Why does reverse sensing occur on a back course localizer approach?

Reverse sensing happens because the localizer indications are mirrored; unless the HSI is set to the front course, left/right indications are opposite to the actual aircraft position.

Is the glideslope used during a back course localizer approach?

No, glideslope indications must be ignored since they only provide valid guidance for the front course approach.

Are back course localizer approaches approved in Europe?

No, back course approaches are generally not approved in European airspace, though they are common in North America.

How should the HSI or course selector be set for a back course approach?

The HSI should be set to the front course (the reciprocal of the back course heading) to ensure correct sensing.

Back Course Localizer

Q: What is a back course localizer?

A back course localizer is the rearward extension of the ILS localizer beam, used for non-precision approaches on the reciprocal runway.

Hard4 min readRadio Navigation

Moderately Examined

Why this matters

Understanding back course localizer procedures is crucial for safe non-precision approaches, especially when flying in regions where these approaches are published. Misinterpreting indications or instrument setup can lead to loss of situational awareness and compromise obstacle clearance.

A back course localizer is the rear-facing signal emitted by an ILS localizer antenna, allowing for non-precision approach procedures on the runway opposite the main instrument runway. Unlike standard ILS approaches, flying a back course approach requires careful attention to instrument settings and signal interpretation due to reverse sensing and the absence of glideslope guidance.

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What is a key characteristic of a back course localizer approach?

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In depth

Explanation

What is a Back Course Localizer?

The back course localizer refers to the mirror-image signal radiated from the rear of an ILS localizer antenna. This signal can be used for published non-precision approaches or departure procedures on the reciprocal runway, particularly in North America. In most of Europe, back course procedures are not approved for operational use, even though the signal may be present.

Signal Characteristics and Reverse Sensing

On a back course approach, the localizer indications are reversed compared to the front course. This means that if the aircraft is right of the centerline, the needle will deflect as if you are left, and vice versa. This phenomenon is known as localizer reverse sensing. Pilots must be aware that the standard course setting on an OBI does not correct this; the instrument will always display reverse sense on the back course.

Instrument Setup for Back Course Approaches

HSI (Horizontal Situation Indicator): To correct the reverse sensing, set the HSI course pointer to the front course (the inbound heading of the main instrument runway). This reverses the indications, making them display correctly for the back course.
OBI (Omni Bearing Indicator): The course setting does not affect ILS indications, so reverse sensing always occurs.
Autopilot/Flight Director: Some systems have a dedicated 'BC' (Back Course) mode to automatically correct the sense. Always consult the aircraft manual for specific procedures.

Operational Considerations

Back course approaches are non-precision: there is no glideslope guidance. If a glideslope signal is received, it must be ignored.
Obstacle clearance is not guaranteed beyond half-scale localizer deflection; immediate go-around is required if exceeded.
The localizer signal is only reliable within its protected area; outside this, false or reverse indications may occur.

Causes of Back Course Localizer Issues

Antenna back-scatter creates the rear-facing beam.
Side lobes and reflections can cause false centerline indications, especially outside protected coverage.
Incorrect instrument setup or misunderstanding of reverse sensing can lead to navigation errors.

The essentials

Key Points

A back course localizer is the rear signal from an ILS localizer antenna, used for non-precision approaches.

Reverse sensing occurs on back course approaches; the needle deflects in the opposite direction.

Set the HSI to the front course (main runway heading) for correct indications on a back course approach.

OBI instruments always display reverse sense on the back course, regardless of course setting.

Ignore glideslope indications on a back course approach—only localizer guidance is valid.

Back course procedures are not approved in most European airspace.

Exceeding half-scale localizer deflection requires an immediate go-around due to obstacle clearance limits.

Watch out

Exam Traps & Typical Mistakes

Assuming the OBI course setting corrects reverse sensing on the back course—it does not.

Believing glideslope guidance is valid on a back course approach—it must be ignored.

Thinking back course procedures are always approved—many regions, including most of Europe, prohibit their use.

Forgetting to set the HSI to the front course for correct sense indications.

Relying on localizer signals outside the protected area, where false or reverse indications may occur.

Test yourself

Example Exam Questions

Question 2Medium

When flying a back course localizer approach using an HSI, how should the course selector be set for correct sensing?

Question 3Easy

What must a pilot do if glideslope indications are received during a published back course localizer approach?

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