Front / Back Course Tracking
Localizer front and back course tracking with normal and reverse sensing: manipulate heading and position and observe the indicator.
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The localizer antenna sits beyond the far end of the runway. Its course is a narrow wedge along the extended centerline in BOTH directions: the front course (the ILS approach, toward the runway threshold) and the back course (the opposite direction, beyond the antenna). Drag the aircraft anywhere, drag the cyan ring to turn it, or press Fly and steer with the 10° buttons.
Watch the needle labels on the map: the left half of the front course gives a needle to the right and the right half a needle to the left — and the same geographic halves continue onto the back course. That is why a conventional CDI, which does not know your heading, is directional when you fly the front course inbound or the back course outbound, and reversed when you fly the back course inbound or the front course outbound: there you must turn away from the needle.
Switch to the HSI and set its course arrow to the front course: the deviation bar is mounted on the arrow, so on the back course the arrow points behind you and the bar sits on the side where the course really is — normal sensing on every leg. Set the arrow to the back-course heading instead and the picture is reversed again. Practice gives you a leg, a display and a needle; choose the correction, then fly it.
Assumption: an ideal localizer — the needle is the angular offset from the extended centerline seen from the antenna, linear to full scale; the back course has the same angular width as the front course (actual back-course widths and usable distances are set by flight inspection). Outside the AIM standard service volume the receiver is shown flagged with the needle parked. Some receivers have a back-course (reverse-sensing) switch; it is not modelled. Use back-course signals for an approach only where a back course procedure is published and ATC authorizes it (AIM 1-1-9).
Course width (full fly-left to full fly-right): tailored to 700 ft at the runway threshold, width = 2·atan(350 ft / antenna-to-threshold distance), capped at 6.0° when that distance is under 6,678 ft (FAA Order 6750.16E para 2-1c; AIM 1-1-9 b: 700 ft at the threshold). 150 µA = full scale = five dots. The IFH (ch. 9) gives “normally 5°”, full scale 2.5° either side; with the default 7,000-ft runway and a 1,000-ft setback the tailored width is 5.01°. A hand-set width (3°–6°) is offered to explore other values.
Why “3° to 6°”: because the width is tailored to 700 ft at the threshold, the angle follows the antenna-to-threshold distance D — 2·atan(350/D) gives 6.0° at D = 6,678 ft (below that the cap holds it at 6.0°), 5° at about 8,000 ft and 3° at about 13,360 ft (the longest runways plus the antenna setback). The 3°–6° range taught in training texts is that consequence, not a separate FAA limit: Order 6750.16E sets the 700-ft tailoring and the 6.0° maximum, and allows a Category I localizer to be non-tailored within 6.0° maximum and 400 ft at the threshold minimum. Platform classification: 700 ft and 6.0° = FAA siting criteria; 3°–6° = training convention. Full scale is half the width either side (2.5° for a 5° course), four times as sensitive as a VOR's 10° — so the localizer needle moves fast near the runway.
Receiver: angle = atan(lateral / along) seen from the antenna (+ = right of the front course as flown inbound, the same geographic side on the back course); needle = −angle / (width / 2), clipped at full scale, independent of heading and OBS (IFH ch. 9 “ILS Function”). With a 5° width on the front course this equals the IFR Trainer’s ILS localizer needle exactly. Coverage: ±10° to 18 NM and 10°–35° to 10 NM (AIM 1-1-9 b), applied to both courses.
Sensing: the correction side is geometric (where the centerline is relative to the heading); the indicated side is the needle (CDI) or the bar's left/right position on the heading-up HSI (needle × cos(course − heading)). CDI: normal when the heading is within 90° of the front course, reverse otherwise (AIM 1-1-9 NOTE, IFH ch. 9). HSI: normal on every heading when the arrow is on the front course (IPH FAA-H-8083-16B p. 4-76), reversed on every heading when it is on the reciprocal. The rule and the geometry are proven identical on 20,000 random cases. Flying: standard-rate turns toward the commanded heading, still air, fixed 0.1-s steps.
- FAA Aeronautical Information Manual, para. 1-1-9 b Localizer (faa.gov HTML, accessed 2026-10-14) — antenna at the far end of the runway; course width 700 ft full scale at the threshold; back course; NOTE on steering opposite the needle inbound on the back course and outbound on the front course; standard service volume 10°/18 NM and 35°/10 NM
- FAA Order 6750.16E Siting Criteria for Instrument Landing Systems (2014), para. 2-1c — 150 µA full scale; tailored 700-ft width; course width capped at 6.0° below 6,678 ft antenna-to-threshold; Category I may be non-tailored within 6.0° maximum and 400 ft at the threshold minimum (no separate 3° minimum — the taught 3°–6° range follows from tailoring); left of course → needle right
- FAA Instrument Flying Handbook, FAA-H-8083-15B (2012), ch. 9 Navigation Systems — “ILS Function” (needle independent of heading and OBS; directional inbound on the front course and outbound on the back course; flying away from the needle otherwise), localizer “normally 5°”; ch. 5 HSI
- FAA Instrument Procedures Handbook, FAA-H-8083-16B (2017), ch. 4, p. 4-76 “Localizer Back Course” — reverse sensing with standard VOR equipment; eliminated on an HSI set to the front course
- Cross-tool: with a 5° course width the front-course needle equals the ILS localizer needle of the IFR Trainer
- Garmin G1000 Pilot’s Guide for Cessna Nav III, 190-00498-08 Rev. A (system software 0563.30 or later), pp. 56-68 — the glass HSI option (Indicator: HSI (glass)): heading-up compass card, course pointer with a two-dot deviation scale (full scale as on a mechanical CDI for VOR and LOC), green for VOR / LOC and magenta for GPS, flight-phase and OBS / SUSP annunciations, glideslope / glidepath indicator; an equipment example, not a statement about every glass display