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You're flying at 5,000' MSL, going 90 knots groundspeed, and you need to descend to a pattern altitude of 2,000' MSL. You plan to descend at 500 feet per minute. How far out should you start your descent?
Or how about this: you're cruising IFR at 10,000' MSL, and ATC gives you a crossing restriction of 5,000' for a fix that's 10 miles ahead. You're cruising at 120 knots ground speed. How fast do you need to descend to meet your crossing restriction?
The 60:1 is one of the most powerful rules-of-thumb in aviation. Even if you think you're terrible at mental math (I think that on a near-daily basis), the 60:1 rule is something anyone can master. And it's not even that hard.
There are more applications of the 60:1 rule in aviation than we could possibly cover here, so we'll stick with descent planning. After all, nobody intends on diving their plane at 2,000 feet per minute to make it to pattern altitude. But if you're not planning ahead, it can happen. And most, if not all of us, have been there at some point.
There are a few basic things you need to understand to use the 60:1 rule.
Figuring out how many miles you're traveling each minute is really the key. Here are some examples that will help you down the road in this article, as well as the next time you fly. Remember, these speeds are ground speed When it comes to figuring out your MPM, ground speed is the only speed that matters.
Now that we have the miles-per-minute stuff out of the way, lets get back to those descent planning questions.
In the first question, we had to descend from 5,000' to pattern altitude at 2,000', for a total of 3,000' of descent. We planned to descend at 500 FPM. And we need to figure out how many miles out from the airport we need to start that descent.
Step 1: First, we need to figure out how many minutes it's going to take us to descend, and that's pretty straight forward. If we need to descend 3,000', and we're doing it at 500 FPM, we divide 3,000 by 500, and we get 6 (3000/500 = 6). It will take us 6 minutes to descend to pattern altitude.
Step 2: Next, we need to figure out how many miles away from the airport we need to start that descent.
Since we're traveling at 90 knots ground speed, it means we're traveling 1.5 miles per minute (MPM). Now all we need to do is multiply our MPM by the number of minutes we need to descend, which was 6. So we'll multiply 1.5 X 6, which gives us 9 NM. We need to start our descent 9 NM out to make it to 2,000' at the airport.
Keep in mind, doing a calculation like this would put you at 2,000' right over the top of the airport. Chances are, you want to get to pattern altitude at least a mile or two before the airport, so you can make a pattern entry and not have to 'chop and drop'.
To do that, simply add a mile or two to your calculation. In this example, if you started your 500 FPM descent 11 miles from the airport, you'd reach pattern altitude 2 miles prior to the airport, which would probably work out well.
Now let's look at our second descent planning question.
We're at 10,000' MSL, and ATC gives us a crossing restriction of 5,000' for a fix that's 10 miles ahead of us. And we're flying at 120 knots ground speed.
Step 1: Our first step is to figure out how much altitude we need to lose. This is pretty easy. We're at 10,000', and we need to get to 5,000', so 10,000-5,000 = 5,000'. We need to lose 5,000 feet.
Step 2: Our next step is to figure out how long we have before we reach the fix. Since we're flying at 120 knots ground speed, we know we're going 2 MPM. With the fix 10 miles out, we'll divide 10 miles by 2 MPM and get 5 minutes (10/2 = 5). So in this scenario, we have 5 minutes to the fix.
Step 3: To finish things off we'll take the altitude we need to lose (5,000'), and divide it by the minutes to the fix (5). 5,000 feet / 5 minutes = 1,000 FPM. We'll need to descend at 1,000 FPM to make the crossing restriction.
Keep in mind, much like the pattern altitude example, this calculation will put you right at the fix at your crossing restriction altitude. It also doesn't account for increased ground speed in the descent. So in this example, it would probably be a good idea to add an extra 100-200 FPM to your descent rate to make sure you get down in time.
It doesn't matter if you're a VFR pilot or IFR pilot, the 60:1 rule makes descent planning easy.
Whether you're trying to impress your passengers with a smooth descent to the airport, or you're trying to make sure you meet an altitude restriction with ATC, the 60:1 rule takes the guesswork out of descending, and makes you look like a pro.
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.