Just reread the bit in the PA-28-181 manual which talks about altimeter error being less than 50ft. It doesn't mean what it was claimed to mean, and I originally missed that too and I have slapped my wrist.
What this actually refers to is when the alternate static source is being used. As we all should know, when alternate is on, the altimeter uses cabin air for it's static pressure, not the air outside.
What the manual actually says is, that when alternate static is on, the altimeter error should be less than 50ft. i.e less than 50ft difference to the altitude indicated when using outside air.
So, totally irrelevant to the subject.
I think.....
