* IFD440

* IFD550

* Stec 55X autopilot

EX5000 PFD and MFD (avidyne)

From what I have learned, this stack does not allow me to use the IFD's VNAV feature. 

What would be the minimum upgrade that I could to in order to have VNAV?

VNAV is a generic term for a range of features. Using GPS guidance with vertical navigation (LNAV/VNAV) is definitely possible IF you have the correct versions of software on your PFD. There are a variety of these types of approaches, including LNAV+V and Visual approaches, both with advisory vertical guidance.

Enroute vertical guidance is also possible with the IFDs, which provide both the green vertical path indicator, a Vertical Speed Required indicator, and on the IFD 550, a vertical track indicator that will allow you to navigate to preset altitudes on a per waypoint basis, either by pitch or by the VS mode of your autopilot. These altitudes are often specified in DPs and STARS, which if you load into an IFD, will be assigned to the charted waypoints. 

What you can’t get is the ability for your current autopilot to fly an enroute VNAV vertical track. This would require a newer autopilot, and support in the PFD which doesn’t exist.

From what you are saying, it does not make sense to change my Stec 55X to another autopilot box, because even with a new autopilot hardware the current PFD would not support an enroute VNAV vertical track.

It’s a complex dance. The IFD interacts with the PFD to display information about lateral and vertical position, and provide data on deviation from pilot selected course and altitude. The autopilot talks to the PFD to adjust heading to track the selected course and required descent angle. In GPSS mode, the IFD sends information directly to the autopilot, which typically results in better tracking of GPS waypoints than using the STec’s analog navigation mode, which must be used for VLOC navigation.

The digital information is sent serially via a datastream that includes “labels” that describe the value, along with the value itself. When the IFD sends the data on vertical track, it does so using 2 different labels, one for approaches, and the other for enroute VNAV. Both the PFD and the AP need to have the ability to process the additional label in order for enroute VNAV to work. Newer versions of the STec can do this, but they are not approved for the Cirrus.

We’re hoping this label will be supported in the new Vantage series of displays. Currently, the DFC100 is the only Avidyne autopilot which supports enroute VNAV, and it only works with Avidyne’s R9. One can hope there will be some sort of solution provided in the future to allow this.

In the meantime, using the VSR, vertical path indicator, and the IFD 550 vertical deviation indicator are all options that combined with VS mode on the autopilot should provide most of the stuff you need to follow enroute STARS or DPs.