When using the alternate static source, which indications will be observed?

• A. Higher on altimeter
• B. Momentary climb on VSI
• C. Higher on ASI
• D. All three indications

Answer: (D)

Using the alternate static source means the three indicators are no longer reading from the true outside static pressure, but from cabin air pressure. Since cabin pressure is typically higher than the outside ambient pressure at altitude, this change affects all three instruments at once.

The altimeter, which uses static pressure to determine altitude, will tend to read higher than your true altitude because it’s sensing a higher pressure than ambient. The vertical speed indicator reacts to the sudden change in static pressure when you switch sources and will show a momentary climb as the system rebalances. The airspeed indicator, which relies on the difference between pitot (total) pressure and static pressure, will also respond to the new static pressure and will indicate a higher-than-actual airspeed during that transient.

Once the system settles, these readings remain off from standard outside-air values until you return to the normal static source.