Why won't approach mode capture the localizer or glideslope?
Fix approach mode not capturing the localizer or glideslope by checking ILS tuning, NAV source, APP arming, altitude and intercept angle.
Approach mode will not capture unless the correct ILS is tuned and identified, the localizer is alive, APP is armed, and the aircraft enters the system’s capture envelope. In aviation and flight simulation, glideslope capture also normally requires localizer capture first and an intercept from below at the published altitude.
A mistake we see constantly is treating APP/APPR as an ILS-finding button. It does not seek the beam; it waits for valid guidance and specific capture conditions, then changes the active lateral and vertical modes.
What conditions does approach mode need for capture?
A successful ILS capture depends on the approach type, radio setup, navigation source, intercept geometry and aircraft mode annunciations all agreeing.
- Confirm that the procedure is an ILS. A localizer-only or back-course approach has no usable glideslope. An RNAV approach normally supplies a GPS glidepath labelled
GP, not an ILS glideslope labelledGS. - Tune and identify the correct ILS. Check the active—not standby—frequency, runway end and identifier. Where the FMS auto-tunes the radio, verify what it actually selected rather than assuming the loaded procedure did it correctly.
- Select the coupled navigation source. Conventional ILS guidance normally requires
VLOC,LOCor the appropriate NAV receiver rather than GPS/FMS. Some glass cockpits change source automatically, but that behaviour is aircraft-specific. - Intercept the front course correctly. Set the published inbound course, even where the modern autopilot does not use the course selector for steering. Use a shallow intercept—roughly 20–30 degrees is typical unless the procedure or aircraft calls for something else—and avoid joining extremely close to the transmitter.
- Approach the glideslope from below. Maintain the charted or cleared intercept altitude until the glideslope moves towards the centre. If the aircraft is already above it, many systems will not capture it safely.
- Arm APP and read the FMA. Arm it before crossing the beam, then confirm that
LOCandGSare armed and subsequently active on the flight mode annunciator. The push-button light alone does not prove capture. Our explanation of the correct point in the intercept to arm APP covers that timing in more detail.
What does each failure symptom mean?
The indications on the navigation display and flight mode annunciator usually reveal which capture condition is missing.
| Symptom | Likely cause | What to check |
|---|---|---|
| No localizer or glideslope indication | Wrong frequency, frequency left in standby, out of range, wrong runway end or unavailable ground aid | Tune the active receiver, verify the identifier and confirm that the selected approach provides an ILS. |
| Localizer needle moves, but the aircraft remains in heading or NAV mode | APP not armed, wrong coupled source, excessive intercept angle or unsupported capture geometry | Check the FMA and NAV source, then establish a shallower front-course intercept. |
| Localizer captures, but no glideslope appears | Localizer-only or back-course procedure, unavailable glideslope or incorrect approach selection | Check the chart and understand how a localizer-only approach differs from a full ILS. |
| Glideslope pointer passes through the centre without capture | APP armed too late, localizer not yet active or aircraft approaching from above | Do not dive after the pointer. Re-establish a valid intercept from below or fly the appropriate missed-approach procedure. |
| LOC or GS is active, but the aircraft does not follow it | Flight director only, autopilot disengaged, manual control force or autopilot fault | Confirm autopilot engagement. If the raw guidance is valid and conditions permit, disconnect and hand-fly it. |
Why does the localizer capture but not the glideslope?
The usual causes are joining above the glideslope, flying an approach without vertical ILS guidance, or crossing the beam before APP and the localizer mode are ready. On many autopilots, the glideslope remains only armed until LOC becomes active.
If the glideslope diamond is above centre and moving down, the aircraft is below the path and approaching a normal intercept. If it crosses the centre and continues below, the aircraft is now above the glideslope. Repeatedly pressing APP rarely fixes this and may cancel an already armed mode.
Can you capture an ILS glideslope from above?
An ILS glideslope should be intercepted from below at the published or cleared altitude, not chased from above. Some avionics may capture under limited conditions, but this is not a sound plan and can expose the aircraft to excessive descent rates or false glideslope lobes above the normal path.
In real-world aviation, maintain the assigned or procedure altitude and request new vectors or fly the missed approach as appropriate. In a simulator, use the same disciplined response rather than forcing the autopilot into a steep dive.
How do you know whether APP has actually captured?
The flight mode annunciator is the decisive indication: armed modes wait in the background, while active modes command the aircraft. A typical sequence is lateral guidance armed, LOC active, GS armed, then GS active as the pitch mode leaves altitude hold.
Labels, colours and capture logic differ between aircraft. Airliner users should follow the specific type’s mode sequence; for example, our 737 mode-annunciation and capture guidance explains the relationship between VOR/LOC, APP and the FMA.
Which simulator problems can prevent ILS capture?
In a flight simulator, a correct-looking cockpit setup can still fail because the aircraft, scenery and navigation database disagree.
- Scenery and navdata mismatch: the FMS procedure may reference a newer runway position or frequency than the installed airport scenery.
- Custom-aircraft logic: some add-ons require tuning through their own FMS or radio-management system rather than the simulator’s default controls.
- Wrong source after loading the approach: loading an ILS does not guarantee that the CDI or autopilot changed from GPS to localizer guidance.
- Duplicate control bindings: a switch or controller assignment may silently change the NAV source, cancel APP or disconnect the autopilot.
- Legacy FSX configuration: the NAV/GPS selector, active NAV1 frequency and intercept altitude are frequent culprits. Use our FSX-specific glideslope capture checks if that is the simulator involved.
Can you continue if approach mode fails to capture?
You may continue only when the raw localizer and glideslope indications are valid, the approach remains stable, and hand-flying is permitted by the aircraft, procedure and operating rules. Many ordinary ILS approaches can be hand-flown, but autoland and low-visibility operations have stricter equipment requirements.
If the signal itself is missing or unreliable, do not follow it. In real flying, comply with the clearance and missed-approach instructions; in simulation, hand-fly valid raw data or practise the missed approach rather than chasing failed automation.