Why does an ILS show the wrong runway or a false signal?

An ILS can appear to guide you to the wrong runway when the wrong frequency or approach is selected, the receiver catches the back side of a localizer, a high intercept captures a false glideslope lobe, or simulator scenery and navigation data disagree. An offset localizer may also be intentional.

In real-world aviation and flight simulators such as MSFS, FSX, X-Plane and Prepar3D, an ILS receiver simply follows the signal on the tuned channel. It does not know which runway the pilot intended. Ground installations are monitored, so selection errors, reception geometry and simulator data conflicts are more likely than a transmitter actually being aimed at the wrong runway.

The localizer and glideslope are separate signals. A moving localizer needle therefore does not prove that a valid full ILS has been received; our explanation of what localizer guidance provides without a glideslope covers that distinction.

What makes an ILS point towards the wrong runway?

The usual cause is that the receiver is following a real signal, but not the front-course signal for the intended approach.

• Wrong approach or frequency: Selecting the neighbouring parallel runway, the reciprocal runway or an outdated procedure can tune a different installation. An FMS may also auto-tune one frequency while the pilot expects another.
• Back-side localizer reception: A receiver behind the localizer antenna may detect rear radiation or an extended course. Indications can be reversed, unstable or apparently plausible. It is not a usable back-course approach unless that procedure is specifically published, and normal glideslope guidance should not be expected.
• Frequency reuse: ILS channels are reused at suitably separated airports. Real-world frequency planning protects the published service volume, but an aircraft high above or well outside that volume may hear an unintended station. Some simulators model signal range too generously.
• Intentional offset: An offset localizer approaches the intended runway at an angle rather than following its centreline. The approach chart shows the final course and how the final alignment is completed. This is not a false signal.
• Side lobes or reflections: Terrain, buildings and large vehicles can distort a signal, especially outside protected coverage. ILS critical-area procedures reduce interference near the antennas, but a needle outside the published approach sector must not be treated as reliable guidance.

Why can a false glideslope look genuine?

A false glideslope can look stable because the antenna pattern may produce additional guidance lobes above the normal approach path. They are most likely to be encountered when joining high, descending towards the beam from above or approaching from outside the designed coverage sector.

The defence is procedural: establish on the localizer, remain at the charted intercept altitude and intercept the glideslope from below. Do not descend aggressively to chase a needle that appears while the aircraft is well above the published vertical profile. We explain the simulator-specific symptoms in our guide to false glideslope lobes and mismatched runway data.

How do I confirm that the ILS signal is correct?

A valid indication should agree with the published frequency, station identifier, final approach course, aircraft position and expected intercept altitude.

1. Check the procedure: Confirm the airport, runway, approach type, localizer frequency and published final course. Watch for runway renumbering or similarly named parallel approaches.
2. Identify the station: Listen for the Morse identifier or verify the decoded identifier displayed by the avionics. A missing or incorrect identifier is a warning that the intended facility has not been confirmed. Some simulator aircraft do not model identification fully, so use the other checks as well.
3. Check the intercept geometry: Approach the front course within the published sector, capture the localizer first and join the glideslope from below. Our guide to setting up a controlled localizer intercept explains the correct geometry.
4. Cross-check independent information: Compare the indicated course with the chart, runway orientation, DME or GNSS position and charted crossing altitudes. Allow for a published offset, but not an unexplained path towards another runway.
5. Verify the receiver and source: Confirm that the display and autopilot are using the radio actually tuned to the ILS, rather than another NAV receiver or the GPS/FMS source. The course selector does not move the transmitted beam, but an incorrect course can confuse the HSI presentation, flight director or autopilot logic.
6. Reject conflicting guidance: In real aircraft, unexplained disagreement calls for discontinuing the approach, flying the published missed approach or following ATC instructions. A centred needle is not sufficient reason to continue when the identifier, position or vertical profile is wrong.

Why does this happen in a flight simulator?

The most common simulator-only cause is disagreement between the airport scenery, navigation database and aircraft avionics. One component may represent a moved threshold or renumbered runway while another still places the ILS on the previous alignment.

If every aircraft shows the same displaced beam at one simulator airport, suspect scenery or navigation data. If only one aircraft is affected, check its avionics, receiver selection and autopilot configuration. A wrong identifier points to another installation; a glideslope captured far above profile points to a false lobe.