In IMC if the static system becomes clogged, which source should you activate to allow static pressure into the system?

• A. Pitot tube
• B. Alternate static source
• C. VSI
• D. GPS

Answer: (B)

When the normal static ports are clogged, you rely on the alternate static source to supply the required ambient pressure to the instruments. This backup path draws pressure from another location (often the cockpit or a separate vent) and feeds it into the static system so the altimeter, airspeed indicator, and vertical speed indicator can continue to function. You’ll typically see some instrument errors with this source, such as the altimeter reading a bit lower than actual and the airspeed indicator acting differently than usual, until normal static pressure is restored. The pitot tube provides dynamic pressure, not static, so it can’t substitute for the static system. The VSI and GPS don’t serve as the static pressure source in this situation.