How to figure out crossing restrictions

[PJO]

And then fudge a little; go down a bit faster than planned, because your gs will rise as you point the nose down.

Think about this. The initial reaction as a result of lowering the nose of the A/C will be that the GS will increase because your IAS increased. If we assume no wind condition as soon as you start leaving you previous altitude, we all know that the TAS is going to reduce as you get closer to the ground. I don’t understand what you meant by “because your GS will rise as you point the nose down”.

This is a formula that I use to compute a x-restriction, using algebra you will be able to adjust this formula to your particular need.
VSI = ALT to lose / Time to the fix
Time to Fix = ALT. to lose / VSI
ALT to Lose = VSI * Time to the Fix

VSI Req. = 12,000'FT/8min. = 1,500 FPM

 

[tumbleweed]

Descent gradients....

Another way to look at it....

Instead of using V/S (fpm), try to just think about the descent gradient (ft/nm) you need to meet the restriction. Say you have to descend from 20,000 to 10,000 and have 20 miles to accomplish it. That is 10,000 in 20 miles. The equation is:

Descent gradient = Alt to lose in ft / distance in nm

DG = 10,000 ft/20 nm = 500 ft/nm = 5 deg of pitch (1 deg = 100 ft/nm)

From there all you do is make slight adjustments for winds. Take a look at Air Force Manual 11-217 Vol 2, Section 6 about the 60-1 rule. A less scientific method is to use the ADI for a WAG (Wild A$$ Guess). You put the distance on the 10 deg nose low bar (so the scale is 20 at that point). Then put the altitude to lose at the appropriate point on the new scale (ie...10,000ft is at 5 deg). If it was 15,000 ft to lose, then you would put it at 7.5 deg nose low. Best thing is, you don't need a fancy fms or gps. Just an ADI, altimeter, and distance.

Here is the link for 11-217 Vol2: http://www.e-publishing.af.mil/pubfiles/af/11/afman11-217v2/afman11-217v2.pdf

Clear as mud?
TUMBLEWEED

 

[midlifeflyer]

I found my reference. Duh - it's the same formula as for departure climb gradient requirements, except that you need to figure out the descent gradient.

Vertical speed required = Altitude to lose/Distance X GS/60

The first half, Altitude to lose/Distance, gives you the required gradient in feet per nautical mile (the same place the climb gradient charts start)

 

[Dr Pokenhiemer]

GravityHater,
You still need to be computing where how far out you need to start down. LOTS of times ATC has "forgotten" about us and starting us down. If you know where you need to start down, you can request lower instead of having to do a screaming dive 10 minutes later. I don't carry pax right now so I can come down @ 6000fpm, but I wouldn't want to do that with a high level executive on board--good way to make someone uncomfortable and lose your job!!