Narrative:

I was on an IFR flight plan on the arrival and was instructed to 'turn to 110 to maintain 3;000 until established on the localizer.' I was at 3;000 and saw the localizer come in as I had activated the ILS approach on my GNS 650. However; my glideslope didn't appear on my pfd and as I was troubleshooting; I descended to 2400. I was in VMC and had the field in sight. ATC came on and told me to climb immediately to 3;000 which I did and I requested to cancel my clearance as I was in VMC. ATC asked why I had descended and I told him I was cleared to descend once established on the localizer. He asked; well what about your glideslope? Were you established on it? I said I wasn't. ATC told me that the MVA in the area was 3;000 and switched me to tower. In hindsight I realize that even though I activated my approach; it did not switch to vloc. I know I was more than two miles from the FAF when I activated the approach so it should have automatically switched over. This is a problem several of us cirrus owners and pilots have encountered with the dfc 90 and gtn 650's -sometimes it automatically switches to vloc and sometimes it doesn't; even when the approach is activated more than two miles from the FAF. This is a problem with the dfc 90 that needs to be understood. It is not good that it sometimes switches from GPS to vloc and sometimes does not. I had thought I was cleared to descend once I was established on the localizer and that is what I did. I may have misheard the clearance and perhaps ATC said 'established' only; but I have always understood established to mean the localizer and the glideslope.

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Original NASA ASRS Text

Title: SR22 pilot reported an issue with the avionics system design that induced an excursion from the cleared altitude on final approach to an altitude below the MVA.

Narrative: I was on an IFR flight plan on the arrival and was instructed to 'turn to 110 to maintain 3;000 until established on the localizer.' I was at 3;000 and saw the localizer come in as I had activated the ILS approach on my GNS 650. However; my glideslope didn't appear on my PFD and as I was troubleshooting; I descended to 2400. I was in VMC and had the field in sight. ATC came on and told me to climb immediately to 3;000 which I did and I requested to cancel my clearance as I was in VMC. ATC asked why I had descended and I told him I was cleared to descend once established on the localizer. He asked; well what about your glideslope? Were you established on it? I said I wasn't. ATC told me that the MVA in the area was 3;000 and switched me to tower. In hindsight I realize that even though I activated my approach; it did not switch to VLOC. I know I was more than two miles from the FAF when I activated the approach so it should have automatically switched over. This is a problem several of us Cirrus Owners and Pilots have encountered with the DFC 90 and GTN 650's -sometimes it automatically switches to VLOC and sometimes it doesn't; even when the approach is activated more than two miles from the FAF. This is a problem with the DFC 90 that needs to be understood. It is not good that it sometimes switches from GPS to VLOC and sometimes does not. I had thought I was cleared to descend once I was established on the localizer and that is what I did. I may have misheard the clearance and perhaps ATC said 'established' only; but I have always understood established to mean the localizer and the glideslope.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.