Which pressure is sensed by the static port for instruments such as the ASI, altimeter, and VSI?

• A. Stagnation pressure
• B. Ambient atmospheric pressure for ASI, altimeter, VSI
• C. Dynamic pressure
• D. Total pressure

Answer: (B)

Static pressure is the ambient atmospheric pressure acting on the aircraft, measured by the static port. This pressure is what the altimeter uses to indicate altitude and the vertical-speed indicator uses to show climb or descent rate, because both rely on environmental pressure rather than the motion of the aircraft. The airspeed indicator, meanwhile, uses both the pitot (stagnation) pressure and this static pressure to derive dynamic pressure, which is what translates into indicated airspeed. The static port does not measure stagnation pressure (that comes from the pitot tube) nor dynamic or total pressure directly. So the pressure sensed by the static port is ambient atmospheric pressure.