What is the alternate static source valve?

• A. A valve that regulates fuel pressure to the engine.
• B. A valve used to vent cabin air in pressurized aircraft.
• C. A valve that controls landing gear hydraulic pressure.
• D. A valve in the instrument static air system that supplies reference air pressure to the altimeter, airspeed indicator, and vertical speed indicator if the normal static pickup should become clogged or iced over.

Answer: (D)

This question tests understanding of the alternate static source valve, which is part of the instrument static system. When the normal static pickup is blocked or iced over, this valve opens to supply reference air pressure to the altimeter, airspeed indicator, and vertical speed indicator from an alternate source. That ensures the instruments continue to operate and provide usable readings despite a blockage in the primary static ports. The other options describe unrelated systems—fuel pressure, cabin-venting for pressurization, and hydraulic pressure for landing gear—so they don’t fit the role of providing alternate reference pressure to the pitot-static instruments.