Biennial Xpdr/encoder check--must it include the altimeter?

robert.p.bowen · Post by **robert.p.bowen** » Mon Jun 25, 2007 3:27 pm

Although I have always had the altimeter and static system tested when doing the transponder and encoder recertification on my VFR-only airplane, my rereading of FAR 91.411 and .413 seemed not to require it. When I told my avionics tecnician to skip the altimeter recert. this time, because I didn't believe it necessary, he said it was--even for a VFR-only airplane--and pointed me to FAR 91.217 as the source.

So what is every one else doing? I want to be legal, but recertifying the altimeter and static system more than doubles the price.

lowNslow · Post by **lowNslow** » Mon Jun 25, 2007 3:34 pm

There was recently a discussion of this issue on a previous thread.
http://www.cessna170.org/phpBB2/viewtopic.php?t=4497

Basically, it is as clear as mud. The guy that does mine does NOT require it as long as the altimeter agrees with the transponder.

jrenwick · Post by **jrenwick** » Tue Jun 26, 2007 1:29 am

Here's the way I read it, and I'm not an expert:

91.217 requires the correspondence between the altimeter and the encoder to be tested "as installed," up to the aircraft's maximum operating altitude. But it doesn't say this has to be done on any time interval -- once, after any installation, seems to be enough.

91.411 requires an altimeter check, but it applies just to IFR operations.

91.413 requires an altimeter check per part 43, Appendix E, "following any installation or maintenance on an ATC transponder where data correspondence error could be introduced." This applies if you operate your Mode C transponder at all, regardless of whether it's IFR or VFR.

So for strictly VFR purposes, it sounds like you need an altimeter check (where they suck it up to a high altitude and verify that the altimeter and encoder agree within limits) only after you've installed some part of the system or messed with it. It doesn't have to be done every 24 months. This may be why there are some shops willing to do "VFR-only" transponder checks.

Then there are the shops that refuse to do it, maybe just because they don't want the liability of their name in the logs if they haven't gone over the whole system. Who can blame an inspector for playing it safe?