Localizer vs. Localizer Back Course: Reverse Sensing Explained, HSI Setup (Set the Front Course), Autopilot BC Mode, and Why You Disregard the Glideslope
Updated: Oct 14
At a glance, a localizer (LOC) approach and a localizer back course (LOC BC) approach look similar: both use the same ILS localizer transmitter for lateral guidance, both are nonprecision approaches flown to an MDA, and both are flown with localizer technique. The difference is which direction you’re using the localizer from — and that one fact changes how your needle behaves, how you set up the HSI, whether your autopilot will couple, and what you do with the glideslope indication. Pilots who understand why the back course reverse-senses never get caught by it; pilots who only memorized “fly away from the needle” sometimes do.
This post covers the front-course localizer and how it’s built, what the back course is and why it exists, reverse sensing explained from the signal up (including the four-quadrant rule that also covers flying outbound on the front course), the HSI fix (set the front course), the CDI reality (reverse sensing, full stop), autopilot BC mode, why you disregard the glideslope, the sensitivity difference near the runway, the AIM’s authorization rule, briefing items, a worked scenario, and the checkride questions.
Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >
The Front-Course Localizer: How It’s Built
Understanding the back course starts with understanding the front course.
The transmitter:
The localizer antenna array sits at the far (stop) end of the ILS runway, on the extended centerline
It radiates two overlapping lobes: one modulated at 150 Hz, one at 90 Hz
Your receiver compares the two; equal strength means on course, and the stronger one tells the needle which way to deflect
The course is the line where they’re equal — the extended runway centerline
The front course:
The side of the array that serves the ILS runway’s approach — the signal intended for landing on that runway
Course width is tailored between 3° and 6° so full-scale deflection is about 700 feet wide at the runway threshold
Flown inbound on the front course, the needle points toward the course: needle left, fly left
The “normal” localizer
Service volume (AIM 1-1-9):
Usable to 18 NM within 10° either side of the course
Usable to 10 NM from 10° to 35° either side
Beyond 35°, indications are not reliable
Intercept from inside the usable sector
A LOC approach:
An ILS flown without the glideslope (glideslope out of service, aircraft not equipped, or a localizer-only installation)
Flown to an MDA with step-down fixes
Aligned with the runway (a few ILSs have a slightly offset localizer, up to 3°, still called an ILS)
The baseline for everything that follows
What the Back Course Is
The other side of the same antenna.
The physics:
The localizer array radiates in both directions; the signal behind it — off the departure end of the ILS runway — is the back course
It’s the same 150 Hz / 90 Hz comparison from the same transmitter, just from the opposite side
On the back course, the lobes are on opposite sides of the course from where they are on the front course — which is the whole reverse-sensing story
Same frequency, same identifier, same antenna
The LOC BC approach:
Uses the back course signal for lateral guidance to the reciprocal runway (the one at the localizer’s end of the field)
Published as “LOC BC RWY XX” with its own plate, step-downs, and MDA
Nonprecision, no glideslope
Flown inbound toward the antenna
Why it exists:
Gives the opposite runway an instrument approach without installing a second localizer
Useful where terrain, cost, or siting prevents a second system
Common at airports with one ILS and a prevailing wind that sometimes reverses
Increasingly replaced by RNAV (GPS) approaches, but still published at many airports
Not every localizer has one:
Some localizer arrays are unidirectional and radiate little or no usable back course
Others radiate a back course that hasn’t been flight-checked and isn’t published
AIM 1-1-9: do not use back course signals for an approach unless a back course approach is published for that runway and authorized by ATC
No plate, no back course
Reverse Sensing, Explained From the Signal Up
Not a quirk — a geometry.
