In which scenarios is the alternate static source commonly used?

• A. When the primary static port is blocked.
• B. When the pitot tube is blocked.
• C. When the primary static port is blocked or during cabin pressurization where cabin differential affects static pressure.
• D. Only during ice conditions.

Answer: (C)

Alternate static source provides a backup static pressure reference for the pitot-static instruments when the normal external static port cannot supply accurate pressure. It’s used when the primary static port is blocked, so the instruments still have a pressure signal to operate with. It’s also used in cabin‑pressurized aircraft because the cabin pressure can differ from outside ambient pressure; using the cabin as the static source helps keep readings functional even though the pressure isn’t exactly the same as outside. In either case, wiring in the alternate static source means the instruments won’t be perfectly accurate—altimeter and vertical speed indicator readings may drift and the airspeed indicator may show some discrepancy—until the underlying condition is resolved. This isn’t limited to ice conditions and it’s not for pitot-tube blockage, which affects dynamic pressure rather than static pressure.