Calculating Top of Descent

Here’s a tip if you’ve ever wondered when you should start your descent to your arrival airport (assuming you don’t have an FMS with top-of-descent calculation capability). A quick and easy way to figure it out is to start with your altitude above field elevation and multiply that number by three. This will give you the approximate distance in nautical miles from the airport to start a 500-foot-per-minute descent in the typical light general aviation airplane and reach pattern altitude.

So, for example, if you’re cruising at 7,500 feet en route to an airport with a 500-foot field elevation, do the math and you’ll come up with 21,000 – drop the last three zeroes, and you can see that you should start your descent when you’re 21 miles out.

Using a time equation is just as easy, and it actually makes more sense when you’re flying faster airplanes. A good rule of thumb is to allow yourself two minutes for each 1,000 feet of altitude you need to lose. So, if you’re cruising at 10,000 feet above field elevation, start descending 20 minutes before your planned arrival.

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Submitted by flyingbob on August 28, 2012.

Good tip, HOWEVER...

Your example is limited to 500 ft/min and 90 kts.
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Now an inquiry for you:

"...do the math and you’ll come up with 21,000..."

Please "do the math" and explain where 21,000 comes from.
It gives me vertigo trying to figure that out.

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Submitted by w.hagen on August 28, 2012.

well lets see... 7500-500=7000 => 7000*3=21000
that wasn't to hard now....

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Submitted by RHalstead on August 28, 2012.

This is fine for VFR, but when depending on ATC who is not going to know just when you need to start down takes the boredom out of it.

Our little airport (KIKW) is near the division between Cleveland and Minneapolis centers air space. So you need to know when you are going to need to start down for the most likely approach. So you call "center", they will transfer you to the local control (MBS) which is not in their air space (unless you are coming from the South) You tell MBS your intentions and hope they give you one of the approaches from the west which are either the VOR-A or the GPS approaches. All of this while you are trucking along in the neighborhood of 200 mph. Depending on where they let you start down your rate of descent may be from 500 to over 800 fpm which is no big deal if you are used to it. Again though, most of these numbers can be done in your head. 120 is 2 miles per minute while 180 is 3 and from there it doesn't take much to figure out how fast you need to descend. It makes no difference if you use knots or mph, just don't mix them.

IFR you need to be down and slowed down before the approach. VFR you need to be down and slowed down (if necessary) prior to the pattern entry. Descending into the pattern is not a good idea.

All pilots who have flown long enough to earn a license should know where they need to start down in the pattern regardless of the wind and adjust their speed and power accordingly. This is one of the reasons I do not like the "always do things the same" approach (no pun intended).

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Submitted by flyingbob on August 28, 2012.

"...well lets see... 7500-500=7000 => 7000*3=21000..."

So, what does that prove ?

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Submitted by Hugh Irvin on August 28, 2012.

I'm glad to see someone FINALLY suggested using time. GPS gives distance (whoopee) and time (yea!) to destination. Time makes MUCH more sense. At 2 minutes per 1000 feet, start your descent to pattern altitude a little before 2 times thousands-of-feet-to-lose. Then airspeed/groundspeed does not change the calculation like it would the distance to descend calculation. And without GPS most pilots don't know with any greater precision time-to-destination or distance-to-destination.

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Submitted by precisionflying on August 29, 2012.

This tip works for larger and faster airplanes as well and keeps them surprisingly fine on a 3 degree descent profile. The only difference is that their rate is usually greater than the mentioned 500 ft/min.

With larger and faster airplanes time-to-destination cannot be taken for its face value at the TOD because it changes considerably the moment you lower the nose. However, regardless of the type of airplane you fly, winds are a big factor in proper timing and they determine whether you'll maybe anticipate or delay your the TOD.

Throughout the descent I find a good rule of thumb is to use a rate that is half my ground speed (for example, 1500 ft/min for 300 KT). And then it’s time to start thinking of speed limits - structural (KT and/or Mach), legal, ATC, gear extension, flap extension, fitting in the pattern, fitting in the flow, etc.

Bottom line is one has to stay well ahead of the airplane. Otherwise, you’ll blush as I did once when I had to ask the tower to execute a 360 to re-join the final.

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Submitted by Winsum on August 29, 2012.

To Hugh Irvin:
"Time makes MUCH more sense."
Can you explain why it makes "more" sense to you? Distance is fixed. Time is variable and is impacted by atmospheric conditions. More substantially as the aircraft size/power decreases.
Just trying to learn something new. I'm a student pilot about a month out of my ticket.
Thanks.

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Submitted by skymachines on August 29, 2012.

It is amazing to me how many pilots cannot compute the "a 3-deg. glideslope is 300 ft. per nautical mile" rule in their heads. A sad reflection on our country's basic education.

I teach all my students: Compute the distance to descend and mult. it by 3, dropping the zeros. So, you're 3000 above the airport, you'll need 9 n.m. If you can't do that in your head, you really shouldn't be a pilot.

But then what? Then one of the least-known rules of thumb comes into play: A 3 deg. glideslope can be maintained if you use 5 x your groundspeed and apply that to the vertical speed indicator (in FPM). So, if you're doing 100 kts. across the ground, a 500 FPM descent will keep you either on the ILS or the VASI. If you're doing 150 kts. in your jet, then 15x5=75, so 750 FPM. It works in ANY airplane for any 3-deg. GS. Makes ILS's and VASI's much easier to fly...just keep checking the groundspeed and multiplying it by 5. (And, again, if you can't do that simple math in your head, I question if you're really qualified to hold the title of "pilot"...you need to practice.) I've taught this to ex-military and current airline and corporate pilots who didn't know this simple rule, and they're amazed at how it helps them. I know it has helped me for over 25 years stay on the GS.

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Submitted by skymachines on August 29, 2012.

I saw an error in my post above: "I teach all my students: Compute the distance to descend and mult. it by 3, dropping the zeros." It SHOULD have read, "Compute the ALTITUDE to descend..." not distance.

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Submitted by jshelton on August 30, 2012.

That's a useful way to know when to start the descent, but since you know the distance from the airport (which implies you have GPS), why not do it the easier way? Using 500 fpm as the target descent rate, simply subtract the desired descent altitude (either the airport or pattern altitude) from your current altitude. That gives the number of feet to descend in thousands of feet. Divide the thousands of feet by 500 and you have the time in minutes to the airport when you should start the descent. For example, cruising at 6000 feet, airport elevation is 1000 feet and the pattern altitude is 1800 feet. Assume you want to reach pattern altitude: 6000-1800=4200 feet. 4200/500 is 8.4 . At 8.4 minutes from the airport you can begin the descent. Since you want to reach pattern altitude BEFORE the airport, I'd start down at 10 minutes in this case. (If I'm feeling math challenged, I simply count down 2 minutes per 1000 feet on the face of the altimeter.)

The really nice thing about this algorithm is that you can pick a different descent rate and it converts easily. For example, at 800 fpm, 4200/500= 5.25 minutes

One other advantage is that it takes wind into account (although not perfectly).

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Submitted by Hugh Irvin on August 30, 2012.

Answer to Winsum,

Because the planes I fly I descend at 500 fpm. See precisionflying's and skymachine's excellent explanations of the additional computations you have to do to use distance. And all our methods are affected by a change in wind while descending. So, 2 x (thousands of feet-to-lose) is easier (for me). YMMV.

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