Definition
A backup source of static (outside) air pressure for the pitot-static instruments, used when the primary static port becomes blocked, typically by ice, water, or debris. Activating it routes static pressure to the altimeter, airspeed indicator, and vertical speed indicator from an alternate location, often inside the cabin in unpressurized aircraft.
Plain English
A backup air-pressure source for the flight instruments that need outside air to work. If the normal opening on the outside of the airplane gets blocked, the pilot can switch to this backup so the altimeter, airspeed indicator, and vertical speed indicator keep showing useful readings.
Context Anchor
You encounter this during pitot-static system checks, icing discussions, and abnormal instrument indications caused by a blocked static port.
Derivation
Alternate' comes from the Latin alternare, meaning 'to do by turns' — here it means 'the other one you switch to.' 'Static' comes from the Greek statikos, 'causing to stand,' referring to still (non-moving) air pressure, as opposed to the moving (ram) air measured by the pitot tube.
Why Pilots Care
Restores accurate altitude, airspeed, and vertical speed indications when primary static ports are obstructed, preventing loss of critical instrument data in instrument conditions.
Intuition Check
Alternate does not mean a second set of instruments; it means a backup pressure source for the same instruments. Static does not mean electrical here; it means still-air pressure around the airplane.
Example Sentence 1
When the airspeed indicator froze during the climb through icing, the pilot selected the alternate static system and the readings stabilized.
Example Sentence 2
The checklist directs the crew to activate the alternate static system and apply a correction factor to the altimeter during blocked-port emergencies.