GPS approach

I was flying a practice GPS LPV approach on full autopilot. The autopilot status under the altitude tape showed both V and L guidance properly set and surrounded by a magenta box. At a point on final past the FAF, the box around the V:NAV went from magenta to red and I lost vertical guidance. I had never seen this before. The plane seemed to descend below the glide slope so I disengaged it and flew it manually.

I haven't seen anything in the manual about this red box though it seems obvious what it means and was wondering what should the A/P do when you lose any kind of guidance on a approach?

The Tru-Trak needs to see an altitude hold condition at least 5 seconds prior to gradient path intercept. One way to do this is to do your final step down with verticle speed, but do not arm the verticlemode to "arm" until the step down altitude is reached for at least 5 seconds, about that time the gradient path will appear, and then arm the verticle portion of the approach. The Tru-Trak manual has a short blurb on this, but my conversation with them was more illuminating. I was getting "FLG" in the pitch window of the auto pilot until I found the error of my ways.

Regards,

Gary Brown

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Originally Posted by jkgoblue

I was flying a practice GPS LPV approach on full autopilot. The autopilot status under the altitude tape showed both V and L guidance properly set and surrounded by a magenta box. At a point on final past the FAF, the box around the V:NAV went from magenta to red and I lost vertical guidance. I had never seen this before. The plane seemed to descend below the glide slope so I disengaged it and flew it manually.

I haven't seen anything in the manual about this red box though it seems obvious what it means and was wondering what should the A/P do when you lose any kind of guidance on a approach?

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It sounds like you lost the GPS signal, it should continue what it was doing for 5 seconds and then it it still does not have the GPS signal it will go back to ARM (altitude bug).