Why does the ILS localizer work but the glideslope is missing?
Learn why an ILS localizer works but the glideslope is missing, how to diagnose it, and when a localizer-only approach is permitted.
An ILS localizer can work while the glideslope is missing because they are separate ground transmitters with different coverage. The approach may be localizer-only, the glideslope may be out of service, the aircraft may be outside its usable beam, or the receiver, avionics setup or simulator scenery may be wrong.
In aviation and real-world flying, ILS combines lateral localizer guidance with a separate vertical glideslope signal. Understanding the separate roles of the localizer and glideslope helps distinguish an unavailable signal from an autopilot capture problem.
Why can the localizer work without the glideslope?
The localizer and glideslope can fail or disappear independently because they use separate antennas, transmitters and coverage patterns. The localizer transmitter is normally beyond the far end of the runway and projects a relatively long lateral beam, while the glideslope antenna sits beside the runway near the touchdown area and projects a narrower vertical beam.
A standard ILS receiver automatically selects the paired glideslope channel when the published localizer frequency is tuned; pilots do not tune a separate glideslope frequency. Receiving and identifying the correct localizer therefore does not prove that its paired glideslope is usable.
- The procedure is localizer-only: A chart titled
LOC RWYprovides no glideslope. A chart titledILS or LOC RWYnormally offers full ILS guidance plus separate localizer-only minima. - The glideslope is out of service: A transmitter fault, maintenance or monitoring shutdown can remove vertical guidance while leaving the localizer operational. Check NOTAMs, ATIS information and approach-status messages.
- The aircraft is outside glideslope coverage: The localizer can often be received farther away and over a wider area. A valid glideslope may not appear until the aircraft reaches the published approach segment.
- The aircraft is approaching the back course: A localizer may be received from the opposite runway direction, but its glideslope is directional and must not be used for a back-course approach.
- Aircraft equipment has failed: A glideslope receiver, antenna, display or associated avionics fault can affect vertical guidance without removing the localizer.
What do the glideslope indications mean?
The raw glideslope display reveals whether the problem is a missing radio signal or merely failed autopilot capture. Display conventions vary by aircraft, but these indications are typical:
| Indication | Likely meaning | Response |
|---|---|---|
No diamond or needle, with a G/S flag or the scale removed | No valid glideslope signal is being received | Check the procedure type, facility status, runway direction, position and receiver |
| Glideslope pointer above centre | The aircraft is below the path | Maintain the published intercept altitude until the pointer moves towards centre |
| Glideslope pointer below centre | The aircraft is above the path | Do not dive to chase it; use the published recovery procedure or reposition |
| Pointer centres but approach mode remains armed | The signal exists, but the flight-guidance system has not captured it | Check NAV source, approach-mode annunciations and capture geometry |
| Erratic or flagged indication | Signal interference, coverage limits or an equipment fault | Treat the guidance as unreliable and follow the applicable missed-approach or fallback procedure |
What should I check first?
Start with the chart and raw radio indications before troubleshooting the autopilot.
- Confirm that the procedure has a glideslope. Read the approach title, notes and minima rather than assuming every runway localizer is a full ILS.
- Verify the runway-end frequency and identifier. The correct frequency, approach direction and Morse identifier must all agree with the published procedure.
- Check the facility status. Review NOTAMs and operational information for an inoperative or unmonitored glideslope.
- Check your position and altitude. Join the published approach within the usable sector and intercept from below. Our guidance on choosing the correct glideslope intercept altitude explains why arriving high commonly prevents capture. Never chase an upper or false glideslope indication.
- Separate signal reception from automation. If no raw glideslope pointer exists, pressing APR cannot create one. If the pointer is alive but the autopilot remains armed, investigate NAV source selection, approach-mode sequencing and aircraft-specific capture conditions.
Can I continue if the glideslope is unavailable?
You cannot continue as a precision ILS without a valid glideslope indication. A localizer-only approach may be flown only when published minima and procedures exist and the flight is cleared and authorised to use them.
That normally means observing the localizer step-down fixes, minimum descent altitude and missed-approach point instead of using the ILS decision altitude. If there are no published LOC minima, or the failure occurs too late to make a stable and authorised transition, execute the missed approach rather than inventing a descent path.
Why does this happen in flight simulators?
In a flight simulator, a missing glideslope is usually caused by geometry, mode selection, runway direction or scenery data rather than an actual transmitter outage. An add-on airport may contain a localizer without a correctly placed glideslope, or its facility data may disagree with the chart or navigation database being used.
- If every aircraft loses the glideslope at one runway, suspect the airport or navigation data.
- If one aircraft has the problem at several known full-ILS runways, check its NAV receiver, failure settings and avionics configuration.
- If the glideslope appears only nearer the airport, the aircraft may simply have been outside its simulated service volume.
- If the localizer is being approached from the reciprocal direction, check for an accidental back-course intercept.
Testing a default aircraft at an unmodified airport helps separate aircraft configuration from scenery faults. Our guide to how flight simulators represent glideslope reception covers source selection and back-course mistakes; FSX users can also follow the focused FSX glideslope troubleshooting checks.