Learn when to use NAV mode versus GPS on an autopilot, how the CDI source selector works, and which setup tracks GPS, VOR and ILS guidance.
On most autopilots, NAV and GPS are not competing autopilot modes. Select GPS/FMS as the navigation source when following a flight plan or GPS approach, or VLOC/NAV when tracking a VOR, localiser or ILS; then engage NAV or approach mode so the autopilot follows that selected source.
Are NAV and GPS separate autopilot modes?
Usually, no: GPS is the guidance source, while NAV is the lateral tracking mode. In real-world aviation and flight simulation, the source selector determines which signal drives the CDI or HSI, and NAV tells the autopilot to capture and follow that displayed course.
A common general-aviation arrangement has a GPS/VLOC or GPS/NAV selector plus a separate NAV button on the autopilot. Selecting GPS does not itself engage the autopilot, and pressing NAV does not automatically choose GPS. Our detailed explanation of NAV capture and tracking covers what happens after the source is selected.
Airliners often use different labels. LNAV normally follows the FMS route, while VOR/LOC or APP follows radio guidance. Some retrofit installations have a dedicated GPSS steering function which may operate through HDG mode rather than NAV; the aircraft flight manual supplement and mode annunciations take precedence over any general rule.
Which setting should I choose?
Choose the source that generates the course you intend to follow, then use the autopilot mode designed to capture it.
| What you want to follow | Navigation source | Typical autopilot mode |
|---|---|---|
| GPS or FMS flight plan | GPS/FMS | NAV on many GA systems; LNAV on transport aircraft |
| VOR radial or airway | VLOC or the appropriate NAV receiver | NAV |
| Localiser or ILS | VLOC with the frequency tuned and identified | LOC or APR/APP; approach mode is normally required for glideslope capture |
| RNAV GPS approach | GPS/FMS with the procedure activated | APR/APP where required for supported vertical guidance; NAV/LNAV may provide lateral guidance only |
| ATC vector or course intercept | Selected heading or track | HDG/TRK initially, followed by NAV or APP when established to capture |
For an ILS, GPS may guide you towards the initial approach fix, but the final localiser and glideslope come from the tuned radio receiver. Some avionics switch automatically from GPS to VLOC; others only display a prompt. See our guidance on selecting the correct source and mode for an ILS before relying on an automatic changeover.
How do I engage NAV without losing the route?
The safe sequence is to prepare and verify the guidance before asking the autopilot to capture it.
- Identify the intended guidance. Decide whether the next segment comes from a GPS leg, VOR radial, localiser or selected heading.
- Make the guidance valid. Activate the required GPS leg or procedure, or tune and identify the correct radio station. Merely loading a flight plan does not always activate its first leg.
- Select the CDI source. Choose GPS/FMS for satellite navigation or VLOC/NAV for radio navigation. Confirm the source label beside the CDI or HSI rather than relying on needle colour alone.
- Set the course where required. Enter the published VOR or localiser course. Many GPS systems set the desired course automatically, but radio-navigation installations often do not.
- Create a sensible intercept. Use HDG or manual flight to approach the course at a moderate angle. NAV may remain armed, overshoot or capture the wrong course if the aircraft starts too far away or nearly parallel to it.
- Engage and verify. Arm NAV, LOC or APP and watch the flight-mode annunciator. Confirm that the mode changes from armed to active and that the aircraft turns towards the intended course.
Why does NAV mode ignore my GPS flight plan?
NAV usually ignores a GPS route because the wrong source is selected or the route is not supplying active guidance. The mistake we see constantly in simulators is moving the GPS/NAV switch to GPS but forgetting to press NAV on the autopilot—or pressing NAV while the source remains VLOC.
- Wrong CDI source: the display still shows VLOC, NAV1 or NAV2 instead of GPS/FMS.
- No active leg: the route is loaded but not activated, or GPS sequencing is suspended by OBS, SUSP or a procedure state.
- NAV is only armed: the aircraft has not reached the capture zone or has an unsuitable intercept angle.
- No valid radio signal: the selected VOR or localiser is mistuned, out of range or feeding a different receiver.
- Installation-specific coupling: an external GPSS converter may require HDG mode even though GPS steering is active.
- Source changed after capture: switching between GPS and VLOC can cancel or alter the active mode on some systems.
On common FSX-style panels, the GPS/NAV switch chooses what the autopilot receives and the NAV button commands it to track that signal. If that arrangement still refuses to couple, work through these FSX GPS/NAV source and coupling checks.
What should I monitor after capture?
The flight-mode annunciator is the authoritative indication of what the autopilot is doing. Check the active lateral mode, the armed mode, the CDI source and whether course deviation is reducing; a lit button alone may show only that capture was requested.
NAV normally controls lateral guidance only. Vertical navigation, altitude capture, glideslope and GPS glidepath tracking require their own supported modes, and their availability depends on the aircraft, navigator and autopilot installation.