Instruments, Attitude Instrument Flying & Aerodynamics Practice Test

The instrument that provides reference air pressure to the altimeter, airspeed indicator, and vertical speed indicator when the normal static pickup is compromised is the alternate static source valve. This device allows the pilot to obtain static pressure from a different location in the aircraft, which is crucial for ensuring that these vital flight instruments operate correctly when the primary static source is blocked or malfunctions.

Using an alternate static source helps maintain accurate readings, which are essential for safety during flight. The alternate static source typically draws air from inside the cabin rather than outside, which may lead to slightly different readings due to the differences in pressure and temperature, but it ensures that the pilot still has the necessary information for altitude and airspeed management.

The other options do not serve this function as effectively. For instance, a pressure relief valve is primarily designed to release excess pressure, not to provide static pressure for instruments. The static discharge valve is responsible for static electricity, and the instrument static valve doesn't directly refer to an existing device used for this specific purpose of providing an alternate reference.