Narrative:

On 1st attempt to execute the RNAV GPS (Y) 12 approach; the ap (autopilot) would not arm and we flew through the final course. Tower advised us of same which we were monitoring on our navigation screen. Tower gave us a new altitude and vectored us around for another approach. During calm moments while being vectored we discovered we only loaded the approach but not armed it. We armed the approach and when cleared for the approach; the ap steered us onto the final inbound course. We were set for a LNAV guidance; but I misread the GS indicator (confused it with the standard default rate of descent icon; which is at bottom of screen; below the GS indicator). Accordingly I assessed that we were above the GS (in fact it had not yet become alive) and started a descent. This resulted in the tower issuing a low altitude alert; which I corrected. I also disconnected the ap and began hand flying. Again; I misread the GS indicator and was temporarily fixated on the standard rate of descent icon; again; and this resulted in the second tower low altitude alert. At this time we were in VMC and at no time were unsafe as relating to terrain. Seconds later we had the runway in sight and were completely VFR with unlimited visibility (about 1.3 miles from runway. Rest of the approach and landing were uneventful.in retrospect; 2 things need more training by me. 1) reliance on a final approach checklist to confirm the approach procedure is not only loaded in navigation but also armed! 2) follow the approach plate procedures and if the GS is not alive at the appointed distance and altitude; declare a missed; reconfigure and do it again. 3) do not fixate on one indicator.

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Original NASA ASRS Text

Title: CE551 pilot reported several low altitude alerts from the Tower after mistakenly flying based off the rate of descent icon instead of the glideslope indicator.

Narrative: On 1st attempt to execute the RNAV GPS (Y) 12 approach; the AP (Autopilot) would not arm and we flew through the final course. Tower advised us of same which we were monitoring on our navigation screen. Tower gave us a new altitude and vectored us around for another approach. During calm moments while being vectored we discovered we only loaded the approach but not armed it. We armed the approach and when cleared for the approach; the AP steered us onto the final inbound course. We were set for a LNAV guidance; but I misread the GS indicator (confused it with the standard default rate of descent icon; which is at bottom of screen; below the GS indicator). Accordingly I assessed that we were above the GS (in fact it had not yet become alive) and started a descent. This resulted in the Tower issuing a low altitude alert; which I corrected. I also disconnected the AP and began hand flying. Again; I misread the GS indicator and was temporarily fixated on the standard rate of descent icon; again; and this resulted in the second Tower low altitude alert. At this time we were in VMC and at no time were unsafe as relating to terrain. Seconds later we had the runway in sight and were completely VFR with unlimited visibility (about 1.3 miles from runway. Rest of the approach and landing were uneventful.In retrospect; 2 things need more training by me. 1) Reliance on a final approach checklist to confirm the approach procedure is not only loaded in NAV but also ARMED! 2) Follow the approach plate procedures and if the GS is not alive at the appointed distance and altitude; declare a missed; reconfigure and do it again. 3) Do not fixate on one indicator.

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.