static drains..
 

I know they are part of the checklist on the PA28 and similar but am wondering what exactly they do or what purpose they serve?

Liam

It's to drain out water which may have condensed in the static pipework over time.

Such water can play havoc with the altimeter etc.

It can also freeze, which is even better ;)

On several occasions I've departed the airfield only to find that I had major instrument errors. These occurred in an airplane which used large static ports (massive, by today's standards), which had accumulated moisture in strong winds and rain. There were no static drains.

Static drains do two things; they drain the static system of contaminates, and in many cases, are used as the alternate static source.

In some systems operating under IFR, opening a static drain means the pitot-static system needs to be recertified, retested, or reinspected. Be sure you know what applies to your system.