Back-course ILS approach: what is it and how do you fly it?

A back-course ILS approach, formally published as LOC BC, uses the rear localizer signal to guide an aircraft towards the reciprocal runway. Fly only a published procedure, configure back-course sensing correctly, ignore any glideslope indication, obey charted step-down altitudes and MDA, then land or go missed at the published point.

In Aviation & Real-World Flying, the chart title is usually LOC BC, meaning localizer back course. Calling it an ILS is convenient shorthand, but it normally provides lateral guidance only and is therefore a non-precision, or 2D, approach. Our guide to lateral-only and 3D instrument approaches explains the distinction.

The same radio-navigation principle applies in Microsoft Flight Simulator, FSX, X-Plane and Prepar3D, although navigation databases, signal modelling and mode labels vary between simulators and aircraft.

What is a LOC BC approach?

A LOC BC procedure uses the localizer signal radiated on the opposite side of its aerial from the normal front-course approach. This rear signal can support an approach towards the reciprocal runway when a specific back-course procedure has been designed and published.

A receivable signal does not by itself create an authorised approach. Confirm that LOC BC is published, available and suitable for the aircraft; in real-world flying, check any applicable outage, equipment requirement or restriction. Never improvise a back course merely by tuning the opposite runway's ILS.

The charted course is approximately reciprocal to the front course, but always use the published figures. For an approach from the normal side, use the standard front-course ILS technique instead.

How do you fly a localizer back-course approach?

Brief LOC BC as a non-precision approach, establish the correct lateral sensing while level, then descend using the charted fixes and altitudes rather than a glideslope.

1. Confirm that the procedure exists. Select the exact runway and LOC BC procedure from the chart or simulator database. An ILS serving the reciprocal runway does not automatically provide a usable back-course approach.
2. Brief the complete approach. Record the frequency and identifier, final approach course, required course-selector setting, intercept altitude, fixes, step-down restrictions, category-specific MDA, missed-approach point and missed procedure. If these details are unfamiliar, review how to read the courses, fixes, minima and missed approach on an instrument chart.
3. Tune and identify the localizer. Activate the correct NAV receiver and confirm the identifier where the simulator models it. Select NAV, LOC or VLOC as required; loading the procedure does not always switch the CDI away from GPS or FMS guidance.
4. Set the course for the fitted equipment. A traditional HSI commonly requires the front-course heading, which is reciprocal to the LOC BC inbound course. For example, an inbound back course of 090 degrees normally means selecting 270 degrees. A basic CDI may retain reverse sensing regardless of its OBS setting, so follow the aircraft or avionics documentation.
5. Intercept while level. Join through the published procedure or an ATC vector, normally using a modest intercept angle. Remain at the charted altitude until established and until the procedure permits descent; needle movement alone is not authority to descend.
6. Select and verify the guidance mode. Arm BC or BCRS where fitted and check the flight-mode annunciation. Without supported back-course logic, hand-fly using the known sensing of that display. Do not use ordinary APP or APR unless the documented avionics logic supports the coded LOC BC procedure.
7. Track and descend from the chart. Make small heading changes as localizer sensitivity increases near the aerial. Cross-check fixes or DME, comply with every step-down restriction and level at the MDA. An advisory VNAV path never cancels a published altitude restriction.
8. Land or execute the missed approach. Continue below MDA only when the required visual references are available and the aircraft can make a normal landing. Otherwise, begin the published missed approach at the MAP.

Why does a back-course localizer show reverse sensing?

Reverse sensing occurs because a conventional localizer indicator is designed around the front course, while the aircraft is approaching from the opposite direction.

A mistake we see constantly is applying “fly away from the needle” to every cockpit. That rule works only with an uncorrected reverse-sensing display. An HSI, flight director or dedicated BC mode may already provide normal command guidance, in which case flying away from it makes the error worse.

Does a back-course ILS have a glideslope?

A standard LOC BC approach has no authorised ILS glideslope, even if the cockpit's glideslope pointer moves. A rear lobe, false signal or simulator limitation can produce a convincing indication that is reversed, unreliable or outside protected coverage.

Use the barometric altimeter, charted fixes or distances, step-down altitudes, MDA and MAP. A charted descent angle, calculated continuous descent or advisory VNAV path can help with vertical planning where permitted, but it does not turn LOC BC into a precision approach.

Can the autopilot fly a LOC BC approach?

An autopilot can fly the lateral portion only when its approved modes support back-course sensing or the loaded procedure provides the correct guidance. A dedicated BC or BCRS mode typically reverses the lateral commands and prevents normal glideslope capture.

Do not assume that pressing APP is correct. Some integrated avionics use APP as part of their documented method for a coded back-course procedure; others interpret it as front-course localizer and glideslope capture. Watch the first capture command, verify the annunciated mode and control altitude or vertical speed separately.

Why will the autopilot not capture the back course?

• Wrong mode or course: front-course APP logic or an incorrectly selected course can command a turn away from the centreline.
• Wrong navigation source: a GPS or FMS source will not capture the raw NAV localizer unless the avionics deliberately integrate the loaded procedure.
• Poor intercept geometry: a steep intercept, late arming or attempting capture after crossing the beam can prevent stable acquisition.
• Unexpected glideslope capture: disengage the mode and return to the documented LOC BC configuration. Never descend on that indication.
• Database or scenery mismatch: the moving-map line, coded procedure and simulated radio signal may come from different data. Do not chase conflicting guidance.
• Missing procedure: the simulator may not contain or model it, or the real procedure may have been withdrawn. Choose another published approach rather than inventing one.

In FSX, the same diagnosis starts with the NAV1, NAV/VLOC and localizer controls used for an ILS. Whatever the platform, disconnect immediately if the autopilot turns away from the published course or commands an unexpected descent.

When should you go missed on a back-course approach?

Go missed at the published MAP without the required visual references, or earlier if reliable lateral guidance cannot be maintained. An unexpected autopilot turn, full-scale deflection, uncertain navigation source or descent below the charted profile is reason to abandon the approach rather than diagnose it close to the ground.