How to figure out crossing restrictions

c152

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I am need to know how to figure out rule for crossing restrictions. and how to figure out how much v/s one should use in the descent.

Thanks
 

CA1900

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Here's what I use:

Thousands of Feet to Lose x 3 = NM from fix to start down
Groundspeed x 5 = Descent rate in fpm

For example: You're at 23,000. You're instructed to cross 30 miles this side of XXX at 11,000.

That means you have 12,000 feet to lose. Multiply by 3, you get 36 miles. (So in this example, you'd add 30 miles to THAT to get 66 miles out.)

You're doing 300 knots over the ground, multiply by 5 to get 1500 feet per minute. (Or, since I can do it faster, I multiply by 10, then cut it in half. Same end result. 300 x 10 = 3000, /2 = 1500.)

There you go. I'm sure there's more to it if you're flying something faster, but this works great in a turboprop.
 

JAC2LAS

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Take your ground speed and times it by five. For example if you have a 300kt ground speed x 5 = 1500. So, 1500 fpm will give you approx. a 3 degree glide path. Hope this helps. Also, there's a "Rule of Thumb" link on the flightinfo homepage, it may have some other suggestions.
 

Flying Illini

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CA1900 said:
Here's what I use:

Thousands of Feet to Lose x 3 = NM from fix to start down
Groundspeed x 5 = Descent rate in fpm

For example: You're at 23,000. You're instructed to cross 30 miles this side of XXX at 11,000.

That means you have 12,000 feet to lose. Multiply by 3, you get 36 miles. (So in this example, you'd add 30 miles to THAT to get 66 miles out.)

You're doing 300 knots over the ground, multiply by 5 to get 1500 feet per minute. (Or, since I can do it faster, I multiply by 10, then cut it in half. Same end result. 300 x 10 = 3000, /2 = 1500.)

There you go. I'm sure there's more to it if you're flying something faster, but this works great in a turboprop.

This is the way I do it. Works good, last long time.

Yesterday coming into HPN, on the Valry 2 arrival, 20 miles from a fix, ATC calls us and tells us, Cross 10 west of xxx (upcoming fix) at 11000. I don't remember our exact altitude but I do remember that it would require an 8700 fpm descent to make it. I replied, "10 west of xxx at 11000? There's no way we can make that." They just replied, "do your best." I still don't understand why they gave us that clearance when it had to be very obvious on his radar that we are 10 miles from the crossing that he wanted us to make and we had over 8000' to lose. Oh well.
 

GravityHater

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

Flying Illini

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GravityHater is correct. I usually re-compute several times in the descent to ensure that I'm "on profile" and adjust the rate if necessary. And if you are going to need to slow since you are descending below 10K, give yourself an extra 5 miles or so (adjust this as necessary...if you're going 280 you won't need to level or slow your descent as much as the guy going 340).
 

minitour

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CA1900 said:
Here's what I use:

Thousands of Feet to Lose x 3 = NM from fix to start down
Groundspeed x 5 = Descent rate in fpm

That's the way to go.

-mini
 

GravityHater

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Originally Posted by CA1900
Here's what I use:

Thousands of Feet to Lose x 3 = NM from fix to start down
Groundspeed x 5 = Descent rate in fpm


minitour said:
That's the way to go.
-mini

I think that only is useful if you are given a pd descent. Where YOU decide when the descent is going to start, no?

Any other clearance, you gotta start down now, not at 22 miles out, 51 miles out or whatever your calculation comes to...

Most I think are just watching the fmc vnav function anyway...
Familiar places, you know the restriction is coming or you can get it off the star so you punch the alt in next to the wp, and just watch the desired vspeed rise. And as it gets to 4000fpm you start to squirm a bit and hover over the mic button hoping to get your request in!
 

midlifeflyer

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GravityHater said:
Originally Posted by CA1900
Here's what I use:

Thousands of Feet to Lose x 3 = NM from fix to start down
Groundspeed x 5 = Descent rate in fpm


I think that only is useful if you are given a pd descent. Where YOU decide when the descent is going to start, no?
Yes. The formulas that have been bandied about are for figuring out things based on a nice comfortable 500 fpm descent (altitude to lose X 3) rate or a 3° glideslope (GS X 5).

I have a feeling the one C152 is asking about is to figure out the required descent rate based on altitude to lose and distance to lose it, as in:

What does my descent rate need to be if I am traveling 150 KTS and need to descend from 8,000 msl to 3000 msl within 8 NM?

I used to have that one handy, but can't seem to locate it right now.
 

PJO

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GravityHater said:
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
 
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tumbleweed

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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
 
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midlifeflyer

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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

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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!!
 
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