Learn why an ILS localizer or glideslope signal disappears on approach, how to isolate the cause, and when a missed approach is required.
An ILS localizer or glideslope signal can disappear when the aircraft leaves valid coverage, passes an antenna, receives a blocked or distorted transmission, or loses correct tuning or navigation-source selection. In Aviation & Real-World Flying and in simulators, a transmitter outage, separate glideslope failure, scenery mismatch or avionics fault can cause the same symptom.
Is the ILS signal actually gone?
First decide whether raw ILS guidance has been lost or only the autopilot has stopped following it. A missing localizer or glideslope pointer, navigation warning flag, or lost station identifier indicates a reception or validity problem. A pointer that remains visible while the aircraft turns or pitches away usually indicates an autopilot mode or capture problem.
| What you see | Likely meaning |
|---|---|
| Both pointers disappear and the identifier is lost | Wrong frequency or receiver, navigation-source change, electrical failure, station outage, or aircraft outside coverage |
| Localizer remains but glideslope disappears | Separate glideslope outage, localizer-only facility, invalid vertical geometry, or passage beyond the glideslope antenna |
| Needles remain valid but APP, LOC or GS mode drops | Autopilot logic, mode selection or capture geometry rather than loss of the radio signal |
| Signal disappears at the same position on every simulated flight | Service-volume modelling, misplaced navaid data, conflicting scenery or a repeatable avionics issue |
A full-scale needle is not necessarily a lost signal: the aircraft may simply be too far from the beam centreline. Glass displays may also hide an off-scale or invalid pointer rather than showing a traditional warning flag. If the raw indications remain alive, use our checks for separating an approach-mode failure from a reception failure.
What causes an ILS indication to disappear?
An ILS indication disappears because the transmission is unavailable, invalid at the aircraft's position, distorted, or no longer connected to the selected flight display.
- Leaving protected coverage: ILS guidance is guaranteed only within a defined service volume. Approaching from behind the antenna, well off the final approach course, too high for the glideslope coverage, or beyond the published range can produce intermittent or absent indications.
- Ground-equipment shutdown: Facility monitoring can remove an unreliable transmitter from service. The localizer and glideslope are separate components, so one can fail while the other remains; our explanation of the independent horizontal and vertical signals covers that distinction.
- Signal distortion: Buildings, terrain and large aircraft or vehicles near an ILS critical area can reflect or disturb the transmission. This more often causes needle bends or fluctuations than a clean, lasting disappearance.
- Cockpit configuration: A frequency transfer, automatic retuning, selection of GPS/FMS instead of VLOC/NAV, use of the wrong receiver, or loss of an avionics electrical bus can remove both indications instantly.
- Simulator data mismatch: The avionics database may describe a different runway position, frequency or approach from the active scenery. Add-on airports can also duplicate or displace a navaid, making the signal vanish at a repeatable point.
Ordinary rain, cloud and wind should not make a serviceable ILS disappear. If weather appears to trigger the problem in a simulator, check failure settings, electrical loads and low-visibility scenery conflicts before blaming radio attenuation.
Why does the glideslope disappear while the localizer remains?
The glideslope can disappear independently because it uses its own transmitter and has a smaller, differently shaped coverage area. A published localizer approach may have no glideslope at all, while an ILS can remain usable laterally when its vertical component is out of service.
The receiver obtains the paired glideslope automatically when the localizer frequency is tuned; there is normally no separate glideslope frequency for the pilot to enter. Persistent lateral guidance with no vertical pointer therefore points towards facility type, glideslope status, reception geometry or simulation data rather than a second untuned radio. Follow the specific checks for a working localizer with a missing glideslope if that is the consistent symptom.
Intercepting from well above the glideslope can also produce unreliable capture or encounters with false lobes. The normal technique is to become established on the localizer and intercept the published glideslope from below; this is explained in our guide to simulated glideslope reception and interception.
Is it normal for ILS guidance to disappear near the runway?
Glideslope guidance becoming erratic or disappearing after the aircraft passes abeam its ground antenna is normal because the vertical beam is intended for the airborne approach, not the landing roll. This occurs beyond the threshold area, when an ordinary landing should already be controlled using the required visual references.
The localizer usually remains available farther along the runway because its antenna is near the opposite end. Some approved low-visibility systems use it for rollout guidance, but that capability must not be assumed for every runway, aircraft or autopilot. A signal that disappears before decision altitude or while still several miles from the runway is not explained by normal antenna passage.
What should a pilot do if the ILS fails on approach?
A pilot must stop relying on invalid guidance and follow the applicable missed-approach or alternative-approach procedure. Before the required visual references are acquired, loss of the localizer normally invalidates the ILS approach.
If only the glideslope fails, changing to localizer-only minima is acceptable only when a localizer procedure is published and regulations, operator procedures, aircraft position and clearance permit the change. Its step-down fixes, missed-approach point and minimum descent altitude may differ from the ILS profile; simply levelling at the ILS decision altitude is not a valid substitute. If a safe transition cannot be made, execute the published missed approach and advise ATC.
How do I diagnose a disappearing ILS in a simulator?
A simulator problem is best isolated by checking the facility, cockpit setup, intercept geometry and scenery data in that order.
- Verify the published approach. Confirm the runway, approach type, localizer frequency and identifier. Do not assume every approach labelled LOC includes vertical guidance.
- Check the active receiver and source. Tune the intended NAV receiver, select VLOC or NAV rather than GPS/FMS where required, and make sure the display and autopilot are using that receiver.
- Use valid intercept geometry. Join the front course within sensible range, at a shallow angle, and below the published glideslope intercept altitude. Do not chase a glideslope received from above or behind the runway.
- Watch the raw needles and mode annunciator separately. If the needles vanish, investigate reception, tuning or data. If they remain valid while LOC or GS mode releases, investigate autopilot configuration and capture logic.
- Check power and simulated failures. Confirm the avionics buses, navigation receivers and relevant antennas have not been failed or depowered.
- Isolate scenery and navigation data. Repeat the approach without conflicting airport scenery or duplicate navaid packages. A failure at exactly the same geographic point strongly suggests coverage or scenery placement rather than pilot technique.