when the regular outside air port that feeds the flight instruments gets blocked. It keeps the altimeter, airspeed, and vertical speed indicator working, though the readings may be slightly off.
Context Anchor
Seen in instrument failure discussions, especially when a pilot must use a backup source or backup equipment after an analog instrument system stops working correctly.
Derivation
Alternate' comes from the Latin alternare, meaning 'to do by turns.' In aviation it carries the sense of 'a second option you can switch to'—not 'different' in a general sense, but specifically a substitute kept ready for use.
Why Pilots Care
Allows continued instrument reference in IMC when a primary system malfunctions, directly supporting safe aircraft control.
Analogy
It is like having a spare flashlight when the main one quits. The spare may not be the one you normally use, but it gives you a usable backup when you need it.
Intuition Check
Alternate does not mean casual, optional, or equally preferred here. It means backup: used when the normal system is not doing its job.
Example Sentence 1
When the airspeed indicator froze during the climb through icing, the pilot selected the alternate system and noted the small altitude correction listed in the POH.
Example Sentence 2
The emergency checklist directs the pilot to confirm the alternate system is supplying power before continuing IFR flight.