When using the alternate static source, the airspeed indicator will indicate higher or lower than normal?

• A. Higher than normal
• B. Lower than normal
• C. No change
• D. Fluctuates widely

Answer: (A)

When the alternate static source is used, the static pressure driving the airspeed indicator is not the true outside ambient pressure. This alternate source typically provides a static pressure that is lower than the ambient outside air. The airspeed indicator measures dynamic pressure, which is ram air (pitot) pressure minus static pressure. If the static pressure is lower, the difference becomes larger, so the instrument indicates a higher airspeed than you are actually flying. In practice you may also see the altimeter read lower than actual altitude and the vertical speed indicator show a momentary climb when the switch is made, but the key point for the airspeed indication is that the alternate static pressure causes the ASI to read higher than normal.