Non Precision Approach Guidance
** Given you're using the UNS, I suspect that you have a Honeywell Primus installation, I don't recall how to set that up exactly. **
We have the Collins Proline 21 like the latest installations in the 560XL and it is possible to use VNAV in one of two ways. In both cases if the approach is based on a ground based aid, you can still legitimately fly using FMS guidance as long as the primary aid for the approach is identified and displayed independently (e.g. as an RMI needle on the HSI) I'll assume you're doing that when describing the LNAV mode in both cases.
The two VNAV options are:
1. Using Approach mode LNAV guidance ("APPR FMS1"/"APPR FMS2"):
The altitude preselect remains at the final assigned altitude (e.g. when vectoring for the approach. VNAV must be armed before the approach and the descent mode will be "GP" (Glide Path) The aircraft will descend to the ground if allowed to.
This makes for a precision approach and is generally used for GPS approaches where the option of a Decision Altitude(Height) is offered when using LNAV/VNAV guidance, In those cases there will be an alternative of an MDA(H) when VNAV guidance is not available.
2. Using NAV mode LNAV guidance ("FMS1"/"FMS2")
The altitude preselect is wound down to the MDA, it is possible to select the precise BARO MIN MDA as shown on the chart (10' resolution) in the Altitude Preselect. This is achieved by posting the MDA on REFS page 2/2 or 2/3 (on the PFD) and winding down the Alerter until that value is displayed.
Again VNAV is armed before the approach however in this case the descent mode is "PATH" which will be accompanied by the aimed mode "VALTS" and the aircraft will level off at the selected MDA.
I hope this helps.