Definition
A secondary source of static (ambient) air pressure that can be selected by the pilot to feed the pitot-static instruments — altimeter, airspeed indicator, and vertical speed indicator — when the primary static port is blocked or otherwise unusable. In most light aircraft it is selected by opening a small valve in the cockpit, which then draws static pressure from inside the cabin instead of from the external static port.
Plain English
A backup way for the airspeed, altimeter, and vertical speed indicator to get the outside air pressure they need, used when the normal opening on the outside of the airplane gets blocked (often by ice).
Context Anchor
Seen in pitot-static system discussions, instrument failure procedures, and cockpit checklists for blocked static ports.
Derivation
‘Alternate’ comes from Latin alternatus, meaning ‘every other’ or ‘substitute.’ ‘Static’ here refers to still or non-moving air pressure (as opposed to the ram pressure of moving air). Together: a substitute source of still-air pressure.
Why Pilots Care
Maintains usable altimeter, airspeed, and vertical speed data during static port blockage, preventing loss of critical flight information and supporting safe continued flight or landing.
Intuition Check
Alternate does not mean optional or less important here; it means backup. Static does not mean electrical static; it means still-air pressure used by the instruments.
Example Sentence 1
When the airspeed indicator and altimeter began showing unusual readings in icing conditions, the pilot opened the alternate static source to restore reliable indications.
Example Sentence 2
The pilot noted the airspeed correction factor listed in the POH before relying on the alternate static source during the approach.