"Teacherjh" wrote in message
...

Because you can never know too much about your plane's systems, you engage
the alternate air one day while the static port is working fine and note
the
offset. Now you are ready with a known correction, when the time comes
that
the primary air is hosed. Is this a valid test?


Only if engaging the alternate cuts off the primary....
(snip LGM scenario)

So, the alternate air had better cut off the primary air!

If it does (as it should) then the test is valid and useful.

Does it, in your spam can? In mine? I probably have to go home and study the
systems, but I admit I can't remember this detail in the POH. Or is there
maybe some regulation behind your use of "should"? Inquiring minds want to
know.

-- David Brooks