The front course, inbound:
The 150 Hz lobe is on the right of the course (as you face the runway), the 90 Hz lobe on the left
Your receiver is built so that “more 150 Hz” deflects the needle left (you’re right of course; fly left) — the needle points to the course
Normal sensing: fly toward the needle
What you learned on the ILS
Turn around — the front course, outbound:
Now you’re flying away from the runway on the same front course; the lobes haven’t moved, but you have
The 150 Hz lobe is now on your LEFT
If you drift into it (left of course), the receiver still says “more 150 Hz, deflect left” — the needle moves left, away from the course
Reverse sensing on the front course outbound — AIM 1-1-9 notes this explicitly
The back course, inbound:
Flying inbound on the back course, you’re facing the same direction as “front course outbound”
Same geometry: the 150 Hz lobe is on your left, the needle deflects away from the course
Reverse sensing: fly away from the needle to correct
The classic trap
The back course, outbound:
Facing the same direction as front course inbound
Normal sensing again
(This is why a procedure turn outbound on the back course feels normal, and the inbound turn is where the sensing flips)
The four-quadrant rule:
Where you are | Direction | Needle sensing (plain CDI) |
Front course | Inbound (toward the ILS runway) | Normal — fly toward the needle |
Front course | Outbound | Reverse — fly away from the needle |
Back course | Inbound (toward the antenna / BC runway) | Reverse — fly away from the needle |
Back course | Outbound | Normal — fly toward the needle |
The one-sentence version:
Your needle senses normally whenever you’re facing the same direction as a front-course inbound — and reverses whenever you’re facing the opposite way
It’s about your heading relative to the front course, not which side of the antenna you’re on
Front course inbound and back course outbound: normal. Front course outbound and back course inbound: reverse
The localizer never changes; you do
The HSI Fix: Set the Front Course
The single most useful thing to know about back courses.
Why an HSI can fix it:
An HSI’s course deviation bar rotates with the course pointer, so the deviation is displayed relative to the course you set
If the course pointer is set to the FRONT course inbound (the ILS runway’s course), the deviation bar is oriented to that direction
When you fly the back course inbound (the reciprocal), the course pointer is now at the bottom of the HSI, pointing behind you — and the deviation bar’s deflection is displayed on the correct side relative to you
The HSI geometrically un-reverses the indication
The rule:
On an HSI, flying ANY localizer approach — front or back — set the FRONT course inbound on the course pointer
Back course approach: set the front course; the pointer sits at the tail; sensing is normal — fly toward the bar
If you set the BACK course on an HSI, you’ve recreated reverse sensing on an instrument that didn’t need to have it
Front course, always
Reading the chart:
The LOC BC plate shows the back course inbound (e.g., 180°) as the final approach course; the front course is the reciprocal and is identifiable on the chart — Jeppesen labels it (“Front Crs 360°”), and FAA charts show it with the localizer feather, which points along the front course toward the ILS runway
Set the front course on the HSI; fly the back course heading
Your heading indicator reads 180; your course pointer reads 360 at the tail
Confirm before the FAF
Verify it:
Before you’re established, check that a drift off course moves the bar in the expected (normal) direction
If the bar moves the wrong way, the course pointer is set wrong
One check, well before the FAF
Setup, verified, then flown
The CDI Reality: Reverse Sensing, Full Stop
A plain CDI (the round OBS-type indicator) has no fix.
Why the OBS doesn’t help:
On a localizer signal, the OBS setting has no effect on the needle — the receiver drives the needle directly from the 150/90 comparison
Setting the front course, the back course, or anything else changes nothing about the needle’s behavior
The needle will reverse-sense inbound on the back course regardless
(Setting the inbound course anyway is good practice for reference)
How to fly it:
Fly away from the needle: needle right, turn left; needle left, turn right
Many pilots pre-brief a mantra (“back course, pull the needle, don’t push it”) and say it out loud before the FAF
Make small corrections and watch the needle move toward center — that confirms you’re correcting the right way
The trap is instinct under workload; the cure is a deliberate, briefed technique
Some navigators reverse it for you:
Some GPS/nav units and glass displays have a back-course mode that reverses the needle output so a plain CDI senses normally
If yours does, know how to engage it and what the annunciation looks like (often “BC”)
If you’re not sure what mode you’re in, assume reverse sensing and verify with a small correction
Don’t let the mode be a surprise
Partial panel and raw data:
On a checkride or in a failure, you may be on a basic CDI with no help
The four-quadrant rule and “fly away from the needle” are the skill
Practice it in VMC before you need it in IMC
Know it cold
The Autopilot: BC Mode and Its Variations
Automation behaves differently across installations.
