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Since pitot static systems are open the outside, how does the system remain clear of water or other contaminants?

fooot
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1 Answers1

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Though it's often omitted from diagrams, most pitot tubes have one or more drain holes connected to the pressure chamber, and are essentially "self-draining" (water doesn't make it into the lines):
Piper pitot blade - Drain on bottom Dynon pitot tube drain holes

In addition to drains on the pitot tube itself some aircraft have separate "drip chambers" and drains for the pitot (and occasionally static) system to remove condensation or rain that got past the drains (marked as 2 in the photo below):
PA-28 pitot/static drains

Regarding other contaminants, at altitude the air is generally pretty clean.
The biggest problem you'll have with contaminants in the pitot tube is usually bugs (either building a nest in the opening, or smacking into it in flight). Pitot covers can be used on the ground to prevent this (and they're also used when washing or painting aircraft to prevent water or paint from getting into the system). Often some tape serves the same purpose - but you need to be sure to remove the tape or cover before flying.
Someone is having a bad day...

voretaq7
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    +1. Nevertheless, it is possible for water to enter the system and cause problems. In these cases the system must be drained from a sump, sometimes by forcing air through the system in a maintenance shop. See stories here and here where this happened. – TypeIA May 20 '14 at 16:49
  • @dvnrrs Yup - there have been cases where the drains in the system are inadequate - in those cases unless there's a pilot-accessible drain the system needs to be opened and drained by a qualified shop, and leak-checked when reassembled. – voretaq7 May 20 '14 at 16:52
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    And blocked tubes will cause havoc for the pilot not expecting it – ratchet freak May 20 '14 at 18:33
  • @CGCampbell That was one of the top 5 results for "bug in pitot tube" -- It was so hilarious I just couldn't resist! – voretaq7 May 20 '14 at 19:34
  • Can you go into more detail about the drip chambers and drains within the system? – fooot May 21 '14 at 16:17
  • @fooot There's nothing really special about the drip chambers - basically they're a small length of tubing and a self-closing valve tee'd into the line at a low point (below the instruments) where water can collect without interfering with the air flowing through the line. Not all aircraft have these drains (in fact the PA-28 series shown in that photo shipped both with and without them). – voretaq7 May 21 '14 at 16:41
  • @voretaq7 I'm interested in how they get rid of the water without affecting the system pressure. – fooot May 21 '14 at 16:43
  • @fooot When the drain valves are closed they make an airtight seal - the drip chambers are effectively just more "space" in the line that winds up at the same pressure as the rest of the system. In the case of the Cherokee drains they're spring-loaded to close so you can't forget to close them after draining the line in preflight. (You wouldn't/shouldn't attempt to operate them in flight as that would mess with the system pressure, though I've never seen them placarded - probably because they're so inconveniently located.) – voretaq7 May 21 '14 at 17:33