Descent Math
 

What's a good formula for figuring out how far away from a fix to start down?

Say you're at 410 and need to get to "X" at 11,000/250knts, how far out should you start down?


j...

Well, it would kinda depend on how fast your airplane will descend, and how fast it will decelerate.





- The truth only hurts if it should -

36 rule

3 times the altitude to loose, 6 times your groundspeed

using your example

30,000 feet to loose = (3*30)=90 miles
250 knots (assuming groundspeed) (250*6)=1500 feet per minute (FPM)

So I would start down around 100 milse from X and maintain 1500 FPM, now as you go down and your ground speed changes, you can adjust your rate, if the groundspeed increases, go down faster decreases, go down slower.

Another way to keep track of your progress is divide your distance from the fix by 3 and see if that equals your altitude.

So you are 30 miles from X (30/3)=10. so at 30 miles, to be on a 3 degree slope, you should be at 10,000 feet.

Remember to take into account your actual height above the ground. So if Im going into denver, 30 miles at 10000 feet only puts me 5000 feet above the ground, maybe a bit low, so I would shoot for 13 to 15,000 on the altimeter. Also keep distance to slow in mind, dont plan your descents so that you arrive over the field at 1500 feet AGL at 300 knots, prolly not gonna work out to well.

3 times the altitude to lose works for me in an MD-80 In your example a .74/280 decent would be 30*3=90. 280 to 250 would add 3 miles. If you decend faster you need less. subtract 10% for a .79/310 decent. In your example (30*3=90)-10%=81miles. Add 6 to slow from 310 to 250. 87 miles. You can also adjust for tailwinds and headwinds. Add 1 miles for every 10 kts tailwind and subtract 1 for every 10 headwind.

I flew the EMB-120 for 4000 hours and can't for the life of me remember how we started down. I do remember about 30% Tq to get zero thrust but as for miles. I cant remember ever doing a 3 to 1 calculation. Huh :confused:

Or, you could simply program the crossing restriction into the FMS...

Right, if you dial in the new altitude and select FLCH you get the carrot and that's where you'll be at the new altitude but the older plane doesn't have that so want a plan. Oh, you mean RNAV and select it don't you.

Be careful asking the FMS and VNAV (Vertical Nav using FMC data) to do too much. Our new guys come out of training having had "use VNAV" drilled into their head by the training department and they have a hard time recognizing when to abandon the technology and revert to old school ways. For descent planning at idle thrust in a Boeing I have always used 100 miles for 30,000 feet and then three miles per 1000 feet for the remainder. Then factor in 1 nm for each 10 knots to slow to 250. For example: I am at 41000 feet and need to cross ABC at 250 knots and 10,000 feet. I am planning a 300 knot descent. I need to lose 31000 feet to cross ABC at 10,000. The extra 1000 feet will require 3 miles and I will need 5 miles to slow from 300 to 250. So 100 + 3 + 5 = 108 miles. Add a mile for each 10 knots of tailwind and you will nail it. It is not perfect but it works well enough for simpletons like me. My pet peeve is when pilots are quick to deploy speedbrakes at the first sign of getting high. Simply lower the nose and increase speed a little and it will accomplish the same thing. The passengers will be rewarded with a smooth decent instead of shaking, rattling and rumbling all the way down. After all your careful planning ATC will surely throw something into the mix to screw it all up.

Most any military heavy flyer I've talked to can vouch for the "3x altitude" technique.

As far as the "old school ways" that cal mentions, there is no substitue for them, because you may be in a situation one day where you're flying on standby instruments in the weather. Knowing how to do a fix-to-fix on your BDHI using the "pencil method" is nice to have in your "bag of tricks" if you ever need it.

When in doubt, punch off the automation and Fly the Plane!