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How To Calculate Your Descent Rate To MDA

Boldmethod

These days, we're pretty lucky when it comes to instrument approaches. Almost every airport has an approach with vertical guidance.

But there are still non-precision approaches out there, and when you fly them, you need to manage your own descent. The fewer and further between they are, and the less often you fly them, the more challenging they are to master.

One of the hardest parts of flying a non-precision approach is managing your descent. Are you descending too slow? Will you actually make it to MDA before the missed approach point?

How about descending too fast? You still have obstacle clearance, but you don't really want to "dive and drive" at MDA all the way to the runway from several miles out.

1 In 60 Rule Saves The Day

You can take the guesswork out of your descent with a rule-of-thumb: the "1 In 60" rule. The rule states that 1 degree of climb or descent closely equals 100'/NM.

For example, if you descend at 1 degree for 1 NM, you'll descent about 100 feet. What does this have to do with an approach, you ask? Look at the RNAV RWY12L descent profile below, then keep reading...

Let's look at this approach starting from HALFF, the intermediate fix, all the way to the runway, because we have a couple different descent angles we're dealing with.

First, let's figure out the descent from HALFF to PYYPP. As you can see in the expanded descent profile (below), you're descending from 7100' to 6700' in 4NM. With some quick math, you can see that you need to descend 400 feet (7100-6700=400), and you need to accomplish it in 4NM.

So if you need to descend 400 feet in 4NM, that means you need to descend 100 feet per nautical mile (400/4=100).

That brings us back to the 1 In 60 Rule. Every 1 degree of descent is going to give you 100 feet per nautical mile. So when you cross HALFF on your way to PYYPP, pitch your plane down 1 degree, and you're on your way.

Now there's a bit of a caveat here. Pitching down 1 degree is doable in a glass-panel airplane. But it's still a bit of an imperfect science determining where 1 degree is on your pitch bars. Most GA aircraft have pitch lines at 0, -2.5, and -5 degrees, so you'll need to do some approximation.

If you're in a round-dial airplane, pitching down 1 degree is tougher to estimate, because of the smaller size of the instrument and limited space between each pitch line. That's where your backup plan comes in.

With one extra step, you can figure out your descent rate in FPM, and use your VSI to help you down.

If you multiply your descent angle (1 degree) by your miles-per-minute, then add two zeros to the end (x 100), you'll have your FPM descent rate. So in this example, if you're flying at 120 knots, you're traveling 2 miles-per-minute (MPM) (120/60=2).

Multiply 1 degree X 2 MPM X 100, and you get a descent rate of 200 FPM from HALFF to PYYPP at 120 knots.

If you fly at 90 knots, you're traveling 1.5 MPM (90/60=1.5), and you'd need to descend at 150 FPM.

Both of those descent rates are pretty slow, and hitting them exactly in bumpy clouds will be a challenge, but it gives you a target to aim for. Keep in mind, you probably want to descent slightly faster than your calculation as well, so you make it to your next fix at the right altitude, and don't leave yourself high on the approach.

PYYP To The Runway

Now that we've made it to the FAF, let's look at our next descent. Lucky for us, in this case, it's printed on the descent profile (thanks FAA chart makers!)

From PYYPP to a threshold crossing height of 41', we've got a 3 degree descent angle.

So once you cross PYYPP, pitch down 3 degrees, and again, you're on your way to the runway.

If you're looking for your backup on the VSI, do the extra math step, and you find that at 120 knots, you'll need a descent rate of 600 FPM (3 degrees X 2 MPM X 100 = 600).

DUUUD Inbound

Once you reach DUUUD at the MDA, assuming you've met the requirements to continue your descent below MDA, you can continue the same descent all the way to the runway.

The gray shaded are from DUUUD to the runway means that the visual segment below MDA is clear of obstacles on the 34:1 slope. Keep your descent rate going, and you'll have a perfect vertical profile all the way to the numbers.


What If There Isn't A Descent Angle Published?

Not every approach is as easy as the one we just looked at. Check out the one below: the VOR-DME-A approach into Kremmling, CO.

On this approach, you need to do a little more math, but it's still pretty straight forward. From the FAF (RLG VOR) to MAPRN, you need to lose about 1,500 feet (10,600-9120=1480).

You have 4.3NM to accomplish the descent. If you round that down to 4NM to make the math a little easier, you'll need to descend at 375 feet per nautical mile (1500/4=375).

Going back to the 1 In 60 Rule, that means you'll need to pitch down 3.75 degrees (375 FPNM/100) to accomplish the descent. Round that to a 4 degree descent, which will be easier to estimate on your PFD, and you're on your way.

If you're backing up the descent with your VSI, at 120 knots, you'll need about an 800 FPM descent rate (4 degrees X 2 MPM X 100 = 800 FPM).

If you're flying at 90 knots, you'll need about a 600 FPM descent rate (4 degrees X 1.5 MPM X 100 = 600 FPM).

Half Art, Half Science

The 1 In 60 Rule isn't a perfect science, but it is a good way to estimate how fast you need to descend to make it to MDA before reaching the missed approach point.

When it come to managing your descent on a non-precision approach, a little guidance can go a long way. Not to mention making your approach smoother, and a lot more stable.

Colin Cutler

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 colin@boldmethod.com.

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