When using the alternate static source, what combination of readings should you expect?

When using the alternate static source, what combination of readings should you expect?

• A. Altimeter reads lower than normal, airspeed lower than normal.
• B. Altimeter reads higher than normal, airspeed greater than normal, and the VSI momentarily shows a climb.
• C. Altimeter reads higher than normal, airspeed lower than normal, VSI shows descent.
• D. Altimeter reads equal to normal, airspeed normal.

Answer: (B)

When you select the alternate static source, the instruments start using cabin/static air pressure rather than the outside ambient. That pressure is typically a bit lower than outside, so the readings shift accordingly. The altimeter interprets a lower static pressure as being at a higher altitude, so it shows higher than the actual altitude. The airspeed indicator relies on the difference between pitot (total pressure) and static pressure; with a lower static pressure, Pt − Ps becomes larger, so the indicated airspeed rises. The vertical speed indicator responds to changes in static pressure; when the alternate source is engaged, the static pressure often drops momentarily, causing the VSI to indicate a climb briefly as the system equilibrates. So you should expect altimeter higher than normal, airspeed higher than normal, and the VSI momentarily showing a climb.

When you select the alternate static source, the instruments start using cabin/static air pressure rather than the outside ambient. That pressure is typically a bit lower than outside, so the readings shift accordingly.

The altimeter interprets a lower static pressure as being at a higher altitude, so it shows higher than the actual altitude. The airspeed indicator relies on the difference between pitot (total pressure) and static pressure; with a lower static pressure, Pt − Ps becomes larger, so the indicated airspeed rises. The vertical speed indicator responds to changes in static pressure; when the alternate source is engaged, the static pressure often drops momentarily, causing the VSI to indicate a climb briefly as the system equilibrates.

So you should expect altimeter higher than normal, airspeed higher than normal, and the VSI momentarily showing a climb.