Back Course Localizer
If you can fly an ILS without the glide slope, you can fly a back course localizer. Because you are closer to the localizer antenna, the course information is more narrow and can seem "sensitive." Just don't forget the switch that allows the flight director to get its mind right about the front course versus the back course. Also remember that most aircraft will not allow you to fly the FMS, "blue needles" with a localizer tuned.
General Description
Equipment Requirements
- The back course localizer approach provides the lateral guidance of an ILS approach without the glide slope information.
- Instrument landing system (ILS): An electronic system that provides both horizontal and vertical guidance to a specific runway, used to execute a precision instrument approach procedure. The ILS system provides both course and altitude guidance to a specific runway. The ILS system is used to execute a precision instrument approach procedure or precision approach. The system consists of the following components:
- A localizer provides horizontal (left/right) guidance along the extended centerline of the runway.
- A glide slope provides vertical (up/down) guidance toward the runway touchdown point, usually at a 3° slope.
- Marker beacons provide range information along the approach path.
- Approach lights assist in the transition from instrument to visual flight.
Equipment Requirements
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Localizer Coverage Limits
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Marker Beacons
Compass Locator
Compass locators are low-powered NDBs and are received and indicated by the ADF receiver. When used in conjunction with an ILS front course, the compass locator facilities are collocated with the outer and/or MM facilities. The coding identification of the outer locator consists of the first two letters of the three-letter identifier of the associated LOC. For example, the outer locator at Dallas/Love Field (DAL) is identified as “DA.” The middle locator at DAL is identified by the last two letters “AL.”
Approach Lighting Systems (ALS)
Normal approach and letdown on the ILS is divided into two distinct stages: the instrument approach stage using only radio guidance, and the visual stage, when visual contact with the ground runway environment is necessary for accuracy and safety. The most critical period of an instrument approach, particularly during low ceiling/visibility conditions, is the point at which the pilot must decide whether to land or execute a missed approach. As the runway threshold is approached, the visual glide path will separate into individual lights. At this point, the approach should be continued by reference to the runway touchdown zone markers. The ALS provides lights that will penetrate the atmosphere far enough from touchdown to give directional, distance, and glide path information for safe visual transition.
- Two VHF marker beacons, outer and middle, are normally used in the ILS system. A third beacon, the inner, is used where Category II operations are certified. A marker beacon may also be installed to indicate the FAF on the ILS back course.
- The OM is located on the localizer front course 4 to 7 miles from the airport to indicate a position at which an aircraft, at the appropriate altitude on the localizer course, will intercept the glide path. The MM is located approximately 3,500 feet from the landing threshold on the centerline of the localizer front course at a position where the glide-slope centerline is about 200 feet above the touchdown zone elevation. The inner marker (IM), where installed, is located on the front course between the MM and the landing threshold. It indicates the point at which an aircraft is at the decision height on the glide path during a Category II ILS approach. The back-course marker, where installed, indicates the back-course FAF.
Compass Locator
Compass locators are low-powered NDBs and are received and indicated by the ADF receiver. When used in conjunction with an ILS front course, the compass locator facilities are collocated with the outer and/or MM facilities. The coding identification of the outer locator consists of the first two letters of the three-letter identifier of the associated LOC. For example, the outer locator at Dallas/Love Field (DAL) is identified as “DA.” The middle locator at DAL is identified by the last two letters “AL.”
Approach Lighting Systems (ALS)
Normal approach and letdown on the ILS is divided into two distinct stages: the instrument approach stage using only radio guidance, and the visual stage, when visual contact with the ground runway environment is necessary for accuracy and safety. The most critical period of an instrument approach, particularly during low ceiling/visibility conditions, is the point at which the pilot must decide whether to land or execute a missed approach. As the runway threshold is approached, the visual glide path will separate into individual lights. At this point, the approach should be continued by reference to the runway touchdown zone markers. The ALS provides lights that will penetrate the atmosphere far enough from touchdown to give directional, distance, and glide path information for safe visual transition.
Regulatory Requirements
Pilots must be instrument rated and the aircraft be equipped with the appropriate airborne equipment to execute back course localizer approaches.
Advantages
The Localizer Back Course provides an accurate centerline.
Disadvantages
The Localizer Back Course provides several opportunities to make mistakes setting the back course instead of the front course or forgetting to press the flight director's BC button, for example.
It also tempts one to "dive and drive." You can, and should, fly a Continuous Descent Final Approach for most Localizer Back Course approaches.
Limitations
The ILS and its components are subject to certain errors, which are listed below. Localizer and glide-slope signals are subject to the same type of bounce from hard objects as space waves.
Pilots must be instrument rated and the aircraft be equipped with the appropriate airborne equipment to execute back course localizer approaches.
Advantages
The Localizer Back Course provides an accurate centerline.
Disadvantages
The Localizer Back Course provides several opportunities to make mistakes setting the back course instead of the front course or forgetting to press the flight director's BC button, for example.
It also tempts one to "dive and drive." You can, and should, fly a Continuous Descent Final Approach for most Localizer Back Course approaches.
Limitations
The ILS and its components are subject to certain errors, which are listed below. Localizer and glide-slope signals are subject to the same type of bounce from hard objects as space waves.
- Reflection. Surface vehicles and even other aircraft flying below 5,000 feet above ground level (AGL) may disturb the signal for aircraft on the approach.
- False courses. In addition to the desired course, glideslope facilities inherently produce additional courses at higher vertical angles. The angle of the lowest of these false courses will occur at approximately 9°–12°. An aircraft flying the LOC/glide-slope course at a constant altitude would observe gyrations of both the glide-slope needle and glide-slope warning flag as the aircraft passed through the various false courses. Getting established on one of these false courses will result in either confusion (reversed glide-slope needle indications), or result in the need for a very high descent rate. However, if the approach is conducted at the altitudes specified on the appropriate approach chart, these false courses will not be encountered.