Why the autopilot cares:
An autopilot tracking a localizer steers toward the needle; inbound on a back course with reverse sensing, that steers it AWAY from the course
It will diverge confidently
The autopilot needs to be told the sensing is reversed
Hence BC mode
The variations:
Some autopilots have a dedicated BC (or REV) mode that reverses the localizer input; engage it and NAV/APR tracking works normally
Some installations with an HSI track correctly in NAV mode as long as the HSI course pointer is set to the front course (because the HSI’s output is already corrected) — but not all
Some autopilots will not couple to a back course at all; the AFM supplement will say so
Some will “couple” but command the wrong turns if the mode or course setting is wrong
The discipline:
Know your specific autopilot’s back-course behavior from the AFM supplement and from practice in VMC
Engage the correct mode before intercepting, and verify the autopilot turns the right way on the first correction
Be ready to disconnect and hand-fly
Never discover your autopilot’s BC behavior at 1,000 feet in IMC
Vertical:
There’s no glideslope to couple to (below); descent is your job — vertical speed or pitch mode, altitude preselect at each step-down and then the MDA
Treat it as a LOC-only approach for vertical purposes
Monitor
Automation flies the needle; you fly the altitudes
The Glideslope: Disregard It
The AIM is explicit, and the reason is physics.
The AIM 1-1-9 instruction:
False glide slope signals may exist in the area of the localizer back course, which can cause the glide slope flag to disappear and present unreliable glide slope information
Disregard all glide slope signal indications when making a localizer back course approach, unless a glide slope is specified on the approach chart
A clear flag does NOT mean a valid glideslope on the back course
“Disregard” means disregard
Why:
The glideslope antenna is beside the front-course touchdown zone, radiating toward the front-course approach
Behind it, reflections and sidelobes can produce a signal strong enough to clear the flag but with no meaningful geometry
Following it could mean descending on a path that has nothing to do with the back-course runway
It isn’t a glideslope; it’s noise that looks like one
In the cockpit:
Expect the glideslope needle to wander, center, or flag intermittently on a back course — ignore it
On glass displays, the glideslope indicator may be suppressed in BC mode; if it isn’t, mentally remove it
Fly the step-down fixes and the MDA
Any “+V” from a GPS is advisory only
The rare exception:
A very small number of back course procedures have a specifically engineered and charted glideslope; the chart will say so explicitly
If the plate doesn’t specify a glideslope, there isn’t one for your purposes
Chart governs
Default: no glideslope
Sensitivity: The Back Course Gets Tighter Near the Runway
A difference pilots feel but rarely understand.
The geometry:
The localizer antenna sits at the stop end of the front-course runway — which is the APPROACH end of the back-course runway
On the front course, you cross the threshold roughly a runway length from the antenna, where the course is tailored to about 700 feet wide
On the back course, you cross the threshold right next to the antenna, where the same angular course width is only a few feet wide
The needle becomes extremely sensitive as you approach the back-course threshold
What it feels like:
Inside the FAF on a back course, the needle responds faster and faster to small drifts
Near the MDA and the MAP, a tiny heading change swings the needle
Pilots who chase it end up S-turning down final
The back course is twitchy where the front course is calm
The technique:
Establish a heading that holds the needle and leave it alone; make 1°–2° corrections, not 5°
Accept a small, stable deflection rather than chasing center near the runway
Transition to visual references early once the runway environment is in sight
The same “don’t chase it inside the FAF” rule, applied harder
The flip side:
Far from the antenna — outside the FAF on the back course — the needle is relatively lazy
Intercept and tracking feel easy, which can lull you before the tightening
Expect the sensitivity to ramp up continuously as you close on the antenna
Brief it
Markers and Fixes on a Back Course
What defines the step-downs and the MAP.
Back course marker (BCM):
Some back course approaches have a back course marker beacon, which identifies the FAF or a fix on the BC final
Its aural identification is a series of pairs of dots on a 3000 Hz tone, with a white marker light
Not all BC approaches have one; many use DME or a GPS-defined fix instead
Check the plate
DME and GPS fixes:
Most modern BC plates define the FAF, step-downs, and MAP by DME (from the localizer-associated DME) or by named fixes you can identify with GPS
Load the approach from the GPS database for fix identification and situational awareness, while navigating on the localizer signal
Know how the MAP is defined before the FAF
Timing from the FAF if nothing else
The front-course markers don’t apply:
The outer and middle markers belong to the front-course ILS; you won’t cross them on the back course
Don’t look for them
Use what’s on the BC plate
Different approach, different fixes
The Briefing Items That Prevent Most LOC BC Mistakes
Brief these explicitly, before the FAF, every time.
