Narrative:

While flying the ILS approach; I experienced a problem with ILS guidance/autopilot coupling. I had loaded the ILS approach into the FMS; tuned and confirmed ILS frequencies in both navigation radios; set the inbound course; and set approach minimums. The initial approach was executed as planned. I was cleared to cross an intermediated intersection at the mandatory altitude of 1500 feet and cleared the ILS approach. As I crossed the intersection at 1500 feet; I selected approach mode on the autopilot controller. The glideslope indicator centered but the autopilot did not capture the glideslope. I selected vertical speed and began to descend the airplane thinking that perhaps the airplane was not underneath the glideslope long enough to capture. My plan was to descend the airplane to the glideslope and then reselect approach to couple to the glideslope. As the airplane was descending in vertical speed mode; I rearmed approach mode. At this time; I could see the ground as well as the runway as I descended through about 1000 feet. The autopilot did not capture the glideslope and descended through the glideslope. At this time; I disconnected the autopilot; leveled the pitch attitude; and intercepted the glideslope. During this time; the tower called and advised to check altimeter setting and advised a low altitude alert. The runway was in sight; the remainder of the approach was stable; and a normal landing conducted.I have conducted many approaches to this runway and had never experienced an issue before. Most of my approaches have been conducted in jets. After landing; I reviewed the procedure and how the avionics were set and observed nothing abnormal. After much thought about the incident I believe the issue was how the ILS navigation source was armed for the approach. When I crossed intersection; the localizer needle was white; which means it was not armed. If it was armed; it should have been cyan and then turned magenta when it was coupled. The indicator next to the localizer CDI indicated localizer 1; which led me to believe I was using that as my active navigation source. In fact; white CDI indicates not armed but is guidance only at that point. A better option at that point would have been to conduct a missed approach or even to descend to localizer minimums. Although I was clear of the clouds at that point I should not have been attempting to continue to use automation. The CDI colors and their meanings had become confusing at that point and I was using my knowledge from a completely different platform to attempt to rectify the situation. Our flight department always flies with two pilots although the airplane is single pilot. Another factor I believe was having a new first officer who is unfamiliar with the avionics. Typically; the non-flying pilot will assist with loading the FMS; briefing; and monitoring the approach; which was not really possible in this instance due to unfamiliarity with the avionics. Another lesson learned here was despite the single pilot capability of the airplane; the airplane should be flown as we train; with two experienced pilots and a review of systems and avionics for pilots flying other platforms who have not flown the airplane recently.

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Original NASA ASRS Text

Title: PC-12 Captain reported that the autopilot was unable to capture the glideslope and ATC advised a low altitude alert.

Narrative: While flying the ILS approach; I experienced a problem with ILS guidance/autopilot coupling. I had loaded the ILS approach into the FMS; tuned and confirmed ILS frequencies in both NAV radios; set the inbound course; and set approach minimums. The initial approach was executed as planned. I was cleared to cross an intermediated intersection at the mandatory altitude of 1500 feet and cleared the ILS approach. As I crossed the intersection at 1500 feet; I selected approach mode on the autopilot controller. The glideslope indicator centered but the autopilot did not capture the glideslope. I selected Vertical speed and began to descend the airplane thinking that perhaps the airplane was not underneath the glideslope long enough to capture. My plan was to descend the airplane to the glideslope and then reselect approach to couple to the glideslope. As the airplane was descending in vertical speed mode; I rearmed approach mode. At this time; I could see the ground as well as the runway as I descended through about 1000 feet. The autopilot did not capture the glideslope and descended through the glideslope. At this time; I disconnected the autopilot; leveled the pitch attitude; and intercepted the glideslope. During this time; the tower called and advised to check altimeter setting and advised a low altitude alert. The runway was in sight; the remainder of the approach was stable; and a normal landing conducted.I have conducted many approaches to this runway and had never experienced an issue before. Most of my approaches have been conducted in jets. After landing; I reviewed the procedure and how the avionics were set and observed nothing abnormal. After much thought about the incident I believe the issue was how the ILS NAV source was armed for the approach. When I crossed Intersection; the localizer needle was white; which means it was not armed. If it was armed; it should have been cyan and then turned magenta when it was coupled. The indicator next to the localizer CDI indicated LOC 1; which led me to believe I was using that as my active NAV source. In fact; white CDI indicates not armed but is guidance only at that point. A better option at that point would have been to conduct a missed approach or even to descend to localizer minimums. Although I was clear of the clouds at that point I should not have been attempting to continue to use automation. The CDI colors and their meanings had become confusing at that point and I was using my knowledge from a completely different platform to attempt to rectify the situation. Our flight department always flies with two pilots although the airplane is single pilot. Another factor I believe was having a new First Officer who is unfamiliar with the avionics. Typically; the non-flying pilot will assist with loading the FMS; briefing; and monitoring the approach; which was not really possible in this instance due to unfamiliarity with the avionics. Another lesson learned here was despite the single pilot capability of the airplane; the airplane should be flown as we train; with two experienced pilots and a review of systems and avionics for pilots flying other platforms who have not flown the airplane recently.

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.