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5 Answers

Lateral Accuracy of Localizer

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Instrument Rating

Hi. I have a quick question. The lateral range of the localizer is 3-6 degrees. Does this mean that as long as the aircraft stays within the 3 to 6 degrees of the localizer signal, the CDI stays in the center, or that you get a full deflection (either to the left or right) when you are precisely off 3 to 6 degrees of the signal?

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5 Answers

  1. Best Answer

    Russ Roslewski on Jun 01, 2017

    It means full deflection is +/- 3-6 degrees.

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  2. John Scarry on Jun 01, 2017

    I think you are referring to an SDA—Simplified Directional Facility installation, which may have a course width of 6 or 12 degrees. Localizers and Localizer Type Directional aids have a width of 10 degrees either side of the course along a radius of 18 NM from the antenna; and
    from 10 to 35 degrees either side of the course along a radius of 10 NM. AIM 1-1-9

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  3. Russ Roslewski on Jun 01, 2017

    John, that’s the operational service volume, meaning the signal is guaranteed to provide reliable guidance within that range – even if that guidance is “full course deflection”. Outside of those limits, what it’s reading may not be accurate.

    The actual signal course width is adjusted to be +/- 700 feet at the runway threshold. (AIM 1-1-9b2. Which, for a 6000 foot runway with the localizer antenna 1000 feet past the departure end, gives a full-scale deflection of 5.7 degrees by my math. A longer runway would have a narrower deflection angle.

    However, there’s no real need for the pilot to actually know the angle at which maximum deflection occurs, as long as they know how to keep the needle in the center. But it is interesting trivia to realize that longer runways will have more sensitive localizers.

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  4. John D Collins on Jun 01, 2017


    The full left to full right scale is 700 feet, but the course is center +/- 350 feet. So your angle is arc tan (350/6000) = 3.3 degrees.

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  5. Russ Roslewski on Jun 01, 2017

    Thanks for the correction John. I knew that, just got carried away by the numbers. And poor proofreading.

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