1. Front course and back course:
State both: “Front course 360, back course 180, I’m flying 180 inbound”
HSI: course pointer set to the front course (360); heading will be 180
CDI: expect reverse sensing; “fly away from the needle”
Say which instrument you have and what it will do
2. Sensing mode:
GPS/nav/autopilot in BC mode or not; what the annunciation shows
Autopilot coupled or hand-flown; what the autopilot does on a back course in this airplane
The first correction will verify it
Decide now, not on final
3. No glideslope:
Disregard the glideslope indication; expect it to behave erratically
Step-down fixes, altitudes, and the MDA
How the MAP is defined
VDP if published
4. Sensitivity:
The needle tightens toward the runway; small corrections
Don’t chase it near the MDA
Transition visual early
Expect twitchiness
5. The missed approach:
Read it twice; back-course missed approaches often turn away from the front-course ILS final to keep you clear of that traffic
Note the initial heading and whether it reverses toward terrain or the front course
Climb first, then navigate
Know it cold
6. Authorization:
The approach is published and ATC cleared you for the LOC BC
A clearance for “the ILS” or “the localizer” is not a back-course clearance; confirm
No published BC → no back course approach
Verify the clearance
Worked Scenario
The setup:
ILS RWY 36 at the airport; LOC BC RWY 18 published. Front course 360, back course 180. HSI-equipped, autopilot with a BC mode. FAF at DME 6.0, step-down at DME 3.0, MDA 1,200, MAP at DME 1.0. Vectors to a 30° intercept from the east
Flying it:
Tune and identify the localizer (same identifier as the ILS). Set 360 on the HSI course pointer — it sits at the tail; heading will be 180. Autopilot BC mode armed; confirm the “BC” annunciation. Glideslope: disregard — brief it out loud
Intercept: the bar comes alive; the autopilot turns toward it — correct direction, verified. Established inbound on 180. Configured and at approach speed before DME 6.0
At the FAF: descend, altitude preselect to the step-down altitude, then to 1,200. The glideslope needle flags, centers, and wanders — ignored. Needle gets progressively more sensitive inside DME 3.0; small corrections only
At the MDA: level, continue toward DME 1.0; runway in sight at DME 1.8, aligned; visual transition; land
Not in sight at DME 1.0: published missed, which turns away from the ILS 36 final
The CDI version:
Same approach with a plain CDI and no BC mode: OBS set to 180 for reference (it does nothing to the needle). Inbound, the needle reverse-senses: needle right → turn left. Hand-flown; mantra said aloud before the FAF; first correction verifies it
Same vertical discipline, same sensitivity, same missed
Common Pilot Mistakes
Setting the BACK course on an HSI (recreating reverse sensing on the one instrument that could have fixed it)
Assuming the OBS setting changes a CDI’s behavior on a localizer (it doesn’t)
Chasing the needle near the runway where the back course is extremely sensitive
Following a glideslope indication on a back course because the flag cleared
Coupling an autopilot without BC mode (or without knowing its back-course behavior) and watching it diverge
Not verifying the sensing with the first correction before the FAF
Looking for the front-course outer/middle markers on a back course
Flying a back course that isn’t published, or on an ILS/LOC clearance that wasn’t a BC clearance
Forgetting that flying outbound on the front course (procedure turn) also reverse-senses
Not briefing the missed approach, which often turns sharply away from the front-course final
Checkride Questions — With Answers
“What is a localizer back course approach?”
An approach that uses the back side of an ILS localizer signal for lateral guidance to the reciprocal runway. Same transmitter, frequency, and identifier as the ILS; nonprecision; flown to an MDA with no glideslope.
“Why does the needle reverse-sense on a back course?”
The localizer’s 150 Hz and 90 Hz lobes are fixed in space. Inbound on the front course, the needle points toward the course. Facing the opposite direction — inbound on the back course, or outbound on the front course — the lobes are on the opposite sides of you, so the needle deflects away from the course. Normal sensing whenever you face the front-course inbound direction; reverse whenever you face the opposite way.
“How do you set up an HSI for a back course?”
Set the FRONT course inbound on the course pointer (it sits at the tail). The HSI’s deviation bar is then displayed relative to that course and senses normally on the back course — fly toward the bar.
“Does setting the OBS fix reverse sensing on a CDI?”
No. On a localizer signal the OBS has no effect on the needle. A plain CDI reverse-senses inbound on the back course regardless of the OBS setting; fly away from the needle (unless the navigator has a BC mode that reverses the output).
“What do you do with the glideslope on a back course?”
Disregard all glideslope indications (AIM 1-1-9) unless a glideslope is specifically charted for the back course. False glideslope signals can clear the flag and present unreliable guidance.
“Can you fly a back course wherever a localizer exists?”
