What happens to the airspeed indicator if the pitot tube becomes completely blocked and the drain hole is open?
Answer
If the pitot tube is blocked but the drain is open, ram air pressure is lost and the airspeed indicator reads zero. If the drain hole is also blocked (e.g., by ice), the ASI may behave like an altimeter — rising in climb, falling in descent.
PHAK Ch. 8 — Flight Instruments
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