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rvdave
12-22-2018, 05:48 PM
I was vfr practicing an rnav approach and the glide slope flagged. I was below the altitude published for faf due to clouds so I?m wondering what the altitude limit is for ap capture of glideslope, I know it?s 30 deg for lateral intercept. This is a Vizion 385 if it makes a difference.

aerovin
12-22-2018, 07:16 PM
Glideslope should not flag "off" if you are below the glidepath within 10 miles of the runway.

BobTurner
12-22-2018, 07:22 PM
Need more details. What did your gps box ?say?? A few months ago I watched an instrument student complain that the GS had flagged on an LPV approach. He hadn?t noticed that the GTN650 had degraded from LPV to LNAV. (As we landed it dropped down to ?no gps?.) Bad geometry or interference, I don?t know which.

rvdave
12-22-2018, 08:33 PM
It was an lpv approach but I don?t recall what was on screen at the time, last I verified it was gps to lpv. I?ll have to duplicate the approach and see if it was an anomaly or not. I was mostly wondering if like heading needing to be within 30 deg if altitude was a similar constraint or not?

rvdave
12-25-2018, 04:03 PM
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?

ChiefPilot
01-02-2019, 06:19 PM
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.