My '73 turbo Aztec has an Altimatic V, actually a Bendix FCS810 relabeled as Piper. I have a manual for it, but of course it does not cover GPS approaches. I seem to be able to get it to track the magenta line point-to point, but not shoot the approach. I would think it would track the HSI needle, which is driven perfectly by the GPS, and the HSI has a good glideslope indication. Is there some button-push sequence I'm missing?