No. Only where a back course approach is published for that runway and authorized by ATC. Many localizers don’t radiate a usable back course.
“Why is the needle more sensitive near the runway on a back course?”
The antenna is at the stop end of the ILS runway — the approach end of the back-course runway — so you cross the back-course threshold right next to it, where the course is only a few feet wide.
Final Thoughts
A front-course localizer is the needle you learned on: toward the course, calm at the threshold, with a glideslope if the ILS is working. The back course is the same signal from behind, and everything that’s different about it follows from that one fact — the needle reverses because you’ve turned around relative to the lobes, the HSI fixes it because setting the front course re-orients the display, the autopilot needs to be told, the glideslope is noise, and the needle tightens because you’re flying toward the antenna instead of away from it.
Brief it that way — front course on the HSI, sensing mode confirmed and verified with the first correction, glideslope disregarded, small corrections near the runway, missed approach read twice — and a back course is just another localizer approach with a couple of extra lines in the briefing. Skip the briefing and it becomes the approach that’s earned its reputation.
On the Written Test and Checkride
Localizer back courses appear on the instrument knowledge test and in instrument orals and, where available, on the practical test. The most commonly tested topics:
Reverse sensing and why (the four-quadrant rule, including front course outbound)
HSI: set the front course; CDI: OBS has no effect, fly away from the needle
Disregard the glideslope on a back course (AIM 1-1-9)
Back course approach must be published and authorized by ATC
Nonprecision — flown to an MDA
Autopilot BC mode
Back course sensitivity near the antenna
Quick Reference
Localizer (front course):
Antenna at the stop end of the ILS runway; 150 Hz (right of course, facing the runway) and 90 Hz (left) lobes
Course width 3°–6°, ~700 ft at the threshold
Service volume: 10° either side to 18 NM; 10°–35° to 10 NM
LOC approach = ILS without glideslope → MDA
Back course:
Same antenna, frequency, and identifier; the signal behind the array
Serves the reciprocal runway; published as “LOC BC RWY XX”; nonprecision → MDA
Use only where a BC approach is published and ATC-authorized (AIM 1-1-9); many localizers don’t radiate a usable back course
Sensing — the four-quadrant rule (plain CDI):
Inbound | Outbound | |
Front course | Normal | Reverse |
Back course | Reverse | Normal |
Normal whenever you’re facing the front-course inbound direction; reverse whenever you face the opposite way
HSI:
Set the FRONT course inbound on the course pointer — always, for any localizer approach
Back course: pointer at the tail, heading is the reciprocal, sensing is normal (fly toward the bar)
Setting the back course on an HSI = reverse sensing you didn’t need
Verify with the first correction before the FAF
CDI:
OBS has no effect on a localizer needle
Inbound on the back course: reverse sensing — fly away from the needle
Some navigators have a BC mode that reverses the output → normal sensing; know the annunciation
Autopilot:
Needs BC/REV mode (or an HSI set to the front course, on some installations); some won’t couple at all — check the AFM supplement
Verify the first turn; be ready to disconnect
Vertical is manual — preselect step-downs, then the MDA
Glideslope:
Disregard all glideslope indications on a back course unless one is specifically charted (AIM 1-1-9)
A cleared flag ≠ a valid glideslope; false signals exist behind the array
“+V” = advisory only
Sensitivity:
Back-course threshold is right at the antenna → the needle becomes extremely sensitive near the runway
1°–2° corrections inside the FAF; don’t chase; transition visual early
Lazy far out, twitchy close in
Fixes:
Back course marker (if any): pairs of dots, 3000 Hz, white light
Most BC plates: DME or GPS-defined fixes; front-course OM/MM don’t apply
Know how the MAP is defined
Briefing (every time):
Front course / back course stated; HSI set to front course; CDI → reverse
Sensing mode (BC) and autopilot behavior; verify on first correction
Glideslope disregarded; step-downs, MDA, MAP definition, VDP
Sensitivity near the runway
Missed approach — often turns away from the front-course final
Published and cleared for the LOC BC (not “the ILS”)
Key Principle:
A back course is the same localizer flown from behind. The needle reverses because you’ve turned around relative to the lobes — so set the front course on an HSI (normal sensing), fly away from the needle on a plain CDI, put the autopilot in BC mode or hand-fly, disregard the glideslope, expect the needle to tighten toward the antenna, and fly it as a nonprecision approach to the MDA — only where it’s published and you’re cleared for it.
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Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.