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3 Answers

Pitot blockage but drain hole NOT blocked

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Aircraft Systems

If the pitot tube inlet becomes blocked but the drain holes remain open (if this is possible) won't this essentially cause the AS indicator to read close to zero if the pressure in the chamber is equalized through the drains? Are there one way valves involved? It seems that the answer I see everywhere is that when the pitot inlet is blocked the system will act like an altimeter. Is that always the case? Thanks and need to know before my instrument practical this weekend.

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3 Answers

  1. Brian on Oct 26, 2011

    You have it right. The sources you’ve read are referring to a complete blockage. Three possibilities:
    Just pitot blocked — ASI reads zero because air inside the tube drains out.
    Just drain hole blocked — ASI reads erroneously high. By how much I am not certain.
    Both holes blocked — ASI reads like an altimeter because air is now traped in the tube and acts just like air inside a water bottle. Ever close up an empty bottle on a commercial flight home only to pick it up on the way out and it’s smashed like a big fat guy sat on it? The trapped air compresses and puts less pressure on the system, ASI reads lower. And vice versa with altitude increase. 

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  2. Wes Beard on Oct 26, 2011

    A good website that graphically shows what is happening is here:  It should help cement what you already know.

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  3. songül on Jan 05, 2014

    Hi, I have some question about pitot static tube. I know ıt has drain hole for water but ı dont know ıts structure. In other words how does drain hole work. ıt drain water how. Static chamber and total chamber have got same drain hole? Plese you help me 🙂

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