Learn what the GPS/NAV source switch controls, when to select GPS or NAV/VLOC, and why the autopilot or ILS may follow the wrong course.
A GPS/NAV source switch chooses which navigation signal feeds the aircraft’s CDI or HSI and, when coupled, the autopilot’s NAV mode. Select GPS/FMS to follow the active flight plan or RNAV course; select NAV/VLOC to track a tuned VOR, localiser or ILS. It does not engage the autopilot by itself.
Is the GPS/NAV switch the same as autopilot NAV mode?
The source switch selects the guidance to use, while autopilot NAV mode commands the aircraft to follow that guidance. Both must be set correctly: selecting GPS does not engage NAV mode, and engaging NAV mode does not guarantee that GPS is the selected source.
This distinction causes many apparent autopilot faults. If the source is NAV/VLOC, the autopilot may chase a tuned radio signal—or show no usable guidance—instead of following the magenta GPS route. If GPS is selected during an ILS, it may follow the flight-plan approximation of the approach rather than the actual localiser.
Names vary by aircraft. Legacy panels may have a physical GPS/NAV switch, Garmin units often use a CDI or GPS/VLOC control, and glass cockpits may offer GPS, FMS, NAV1 or NAV2 through an on-screen source selector. Our Garmin GNS 530 source-control walkthrough shows a typical GPS-to-VLOC implementation.
Which navigation source should I select?
Select GPS/FMS for computed route guidance and NAV/VLOC for a signal received through a navigation radio.
| What you want to follow | Source to select | What must be prepared |
|---|---|---|
| En-route GPS flight plan or Direct-To course | GPS or FMS | An active flight plan leg or Direct-To waypoint |
| GPS/RNAV approach | GPS or FMS | The correct approach loaded and activated or sequenced |
| VOR radial | NAV or VLOC | The correct NAV receiver tuned and the required course set |
| Localiser or ILS final approach | NAV, VLOC or ILS | The localiser frequency, front course and appropriate approach mode |
GPS selection does not make every displayed waypoint active. Check that the desired leg is highlighted and that sequencing is not suspended. Likewise, NAV/VLOC does not tune a station automatically unless that aircraft’s avionics specifically provide auto-tuning.
Airliners may not have a literal GPS/NAV switch. Their FMC route normally feeds LNAV, while localiser and approach modes use radio guidance. In those aircraft, the flight-mode annunciator and navigation-source indication are more useful than looking for a cockpit switch.
How do I switch from GPS guidance to an ILS?
Stay on GPS while it supplies useful arrival guidance, prepare the ILS, then change to NAV/VLOC before localiser capture and verify the source annunciation.
- Load or follow the arrival. Use GPS/FMS for the en-route and initial approach legs when appropriate.
- Prepare the ILS. Tune the correct frequency, identify the localiser if the simulation models identification, and set the published front course.
- Approach the intercept. Use heading mode or hand-fly an intercept that allows the localiser to be captured without an excessive crossing angle.
- Select NAV/VLOC. Do this when the localiser is receivable and before capture. Some avionics can change sources automatically, but always verify the displayed source.
- Arm approach mode. Use APR rather than ordinary NAV mode when you need both localiser and glideslope capture, and intercept the glideslope from below.
A common mistake we see is leaving the display on GPS because the magenta course appears to line up with the runway. That course is not the radio localiser. For the complete transition and capture sequence, see our explanation of choosing GPS or NAV during an ILS approach.
Why will the autopilot not follow the selected source?
The aircraft follows only a valid selected source, and NAV or APR mode must still be armed or active.
- Wrong source: the indicator says VLOC or NAV while you expect GPS guidance, or vice versa.
- Wrong receiver: NAV2 is tuned but the HSI and autopilot are using NAV1.
- No active GPS leg: the route is loaded but not activated, a discontinuity exists, or waypoint sequencing is suspended.
- Another lateral mode is active: HDG, ROL or another mode remains in command even though NAV is armed.
- Poor capture geometry: the aircraft is too far from the course, crossing it too steeply or flying away from it.
- Conflicting control bindings: a duplicated button or switch repeatedly changes the source after you select it.
- Aircraft-specific avionics: some add-on aircraft use custom systems and ignore the simulator’s legacy GPS/NAV control.
Read the source text and flight-mode annunciations rather than relying only on needle colour. Many Garmin-style displays use magenta for GPS and green for VLOC, but that convention is not universal. FSX users with the correct route displayed but no capture should also check our FSX GPS and NAV autopilot troubleshooting steps.