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4 Answers

What are the risks of intercepting a ILS GS from above?

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4 Answers

  1. Best Answer


    John D Collins on May 20, 2015

    1) Some autopilots will not intercept from above.
    2) Autopilots that intercept from above can give a wild ride in order to capture as the descent rate has to exceed normal.
    3) Depending on how high the intercept is from, you can get a false glideslope.
    4) There are certain failure modes of the GS/CDI/Nav radio that can’t be detected if you intercept from above.
    5) It is easy to still be above the GS when passing over the localizer FAF and therefore you can’t do the cross check of the altimeter at this point.
    6) Intercept from below with a stuck indicator with a full fly up is not dangerous, as you reach the FAF and don’t get an indication to descend, IOW you never see the GS indicator go from full scale to centered. This is a soft failure mode. A stuck indicator from above, there will not be any indication that can be relied on as the indicator will not change from a fly down indication. KCS55A systems are prone to this kind of failure.

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  2. Drew on May 21, 2015

    John, I appreciate your comprehensive answer. In reference to #3, could you explain how it is is possible to get false glideslope indications?

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  3. John D Collins on May 22, 2015

    On an ILS, there are false glideslope indications at multiples of the primary GS angle. A three degree GS will have false on glideslope indications at 6, 9, 12, and 15 degrees. The false GS at 6 degrees and 12 degrees will be reverse sensing, in that if you are below the GS, you will get a fly down and above it a fly up. The one at 9 and 15 degrees is correct sensing but obviously way too steep to fly safely, but the autopilot will try. A high intercept above or on the 6 degree false glideslope will indicate to the autopilot to fly up and it will attempt eventually to intercept the 9 degree slope. Below the 6 degree slope, the indication will give a fly down indication and the autopilot will dive to follow it. This can’t occur if you intercept the glideslope from below at the charted altitude.

    There is no corresponding test of the GS receiver and indicator as is required for VOR use by 91.171. So, each use of the GS should involve a verification that it is operating reasonably. That is one of the reasons for verifying the crossing altitude of the GS at the localizer only FAF. Another check of reasonableness is to verify that the GS deflects full up as you intercept from below and that the needle moves smoothly towards the center as you intercept the GS.

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  4. Drew on May 22, 2015

    Makes sense. Thanks, John.

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