@midairmelody: What are we looking for? When you open the alternate source, you’re venting your instruments to the cabin air instead of the outside air. Because of the airflow over the fuselage, cabin pressure is slightly lower than atmospheric pressure. The Momentary Jump: • Altimeter: Increase in altitude • VSI: Should show a momentary climb. • Airspeed: Increase from 0

Melody Marti | CFI

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Monday 11 May 2026 14:17:34 GMT