Definition
A backup air inlet that supplies static (ambient, non-moving) air to the pitot-static instruments — the altimeter, vertical speed indicator, and airspeed indicator — when the primary static port becomes blocked, typically by ice or other obstruction. The pilot selects it using a cockpit valve, allowing the instruments to continue functioning by drawing static air from an alternate location, often inside the cabin.
Plain English
A backup air opening the pilot can switch to so the altitude, airspeed, and climb instruments keep working if the normal outside air port gets blocked.
Context Anchor
You encounter this during system operation, instrument checks, and icing discussions, especially when the normal static port may be blocked by ice or debris.
Derivation
‘Alternate’ comes from Latin alternatus, meaning ‘every other’ or ‘a substitute.’ ‘Static’ comes from Greek statikos, meaning ‘standing still’ — referring here to ambient air pressure that isn’t moving relative to the aircraft. So an alternate static source is simply a substitute supply of still-air pressure when the main one is unavailable.
Why Pilots Care
Selecting the alternate source prevents total loss of altitude, airspeed, and vertical speed indications but introduces small errors because cabin pressure is not identical to outside pressure; pilots must correct for these during flight.
Intuition Check
Do not read “static” as meaning electrical static or radio noise here. It means the surrounding air pressure the airplane’s instruments need for their readings.
Example Sentence 1
After noticing the altimeter freeze during climb in icing conditions, the pilot opened the alternate static system source to restore accurate readings.
Example Sentence 2
The instructor demonstrated how selecting the alternate static system source affects indicated airspeed and altitude in the training aircraft.