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In CAT II and CAT III Approach Charts, the Jeppesen version of charts depicts both the "DH" and the "RA".

And, I found out that they are somewhat different.

So, why do these differences occur?

Averagely about 2 ~ 3 ft deviations are existing.

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    $\begingroup$ Please edit and give some examples. $\endgroup$
    – GdD
    Jan 25 at 15:00

1 Answer 1

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This entry is defined in the Jeppesen Charts Legend as follows:

Jeppesen Cat II minima

The difference between radio altimeter (RA) and decision height (DH) is the different baseline. The radio altimeter will measure height above ground (directly below you) whereas the decision height uses the runway as a baseline. I made a sketch to illustrate the difference with the numbers of the example above:

example sketch

It is usually not an issue for CAT III because at a typical DH of 50' you are already above the runway threshold on a normal glideslope (and for no DH it obviously doesn't apply). For CAT II with a typical DH of 100' there could be sloping terrain in front of the runway, but typically not much, that is why you see only a few feet difference. If there is a steep slope or a ditch in front of the runway, radar reflectors can be used to imitate a flat surface as shown in this related answer.

This example from Munich (EDDM) shows the RA entry only for the CAT II:

EDDM ILS 08L

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    $\begingroup$ Also, some CAT III approaches have no DH (just an RVR). $\endgroup$
    – user22445
    Jan 25 at 16:31
  • $\begingroup$ Just a reminder that RA could be considerably larger than DH, should the ground rise to meet the end of the runway. Lukla, I'm looking at you... $\endgroup$
    – FreeMan
    Jan 25 at 19:30
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    $\begingroup$ The FAA CAT III A,B,C definitions no longer apply and "Operators will use the published minima in conjunction with their Operations Specifications to determine the lowest landing minima allowed to them, as is currently done." see this reference $\endgroup$
    – user22445
    Jan 25 at 21:16
  • $\begingroup$ Thx for the detailed explanation! I was able to totally understand what you wrote. Have a good day! $\endgroup$
    – moon
    Jan 26 at 0:14
  • $\begingroup$ @757toga Got it, I removed all references to CAT IIIA or CAT IIIB (they were not relevant for the answer anyway). Jeppesen still prints separate IIIA and IIIB minima for other countries though (as shown in the Munich example). $\endgroup$
    – Bianfable
    Jan 26 at 7:24

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