Descent Planning

[Way2Broke]

I have heard of a couple of ways, which one is right?

To estimate the point from the field to begin your descent, multiply your altitude (whole numbers)by three and add ten. To descend from FL 350: 3 x 35 = 105 + 10 = 115 miles out Why do you add the ten?

If you do the same problem and work it this way....

35000 (feet to lost) divided by 300 = 116.666666

You get pretty much the same answer. I have always done it the second way. Am I wrong?

 

[swa737-700]

Way2Broke,
That is a pretty good wag...3 times your altitude to lose. I add 1 mile for every 10 knots of tailwind and subtract 1 mile for every 10 knots of headwind. Also, if your descent will take you below 10K, add 1 mile for every 10 knots you neeed to lose to get to 250 knots. Sooo, to cross 35 miles outside of STL at 8000 feet, and assuming you are at 35K, you would plan on your top of descent at 35 - 8 = 27. 27 x 3 = 81 miles. To cross 35 outside STL at 8K, you would start at 81 + 35 = 116 outside STL. Assuming you plan the descent at 280 knots, you will need to add an additional 3 miles (slowing from 280kts to 250 kts). Then you will need to consider the headwind/tailwind component. Remember to add distance for tailwinds and subtract distance for headwinds. Hope this helps.

 

[Doc Holiday]

There's really no wrong way to do it, you just need to adjust your rate of descent to match the method you use.

In your first example, you are accomplishing a descent gradient of 1000 feet for every three miles you travel. This works out to 333 feet per NM. If you divide your second example by 333 instead of 300, you will come up with 105 NM.

Multiplying your GROUNDspeed (in knots) by 5 will give you the required descent rate to maintain that gradient of 333 feet per NM. Now the reason 10 was added to the 105 in the first example is probably just someone's technique for adding a fudge factor. This is to account for the airplane beginning its descent and leveling off (the airplane won't go from level flight to 1500 FPM back to level flight instantaneously, you need a little 'wiggle' room to allow for the airplane to change its flight path). For such a big descent like FL350 to the ground, 10 miles is probably a good number. For normal descents of 10,0000 to 15,000 feet I usually add five. Finally, I think it's just coincidence that adding 10 to the first example very nearly equalled to number derived from the method you normally use. For smaller descents, adding that 10 would get a larger number than your prefferred method would yield.

Hope this helps.

 

[Fedora]

I've watched guys get so wrapped up in figuring this out, that they forgot to fly the jet.
The typical jet descent speed is 320, so the ten came from guys adding a few miles to the 7-8 miles realy required to slow to 250 at 10k. Add a few too many, and you're powering up too soon after level off. A little "practicing" on your jet will work out the numbers for you. Nice when you can close them at top of descent and not have to power up too soon when you get down. It can really save fuel. I look for my "windows" to fly through: 100nm/30,000 agl; 30nm/10,000@250; 10nm/3000. If you make the first two OK, you won't be scrambling because your high and fast. Jets won't slow down like turboprops, so you may have to compensate.
One last tip: Use what you've got! For example if you have to lose 24000 feet by the next fix, and Mr. GPS says 8 minutes to the fix, set 3000 fpm in the vsi and like the man says "set it and forget it". A check about half way down to fine tune, but usually it's a tiny correction because the box figures ground speed continuously. Not counting Magellan, most start down 100-110 miles out. Don't be afraid to "play" with it a little, just hit your desired targets on the way down.

 

[Fedora]

I've watched guys get so wrapped up in figuring this out, that they forgot to fly the jet.
The typical jet descent speed is 320, so the ten came from guys adding a few miles to the 7-8 miles really required to slow to 250 at 10k. Add a few too many, and you're powering up too soon after level off. A little "practicing" on your jet will work out the numbers for you. Nice when you can close them at top of descent and not have to power up too soon when you get down. It can really save fuel. I look for my "windows" to fly through: 100nm/30,000 agl; 30nm/10,000@250; 10nm/3000. If you make the first two OK, you won't be scrambling because your high and fast. Jets won't slow down like turboprops, so you may have to compensate for type.
One last tip: Use what you've got! For example if you have to lose 24000 feet by the next fix, and Mr. GPS says 8 minutes to the fix, set 3000 fpm in the vsi and like the man says "set it and forget it". A check about half way down to fine tune, but usually it's a tiny correction because the box figures ground speed continuously. Not counting Magellan, most start down 100-110 miles out. Don't be afraid to "play" with it a little, just hit your desired targets on the way down.

 

[Amish RakeFight]

Decent planning is really just a rule of thumb which should be adjusted accordingly. This provides me with ballpark.

3 x ALT to lose = Miles out
GS/2 = Descent rate


35,000 to lose = 105 miles out

300 knots GS = 1500 FPM

 

[Princedietrich]

I've heard the 3-for-1 rule used many times. We used the 2-for-1 rule being freightdawgs.

Personally, I'd do the 2-for-1 technique and then round it up to the next 10, so if I came up with 73 I'd fudge it to 80 to give some wiggle room. I'd also tack on 5 miles for each major speed reduction I had to do, like getting below 250 at 10k or slowing the learjet down to 300 below 14.5k. Every couple thousand feet I'd check my progress, which was easy for me since I can do simple add/subtract math pretty quick in my head. If I was coming down too fast, I'd reduce the rate of descent a smidge and add some power to tack on some forward speed. If I wasn't coming down fast enough, I'd push the nose forward a bit to increase the rate of descent but keep the forward speed up, I'd pull some power out if necessary to keep from overspeeding or to get ahead of a 2-for-1 again.

A freighter lear can easily do 1-for-1 if desperation and/or bad planning calls for it. Just pull the power to idle, pop the boards, shove the nose down, and hang on for dear life. I don't know if a freighter falcon can perform like that but if it can, you've got a trump card stashed away just in case.