Since pitot static systems are open the outside, how does the system remain clear of water or other contaminants?
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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):
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):
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.
answered May 20, 2014 at 16:48
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4$\begingroup$ +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. $\endgroup$– TypeIAMay 20, 2014 at 16:49
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$\begingroup$ @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. $\endgroup$– voretaq7May 20, 2014 at 16:52
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1$\begingroup$ And blocked tubes will cause havoc for the pilot not expecting it $\endgroup$ May 20, 2014 at 18:33
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$\begingroup$ @CGCampbell That was one of the top 5 results for "bug in pitot tube" -- It was so hilarious I just couldn't resist! $\endgroup$– voretaq7May 20, 2014 at 19:34
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$\begingroup$ Can you go into more detail about the drip chambers and drains within the system? $\endgroup$– foootMay 21, 2014 at 16:17