Originally Posted by rvdave
I was able to duplicate the symptom. It seems the ifd540 requires the correct altitude at the fix even though intercept is below gs it gives a check altitude error. When I flew at the correct altitude it coupled, when i was 100 to 200 low it would flag gs. The navigator remained in lpv mode so I could still hand fly but not with ap. Is this an ap error or navigator error or correct operation?
Something sounds off here - I've definitely intercepted the GS on an LPV from my IFD540 when not at the published altitude; it's common, in fact, for ATC to assign a slightly different altitude than what's on the chart. Some descend to published altitude, some "ride the glide".
Maybe check that you've got the approach armed and that it's actually doing an LPV approach and not an LNAV approach is about all I can think of off the top of my head.