5.localizer-back-course-procedure. Localizer Back Course Approach
The localizer back course (LOC BC) is a non-precision approach that uses the back azimuth of the ILS localizer antenna array. When a localizer transmitter radiates its course signal, energy is also broadcast off the back of the antenna in a mirror-image pattern. Some airports take advantage of this by publishing a separate instrument approach to the opposite runway end. The result is a usable course alignment without the cost of a second localizer installation. A back course is not always available: it must be flight-checked, charted, and published in the Chart Supplement and on an FAA approach plate before it can be flown.
Because the back course is the mirror of the front course, two operational realities dominate every back course procedure:
- There is no glideslope. The glideslope antenna is highly directional and is not usable from the back side. Vertical guidance must come from step-down fixes, the MDA, and timing or DME.
- The CDI exhibits reverse sensing unless the avionics compensate. With a conventional VOR/LOC indicator, the needle deflects away from the runway centerline rather than toward it, so corrections appear backward to the pilot.
Eliminating reverse sensing. Modern HSIs and most IFR-capable GPS navigators offer a back course mode. On an HSI, when the pilot sets the front course in the course pointer (not the back course inbound heading), the instrument automatically resolves the geometry and the CDI behaves normally—needle toward the centerline. With a conventional CDI that has no back course feature, the pilot must mentally reverse the needle: fly away from the needle to return to course. Many older installations include a BC button or annunciator; selecting it inverts the needle, but the OBS setting requirement depends on the manufacturer—consult the AFM/POH.
Tuning and identification. A back course uses the same localizer frequency (108.10–111.95 MHz, odd-tenth frequencies) as the front course ILS. The Morse identifier is the same three- or four-letter ident published on the chart. There is no separate audio identifier for the back course, so the pilot must confirm by chart that the back course is authorized and that the correct procedure is loaded.
Course width. Localizer course width is fixed in angular terms at the runway threshold, normally 700 feet wide (typically 3° to 6° total, tailored so the full-scale deflection equals 700 ft at threshold). Because the antenna is located at the far end of the runway from the front-course landing direction, the back-course beam is much narrower than the front course at the missed approach point. This makes the CDI extremely sensitive during the final segment of a back course approach—small heading changes produce large needle movement. Smooth, small corrections (typically 2°–5°) are essential.
Procedure flow.
- Brief the approach. Note that it is LOC BC, that there is no glideslope, the FAF, MDA, missed approach point (timed from FAF or DME), and the missed approach procedure.
- Tune the localizer frequency, identify by Morse code, and set the front course in the HSI course pointer (or activate BC mode per the avionics).
- Intercept the back course at the published altitude. Track the course inbound, applying small wind corrections.
- Cross the FAF, start the timer, and descend to the MDA. Comply with any step-down fixes.
- At the missed approach point (MAP)—identified by time, DME, or a fix—if the runway environment is not in sight under §91.175, execute the published missed approach.
Common errors.
- Setting the back course inbound heading in the course pointer of an HSI, which defeats the auto-sensing logic and produces reverse needle indication.
- Forgetting that the glideslope flag will be present; some pilots inadvertently chase a false glideslope indication from the front-course transmitter. The glideslope is unusable and must be ignored on a back course.
- Overcontrolling near the MAP due to increased course sensitivity.
- Failing to verify the avionics actually support back course operation. Some WAAS GPS units fly the LOC BC as an overlay; others require a specific procedure load.
Example. An aircraft is inbound on the LOC BC RWY 13 approach. The front course (the published ILS RWY 31) is 310°. The pilot sets 310° in the HSI course pointer even though the airplane is flying a 130° heading inbound. The CDI now displays normal sensing: a left deflection means the airplane is left of centerline, correct right. Crossing the FAF, the pilot starts the timer, descends to the published MDA of, say, 760 ft MSL, and looks for the runway environment at the MAP based on the timing table for the groundspeed flown.
The back course is a valuable tool at airports without full ILS to both runway ends, but its quirks—no vertical guidance, reverse sensing on conventional CDIs, and high course sensitivity—demand careful preparation and disciplined scan technique.