Ask the Expert: Descent Planning

There is really nothing new in this article. If you are totally comfortable with your descents and have learned the techniques presented in my or King Air Academy’s Flight Training Notes, then you are wasting time here. Move along; nothing to see here.

But if you have not yet learned these tips, you’re about to learn them now (and how useful they are)! One of the nifty things is that when you learn the tips, you can apply them to any pressurized airplane from your King Air to the 747 your airline operates. In fact, I first learned these techniques back in the ’70s from a great book about flying 747s, “Handling the Big Jets” by David P. Davies.

There are two pieces of mathematics that are necessary to solve our descent planning question: Altitude to Lose and Ground Speed.

Rule #1:

Distance Required (nm) = [Altitude to Lose (in thousands) x 3] + 10%

We are cruising at FL270 and the ATC-crossing restriction is at 11,000 feet. That means we have (27,000 – 11,000) 16,000 feet to lose; 16 x 3 = 48, plus 10% more is 52.8. For our realistic (lazy?) computation, anywhere between 52 nm and 60 nm is fine.

Another example: We are at 12,000 feet, landing at Sea Level, VFR. We have 12,000 feet to lose; 12 x 3 = 36, plus 10% more is about 40. We need to start down about 40 nm out.

One more: Cruising at 21,000 with a crossing restriction of 7,000 would mean 14,000 feet to lose; 14 x 3 = 42, plus 10% = 46. We’ll start down about 46 to 50 miles out.

Rule #2:

Rate of Descent (fpm) = [Ground Speed (knots) ÷ 2] X 10

What we are really doing is multiplying our ground speed by five. But for most of us mathematically-challenged types, it is easier to divide by two instead of multiplying by five. That works perfectly so long as we move the decimal point appropriately. So just half your ground speed and add a zero at the end.

Descent ground speed is 260 knots? Then descend at 1,300 fpm. At 300 knots? 1,500 fpm and 200 knots would be 1,000 fpm. You get the idea. Easy, right?

So at the appropriate distance out, you nose over to get the vertical speed desired, tap the VS button on the autopilot – if you have one – and go back to sipping your coffee, right? What could go wrong?

Well, winds can change, that’s what can go wrong. And the wind change will cause your ground speed to vary. Also, you may make a significant power reduction due to turbulence encountered and that, too, will have an effect on ground speed.

So it is important to stay on top of this descent planning and to make the necessary adjustments. In other words, make a “How Goes It” (Howgozit) check every few thousand feet. For example, you are descending to Sea Level from 16,500 feet, so you started down about 54-60 miles out. Your ground speed was 240 knots, so you began your descent at 1,200 fpm. Now, passing 10,000 feet you make your howgozit check. At 10,000 feet to go, I need 33 miles. The GPS tells me I have 35 miles to go. I am slightly ahead of the profile, so no need to change anything.

But now I am passing 7,000 feet. I need 23 miles and the GPS says I have 28 miles left to go. I am getting so far ahead of the profile that I will dial the descent rate from 1,200 back to 1,000 fpm and see how it goes. Later, at 4,000 feet, I need 13 miles and I have 14 to go. Ah, all looks good in my descent world!

Another example: We are descending from FL280 to make a crossing restriction at 16,000 feet. We started down 40 miles out with an initial descent rate of 1,500 fpm. (Our ground speed in cruise had been 280 knots and we figured we’d pick up a little in the descent.) Passing FL230, we notice that we have 20 miles remaining to the fix, yet we need about [(23 – 16) X 3 + 10%] 23 nm. We better increase the VS from 1,500 to 1,800 or so. Later, passing 180, we need seven and we have eight nm remaining. Cool! We are slightly ahead of the game. No need to make any adjustment.

Are there other, perhaps even better, ways to conduct our descent planning? Of course. Modern VNAV (Vertical Navigation) capabilities make this a snap for the autopilot to accomplish. The old method of time-to-go and altitude-to-lose works well if we always descend at 1,000 fpm (20,000 feet to lose? I’d better start down 20 minutes out.). But always descending at 1,000 fpm is certainly not optimal for every situation, especially in the 200- and 300-series King Airs. Furthermore, when ATC assigns something like “Descend so as to cross 15 miles west of Salom at 17,000 feet,” the time-to-go reading is not known to us unless we create a new waypoint at that offset point. But distance is always a known quantity. (In this case, approaching Salom from the west, we’d need to add the extra 15 miles to our calculation. Need to lose 10,000 feet? Then instead of 33 out, we’d start down about 48-50 nm from Salom.)

Try it. You’ll like it! And to make it easier, I will include a couple of tables. Feel free to copy them and attach them to your kneeboard or whatever for easy reference.