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You're getting ready to brief your GPS approach, and you see something strange: the LNAV MDA minimums are lower than the LNAV/VNAV DA minimums. How can that be? And which one should you fly?
You would assume that approaches with vertical guidance will get you lower than non-precision approaches, but that's not always the case. Here's why.
There's a lot that goes into calculating the minimum altitudes for an approach, but there are a few general rules that all of them follow. For the purposes of this article, we won't go into LPV approaches - we'll stick to LNAV/VNAV, and LNAV only approaches.
First, let's look at approaches with no vertical guidance. For LNAV (lateral navigation) approaches, the FAA looks at the obstacle heights along the approach path, and draws a straight line called the Obstacle Clearance Surface (OCS). Then, they add 250 feet to that line for the LNAV Required Obstacle Clearance (ROC). That altitude becomes the LNAV MDA. So at a minimum, the LNAV MDA on an approach is at least 250 feet above the highest obstacle in your path. But, there are several factors that can cause the MDA to be higher. If the MDA needs to be raised, they do it in 20 foot increments.
Next up, let's look at approaches with vertical guidance. For LNAV/VNAV approaches, the FAA draws the same Obstacle Clearance Surface horizontal line. At the controlling obstacle, they draw a horizontal line away from the runway until it reaches the Obstacle Clearance Surface (OCS) plane, which in our diagram is the the gray angled line. From there, they draw a vertical line that reaches the glide path. That point (the orange arrow) becomes the Decision Altitude (DA).
Why do they do it that way? By moving the DA outside of the controlling obstacle, it gives you the chance to see and avoid the obstacle while you're descending on the approach.
So with all of that in mind, it brings you back to the original question, which descent minimums should you choose? The LNAV, or the LNAV/VNAV?
It depends on your situation. First off, using vertical guidance on an approach is almost always the best idea. When you fly an approach on a glide path, you have a continuous vertical path to follow. And on top of that, you don't have to make as many power changes as you would on a "dive-and-drive" approach. Once you're configured for your descent, it only takes minor changes to maintain your glide path.
As for a non-precision approach, you have a lot more workload to deal with. You need to keep track of your step down altitudes, and manage your power settings as you transition from level flight to descents through your step down fixes. That makes the approach more prone to error.
On an approach like this, your decision is really going to come down to the weather. Where are the ceilings at, and will the LNAV/VNAV approach get you under them?
On the Harrisburg approach, the LNAV/VNAV approach will get you down to 1,264 feet above touchdown, and the LNAV will get you down to 872 feet above touchdown. That's a difference of 392 feet, which is quite a bit when the ceilings are low.
To give yourself some breathing room on the approach, you would probably want at least 1,500 foot ceilings to shoot the LNAV/VNAV approach. That would give you a little over 200 feet of room between you and the clouds when you reach DA. If the ceilings were lower than that, you probably would want to opt for the LNAV only approach, which gets you quite a bit lower.
The next time you see an approach with MDA minimums lower than DA minimums, take a minute to think about what your best choice is going to be. If you're a WAAS capable airplane, and there are LPV minimums, that's probably going to be your best route. But if you're at an airport with LNAV/VNAV and LNAV only, you have a choice to make.
If the DA minimums will easily get you out of the clouds, following the glide path is almost always the best choice. A constant glide path will lighten your workload, reduce your chance of error, and make your passengers more comfortable during the descent.
But if the clouds are near your DA, the MDA is probably the way to go. Managing your descent profile and making sure you break out of the soup is better than having to go missed and try the approach all over again.
Colin is a Boldmethod co-founder, pilot and graphic artist. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed development of numerous commercial and military training systems. You can reach him at firstname.lastname@example.org.