Which VSI indication occurs when using the alternate static source?

• A. Momentary climb
• B. Steady climb
• C. Steady descent
• D. No change

Answer: (A)

When you switch to the alternate static source, you change the static pressure reference used by the VSI. The VSI senses how quickly static pressure changes to show vertical speed. The alternate source taps cockpit air, which is not exactly the same pressure as the outside air, so there’s an abrupt change in the pressure the VSI uses. If the alternate source pressure is lower than outside, the static pressure at the instrument momentarily drops, which the VSI interprets as a climb. Once the pressure stabilizes to the new source, the indication settles back.

So, the immediate transient is a brief climb indication, which is why a momentary climb is the best answer.