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2 Answers

Minimums in CAT II & III chart

Asked by: 794 views Instrument Rating

Have a look at this chart: SEA, ILS RWY 16R (CAT II & III) (

While reading this chart, I'm kind of lost on the minimums for the CAT II approach. Why the RA (which I guess refers to the radio altimeter), which is 139 AGL, is different from DH, which is 100 AGL? Does the difference come from the difference in ways of measuring altitudes between the radio altimeter and barometric altimeter?

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2 Answers

  1. Best Answer

    Russ Roslewski on Jan 31, 2017

    Local terrain is the reason.

    DH is the height above the Touchdown Zone of the runway when the airplane is at that minimum altitude. So, in this case DH is 100 feet above the touchdown zone. Easy enough.

    RA, however, is the reading on the radar altimeter when the airplane is at that point. Since the earth isn’t flat, it often varies, sometimes significantly, from the DH. At SEA, for example, the ground drops off quite steeply at the end of the runway. In fact, there is an tall retaining wall built between the runway and a local street.

    Take a look at the Google Street View:

    You can see the approach light “bridge” structure sticking out, that’s where the runway centerline is, so it’s pretty clear in this case that there’s a large difference between how high the airplane is above the ground directly below it, and how high it is above the runway.

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  2. Lemontree on Jan 31, 2017

    Thank you Russ. Very clear now.

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