Autopilot navigation source reference

As we all know, the source for the autopilot “NAV” and “APR” modes is set with the datarefs sim/cockpit2/radios/actuators/HSI_source_select_pilot and HSI_source_select_copilot respectively for the autopilot side. But what EXACTLY supplies the lateral and vertical deviation indications we see on the PFD and that the autopilot sees? What should the Nav ID and DME distance readout be?

This article summarizes X-Plane’s capabilities when it comes to modern integrated approach navigation and sheds light on the new datarefs.

ILS approach

Tune the ILS on the NAV1 radio (for left autopilot, or NAV2 radio for right autopilot). Whenever a valid ILS signal is received by the on-side NAV receiver, it locks out ALL other approach modes!

RNAV (GPS) and RNAV (RNP) approaches and overlay approaches

This approach is loaded into the GPS or FMS. It is flown with either Baro-VNAV or SBAS-derived VNAV. No FAS block is provided. No channel is tuned into the WAAS or the GLS receiver.

This includes all cases of “overlay” approaches, i.e. VOR, DME, NDB and TACAN approaches that can be flown with integrated approach navigation, if the airplane is so equipped.

In this case, the DME ID of the GPS receiver tells you the approach ID (e.g. “RNV23LY”) and the DME ID of the augmentation receiver tells you the missed approach point (e.g. “RW23L”). The distance readout of the augmentation receiver tells you the distance to that point (usually the runway threshold).

RNAV (GPS) or RNP approach with SBAS FAS

This approach is loaded into the GPS or FMS. The FAS block determines the SBAS system to be used (WAAS, EGNOS or MSAS). The SBAS receiver is tuned to the respective channel and provides FAS navigation, both lateral and vertical.

In this case, the DME ID of the GPS receiver tells you the FAS block ID (e.g. “W05A”) and the DME ID of the augmentation receiver tells you the missed approach point (e.g. “RW05L”). The distance readout of the augmentation receiver tells you the distance to that point (usually the runway threshold).

GLS

This approach is loaded into the FMS. A FAS block is provided. The GBAS receiver is tuned to the GLS channel and provides the FAS navigation.

In this case, the DME ID of the GPS receiver tells you the FAS block ID (e.g. “G25A”) and the DME ID of the augmentation receiver tells you the missed approach point (e.g. “RW25R”). The distance readout of the augmentation receiver tells you the distance to that point (usually the runway threshold).

LOC or LOC backcourse approach with baro VNAV

This approach is loaded into the FMS. No SBAS or GLS channel is tuned. No FAS block is provided. Final approach navigation comes from the localizer analogue signal. VNAV is provided by the FMS using baro altitude. The autopilot is in VLOC mode laterally, but GP mode vertically.

In this case, the DME ID of the GPS receiver tells you the approach ID (e.g. “LOC31”) and the DME ID of the augmentation receiver tells you the missed approach point (e.g. “RW31”). The distance readout of the augmentation receiver tells you the distance to that point (usually the runway threshold).