Modern aircraft have severals devices used for navigation, usually VOR/LOC receivers for radio navigation and a GPS navigator or FMS computer for area navigation (RNAV). These devices all provide information on the course to fly, as well as some lateral offset to the planned course.
CDIs and HSIs display that information, and also feed it to the flight director or autopilot, so the pilot can track a course or radial using the desired autoflight system.
Which means of navigation is displayed on the HSI and fed into the autopilot is determined by a source selector, also called the NAV/GPS switch. Depending on the equipment in your plane, X-Plane simulates two types of source selectors found in the real world. The difference is very subtle, so here’s the full explanation:
Analog source switching
Analog source switching is found in most general aviation aircraft that have an older GPS navigator or a GPS retrofitted into the panel. In that case, you have usually two sets of wires going into the source selector, one from a VOR receiver and one from the GPS, and only one set of wires going out to the HSI. The source selector then determines whether the NAV/VOR/LOC or the GPS signal is fed to the HSI. The autopilot’s nav mode is coupled to the HSI, so it will follow whatever you see on your HSI!
In X-Plane, that type of switch can be placed on the panel, variants exist that have either NAV1/NAV2/GPS/FMS or subsets of sources selectable.
The key point to remember here is: Several signals from different devices go into the source selector, and you select one output.
Integrated NAV/GPS switching
Integrated switching is found in more modern equipment with GPS units that are approach-capable and have built-in nav receivers. An example is the very popular GNS430W by Garmin. These devices combine a NAV receiver and a GPS navigator in one piece of panel hardware. In these approach capable navigators there is usually one switch to select whether the device should output VOR or GPS navigation information. On a GNS430W that switch is labelled “CDI” and toggles a selection that can either read “VLOC”, meaning VOR or LOCalizer signal from the nav receiver, or “GPS”, meaning the GPS navigator. Because here the nav receiver is integral part of the avionic, there is no separate wiring and no separate source selector outside the device itself. The device has only one signal output, and that is fed to the HSI, CDI and/or autopilot.
The key point to remember here is: Only one signal comes from the device, and the signal’s source is determined internally.
What’s the difference in X-Plane 10.30+ ?
In X-Plane, both of these switches are controlled by the same type of panel instrument, the HSI source selector, and the same datarefs sim/cockpit2/radios/actuators/HSI_source_select_pilot for the pilot’s side and sim/cockpit2/radios/actuators/HSI_source_select_copilot for the copilot’s side.
The difference becomes obvious when you install a non-switched analogue CDI instrument in your panel:
- In case of the analog HSI switch and a non-approach capable GPS or old-school FMS, the CDI is not switched and simply displays the source it has been wired to in Plane-Maker.
- In case of the integrated switch, the CDI is wired to the integrated unit and it will display whatever has been selected inside this unit. You may install the CDI source indicator instrument in the panel as a visual reminder to the pilot. More modern CDIs also feature an integral green light for that.
The latter is necessary to comply with FAA-regulations: For a GPS to be approach certified, it needs to drive an external CDI or HSI. That means in panels without an HSI, the CDI has to be capable of indicating the GPS navigation, otherwise the installation cannot be certified for GPS approaches under IFR.