You’ve likely heard of Minimum Descent Altitude (MDA) and Decision Altitude (DA), but what exactly are they, and how do they differ? Let’s explore.
Minimum Descent Altitude
Starting with MDA. What is a Minimum Descent Altitude (MDA)? According to the AIM, it’s “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.” It might seem a bit confusing at first, but it doesn’t have to be. In simpler terms, MDA is the minimum altitude you can descend to on a non-precision approach. Between the Final Approach Fix (FAF) and the Missed Approach Point (MAP), you can descend to your MDA and stay there until you can see the runway environment. If visibility is restricted due to clouds, you’ll continue flying at MDA until the MAP. What’s needed to go below MDA and land? You need to be in a position to land, have the required flight visibility, and be able to see the runway environment, which we’ll discuss further in this article. The challenge with MDA is that if visibility is poor, you might have to get close to the runway before you can see it clearly, potentially leaving you too high when you spot the runway for a safe descent and landing. To assist in your descent and landing, many non-precision approaches include a Visual Descent Point (VDP) as a cue on when to 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 applies only to approaches with electronic vertical guidance, such as those with a glide slope (ILS) or a glide path (LPV, LNAV/VNAV). Unlike MDAs, which are absolute floors, when flying to a DA, you make your “continue-to-land” or “go missed” decision at DA while still on the glideslope. This means you might briefly go below DA while deciding whether you have the requirements to continue down the glidepath and land or if you need to go missed and start climbing. But it doesn’t mean you can delay your decision to get lower. You need to make your decision at DA while on the glideslope. Here’s how it works: as you descend down the electronic glideslope of an approach, when you reach DA, you look up, assess if you meet the three landing requirements, and then make your decision, which could be either 1) continuing your descent down the glidepath and landing or 2) going missed and starting climbing. In practical terms, typically you’ll descend out of the clouds and have good enough visibility to see your landing runway well before reaching DA. But if the weather is at minimums, it’s quite possible you’ll be making your continue/go-around decision at DA.
How To Identify Your MDA Or DA
Now that you understand the theory behind MDAs and DAs, let’s look at how to identify them on approach charts. To find MDA or DA, you’ll look in the Minimums section at the bottom of the chart. Under the minimums, you’ll see different lines of minimums you can fly (depending on the equipment you have onboard) and whether they correspond to a Decision Altitude or a Minimum Descent Altitude. Take, for example, the RNAV (GPS) RWY 34 at Newport Municipal. In the minimums section, you’ll see “LNAV MDA” written. LNAV stands for Lateral Navigation, and MDA as you now know, stands for Minimum Descent Altitude. On this approach, you can descend as low as 860′ MSL until you see the runway or reach the MAP. Keep in mind, not all approach charts with MDA’s will specifically state “MDA”. Generally, 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. On the KAPA ILS or LOC RWY 35R below, the ILS approach has a DA of 6,085 feet MSL. This means you can descend on the glide path to 6,085′, make your decision while still on the glide path, and then either continue your descent to land or start climbing and execute the missed approach. To summarize, MDAs are used on non-precision approaches and are a minimum altitude floor that you can’t go below until you see the runway environment. DAs are altitudes where you make your continue-to-land or missed approach decision while descending on the glideslope, and you might briefly go below DA while making that decision.
