FAR 23.1325 Static pressure system.
(b)(3) If a static pressure system is provided for any instrument, device,
or system required by the operating rules of this chapter, each static
pressure port must be designed or located in such a manner that the
correlation between air pressure in the static pressure system and true
ambient atmospheric static pressure is not altered when the airplane
encounters icing conditions. An antiicing means or an alternate source of
static pressure may be used in showing compliance with this requirement. If
the reading of the altimeter, when on the alternate static pressure system
differs from the reading of the altimeter when on the primary static system by
more than 50 feet, a correction card must be provided for the alternate static
system.
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The way I read this is that if you don't have a heated static port, and
there's any chance that ice will block your static port, you need an alternate
static, VFR or IFR. I know this seems to apply to "normal, utility,
etc., and not experimentals, but your op limits will say you need a static
check per part 43 appendix E, which then refers back to part 23.
We had an a customer's plane go all goofy when water in the static line
would freeze at altitude. Utterly VFR but an alternate static source
still would have come in handy.
McMaster-Carr makes an nice little valve (P/N 6791T21) that looks like a
toggle switch that fits the bill nicely. It's about $15.
Dave Saylor
831-750-0284
CL