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The Difference Between Decision Altitude (DA) and Decision Height (DH)

DA and DH aren't interchangeable phrases. They're two completely different things. Here's what you need to know...

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In the lowest of weather conditions, you'll likely find yourself flying a CAT I ILS or RNAV LPV approach. And if your airplane is certified for extremely low weather conditions, you might even fly a CAT II/III approach. The type of approach flown determines whether you'll use a DA or DH for your minimums. For simplicity, we'll explain the concept using ILS approaches.

Decision Altitude (i.e. Standard Cat I ILS)

Decision Altitude (DA) is an MSL altitude. When you fly a Category I ILS, which is what almost all general aviation pilots will fly, you fly to a DA.

Looking at the ILS approach in the image below, the published minimums for the straight-in ILS 35R are "6085 - 1/2".

That means DA is at 6,085' MSL. As you're descending on the glideslope, when you reach 6,085', you're at DA.

You'll see in smaller numbers "200" published next to the minimums for the approach. That's the decision height (DH). That means you'll be 200' above the touchdown zone elevation (TDZE) when you reach DA.

Most Cat I ILS approaches get you down to 200 feet above TDZE, but that's not always the case.

Looking at the Olympia Regional Airport (KOLM) ILS 17 below, the ILS DA is published at 425' MSL. That's 218' above TDZE. So why is this the case? As pilots, we don't know for sure, but it's likely due to an obstacle on short final.

Decision Height (i.e. Radar Altimeter ILS CAT II/III Approaches)

Decision Height (DH) is your height above the touchdown zone elevation (TDZE). If you fly a CAT II/III precision approach, you'll fly it to a radio-altimeter (RA) based DH. This requires your aircraft to be equipped with a radar altimeter, which measures your height above the terrain presently beneath your aircraft.

CAT II/III approaches are flown to DH minimums less than 200' above the runway's TDZE. That's why they're published as a CAT II/III approach, and not a standard CAT I approach.

As you fly a CAT II/III approach, you'll reference your aircraft's RA reading to determine when you've reached DH. Minimums are always less than 200' above the threshold, in part, to ensure varying terrain/obstacle elevation below your aircraft doesn't interfere with your RA reading.

By the time you're less than 200' above the runway, you'll be flying over a flat surface just before the runway threshold, which is one requirement for an airport to certify a CAT II/III approach.

Use Correct Phraseology

Whether it's a checkride, working with a student, or briefing an approach to another crew member, remember to use the correct phraseology.

If you're flying a Cat I approach, which most of us are, you're flying to DA minimums. If you're flying a CAT II/III approach, you're flying to DH minimums using a radar altimeter.

Why Does This Matter So Much?

Let's say you're flying the ILS to Runway 23 at the Charleston, WV airport (KCRW). It's a CAT I ILS with minimums of 1,181' MSL. On the approach chart, it shows this is 250' above the runway's threshold.

The problem is, the Yeager Airport is built essentially on a chopped-off hilltop sitting 300' above the valleys below. If you were going to rely on an RA to determine your minimums for the approach, you'd find the RA plummeting as you pass over the cliff at the end of Runway 23.

What Do You Think?

When's the last time you flew an approach to DA minimums? Have you ever flown a CAT II/III approach using a radar altimeter? Tell us about it in the comments below.

Swayne Martin

Swayne is an editor at Boldmethod, certified flight instructor, and an Embraer 145 First Officer for a regional airline. He graduated as an aviation major from the University of North Dakota in 2018, holds a PIC Type Rating for Cessna Citation Jets (CE-525), and is a former pilot for Mokulele Airlines. He's the author of articles, quizzes and lists on Boldmethod every week. You can reach Swayne at swayne@boldmethod.com, and follow his flying adventures on his YouTube Channel.

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