CDI scaling for RNAV

[danburdette]

The Skyview Pilots User Guide rev W only reference to GPS CDI scaling is on page 4-15. It states that when following a GPS course the full scale deflection is 5nm. I have a Garmin 420W (TSO-C146) navigator feeding position, route, and approaches to Skyview. The CDI deflection seems more like the scaling I would expect for IFR enroute, terminal and approach, including LP and LPV. Multiple Advisory Circulars specify CDI scaling for RNAV operations. Could you elaborate on the CDI scaling for SV RNAV operations with a TSOed GPS/WAAS navigator? 

 

[Dynon]

In these instances the CDI is sent to the HSI as a needle value relative to full deflection. In other words, the needle position in the HSI is managed and set by the navigator, and so its scaling will be per that device's documentation.

 

[dynonsupport]

Also, the current scaling is indicated by the text in the HSI window. It will say 5.0 NM when enroute, but you'll see this change when you go to terminal mode and then it will decrease slowly as you are on the approach.

 

[danburdette]

I would like to suggest that you consider to changing the GPS enroute scaling from 5nm to 4nm for full scale. RNAV routes are RNAV 2 (RNP 2). Some of our navigators do not have airways in the database, so we have to program waypoint to waypoint. The reason for the suggestion is that we are transitioning into a Performance Based Navigation (RNAV and RNP) system. The routes that we get cleared to fly are RNAV 2 or RNP 2. Advisory Circulars AC 90-100 (Terminal and Enroute) and AC 90-105 (RNP Operations) give us operators guidance for enroute GPS RNAV operations.

 

[dynonsupport]

Even in enroute, what we are sent by the navigator is % full scale deflection to display, and what full scale represents. We aren't sent X miles left or right. We're just doing what the navigator tells us to, and I don't think we can do anything but that if we want to be compliant with the TSO and thus legal for IFR ops. We also wouldn't match any other NAV head hooked to that navigator which would be confusing.

If the navigators start sending 4.0 NM for enroute, we'll show that automatically. For them to do so, I'd imagine the TSO would need to change.