Learn when to switch from GPS to NAV/VLOC for an ILS, which autopilot mode to arm, and how to fix localiser or glideslope capture failures.
For an ILS approach in a flight simulator, use the NAV/VLOC radio source for final approach, not GPS. GPS may guide you along the arrival and towards the intercept, but switch to the ILS source before capture and arm APP/APR mode so the autopilot can follow both localiser and glideslope.
Why does an ILS need NAV/VLOC?
An ILS provides radio guidance from ground-based localiser and glideslope transmitters, so the aircraft's navigation receiver must be tuned to the runway's ILS frequency. GPS can sequence the approach fixes and position the aircraft for interception, but it does not replace the ILS radio signal on the final approach.
If the published procedure is RNAV or GPS rather than ILS, remain on the GPS/FMS source instead. Our explanation of the difference between ILS and RNAV guidance covers which source each approach uses.
What do GPS, NAV and APP actually control?
The GPS/NAV selector chooses the navigation source, while the autopilot buttons tell the aircraft how to follow that source. Confusing those two functions is one of the most common reasons a simulated ILS fails to capture.
| Selection | What it does | Use during an ILS |
|---|---|---|
| GPS/FMS source with NAV or LNAV | Follows the programmed magenta route | Useful for the arrival and positioning before the intercept |
| NAV/VLOC source with NAV mode | May follow the selected radio course laterally | Can track the localiser on some autopilots, but normally provides no glideslope coupling |
| NAV/VLOC source with APP/APR mode | Arms localiser and glideslope capture | The normal selection for the ILS final approach |
In a Garmin-equipped light aircraft, the source may be changed with a CDI button or a GPS/VLOC selector. Older panels often use a physical NAV/GPS switch. Modern Airbus and Boeing simulations usually manage the source differently: the FMS follows the arrival, the ILS is tuned or identified, and APP or APPR is armed. The flight-mode annunciator should then show LOC and G/S armed and subsequently captured.
When should I switch from GPS to NAV/VLOC?
Switch to NAV/VLOC before the expected localiser capture, once GPS or heading mode has placed the aircraft on a suitable intercept. Changing too early removes useful route guidance; changing after passing through the localiser may leave the autopilot following the wrong source.
A practical intercept is normally made from below the glideslope and at a modest angle to the localiser, commonly 30 degrees or less. Approaching from above the glideslope is a frequent cause of failed vertical capture.
How do I set up the ILS capture?
A reliable ILS setup uses GPS only for positioning, then transfers final guidance to the radio receiver and approach mode.
- Load the approach. Select the correct runway and ILS procedure in the GPS or FMS if the aircraft supports it. This supplies the fixes and helps with arrival sequencing.
- Check the ILS data. Confirm the runway, frequency and inbound course against an appropriate approach chart. We explain how to find and verify simulator ILS frequencies and courses.
- Tune the coupled receiver. Enter the ILS frequency into the NAV receiver used by the autopilot, commonly NAV1 in a general-aviation aircraft. Set the published front course and verify the identifier if the simulation models it.
- Fly towards the intercept. Use GPS, heading mode or the aircraft's managed lateral mode to reach the localiser intercept at the published altitude.
- Select NAV/VLOC. Confirm that the course-deviation indicator now displays the radio localiser rather than the GPS route. The localiser needle should become active as the aircraft enters its coverage.
- Arm APP/APR. Do this while established on a sensible intercept and below the glideslope. Monitor the annunciations for localiser capture first and glideslope capture afterwards; button illumination alone does not prove capture.
Why will the localiser or glideslope not capture?
A failed capture usually comes from the wrong source, poor interception geometry or an incomplete radio setup.
- The autopilot keeps following the magenta line: GPS/FMS is probably still selected, or NAV/LNAV remains active without APP/APR being armed.
- The localiser needle never appears: Check the active frequency, selected NAV receiver, runway direction and navigation-data compatibility. Some scenery and navdata combinations assign different frequencies.
- The localiser captures but the glideslope does not: The aircraft may be above the beam, APP/APR may not be armed, or the procedure may not provide a usable glideslope. A back-course approach does not use the normal front-course glideslope.
- The aircraft turns away from the runway: Confirm that the correct runway ILS and front course are selected. Nearby parallel-runway frequencies and reciprocal courses are easy to confuse.
- APP is armed but nothing captures: The intercept may be too steep, too far from the transmitter or beyond the localiser. Re-establish a shallow intercept rather than waiting for the autopilot to rescue a poor setup.
FSX adds another common trap: its NAV/GPS switch selects the source, while the autopilot's NAV button selects a tracking mode. Our FSX GPS and NAV autopilot troubleshooting explains that distinction in the older simulator's panels.