Interesting Glideslope capture

[Rich]

This is something I intend to explore more, but I wondered if anyone else has seen what is a first for me.

First of, note below I have GNS530W/Aspen E5/KAP140 to do coupled approaches. Today I set up the LP approach (which executes as "LP+V" into S39 RWY 29. Flying at 6000 ft lined up flying level well below the GS. The GS indicator on the Aspen was "ghosted" off scale at the top as expected. GSS enabled. I pushed the "APR" button on the autopilot, an operation I've performed many times, expecting to get "GS Arm" on the A/P display and continue level until the GS indicators showed on GS. The AP would switch to "GS" and start to descend on GS.

Instead the AP immediately displayed "GS" and commenced to climb up to get the GS. I shut off the AP and then reconfigured the AP in ALT mode and just hold heading until the GS diamond symbol filled in (lost the "ghost" effect). Then hit APR and everything behaved normally.

[Rich]

Numerous tries later, I haven't been able to duplicate this.

I have worked out the process for capturing from above without using CWS. Just set VS mode in the AP and set an aggressive descent rate (like 1100 FPM) and pull the power well back to get a much steeper than normal angle. Works great.

[Lance Murray]

Most operators consider capturing from above as a bad practice. Just an FYI. Diving for a glideslope isn't the best thing to do. It's hard to be stabilized when trying to capture from above.

[Rich]

Most operators consider capturing from above as a bad practice. Just an FYI. Diving for a glideslope isn't the best thing to do. It's hard to be stabilized when trying to capture from above.

This is true, especially in a really slippery airplane. But using the AP in my airplane it's stabilized and not at some hairy break-neck speed. Controlling speed is the key.

In an LP+V (as here) it is technically required to be able to deviate above GS, as it's possible for the derived GS to bust the minimum altitudes of waypoints.