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The Difference Between MDA and DA

Boldmethod

You've probably heard of Minimum Descent Altitude (MDA) and Decision Altitude (DA), but what are they, and how do they differ?

Minimum Descent Altitude

Let's start with MDA.

What is a Minimum Descent Altitude (MDA)? According to the AIM, MDA is, "the lowest altitude expressed in [MSL] to which a descent is authorized...in execution of a standard instrument approach procedure, where no electronic glideslope is provided." Sound confusing? It doesn't have to be.

In more simple terms, MDA is the minimum altitude you can descend to on a non-precision approach.

Between the Final Approach Fix (FAF) and Missed Approach Point (MAP), you can descend down to your MDA and remain there until you spot the runway environment. If you can't see the runway because of clouds or visibility restrictions, you'll continue flying at MDA until the missed approach point (MAP).

What do you need to go below MDA and land? You need to be in a position to land, have the required flight visibility, and have the runway environment in sight, which we discuss in this article.

The challenge with an MDA is that if the visibility is poor, you'll need to get relatively close to the runway before you can visually see it. That can leave you too high once you spot the runway to descend and land safely.

To help you continue your descent and land, many non-precision approaches include a Visual Descent Point (VDP) as a cue when you should begin a normal 3-degree descent from your MDA to the runway.

Decision Altitude

DA, or Decision Altitude, is defined by the AIM as, "A specified altitude or height in the precision approach at which a missed approach must be initiated if the required visual reference to continue...has not been established."

DA only applies to approaches with electronic vertical guidance. In practical terms, that means approaches with a glide slope (ILS), and approaches with a glide path (LPV, LNAV/VNAV).

DA's are different than MDAs. MDAs are absolute floors, but when flying to a DA, you make your "continue-to-land" or "go missed" decision at DA, while you remain on the glideslope. This means you may go slightly below DA while you're deciding if you have the requirements to continue down the glidepath and land, or if you need to go missed and start climbing. This doesn't mean that you can procrastinate your decision so that you can 'get lower'. You need to make your decision at DA, while you remain on the glideslope.

Here's how it works: as you descend down the electronic glideslope of an approach, as you reach DA, you look up, determine if you have the three requirements to land, and then make your decision, which is either 1) continue your descent down the glidepath and land, or 2) go missed and start climbing.

In practical terms, you typically descend out of the clouds and have good enough visibility to see your landing runway well before you reach DA. But if the weather is at minimums, it's entirely likely that you'll be making your continue/go-around decision at DA.

How To Identify Your MDA Or DA

Now that you know the theory behind MDAs and DAs let's identify them on approach charts.

To find MDA or DA, you'll look to the Minimums section toward the bottom of the chart. Under the minimums, you will see different lines of minimums you can fly (this will depend on the type of equipment you have onboard) and whether they correspond to a Decision Altitude or a Minimum Descent Altitude.

Looking at the example below, the RNAV (GPS) RWY 34 at Newport Municipal, you'll see that under the minimums section there the words "LNAV MDA" are written. LNAV stands for Lateral Navigation, and MDA as you now know, stands for Minimum Descent Altitude. On this approach, you can descend to as low as 860' MSL until you see the runway, or until you reach the MAP.

Keep in mind, not all approach charts with MDA's will specifically state "MDA". In general, approaches that are always non-precision approaches with MDAs, like VOR approaches, don't include the words "MDA" in the minimums section.

Now let's look at DA minimums. Looking at the KAPA ILS or LOC RWY 35R below, the ILS approach has a DA of 6,085 feet MSL. That means you can descend on the glide path down to 6,085', make your decision while you remain on the glide path, and then either continue your descent to land, or start climbing and execute your missed approach.

To review, MDAs are used on non-precision approaches, and they are a minimum altitude floor that you cannot go below until you see the runway environment. DAs are altitudes where you make your continue-to-land or missed approach decision as you descend on the glideslope, and you may go slightly below DA as you make that decision.

Nicolas Shelton

Nicolas is a private pilot from Southern California. He is currently studying at Purdue University, where he is working on advanced pilot ratings. You can reach him at nicolas@boldmethod.com.

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