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3 degree descent planning, Time vs. Distance

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3 degree descent planning, Time vs. Distance

Old 8th Sep 2019, 17:46
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3 degree descent planning, Time vs. Distance

So Iíve had differing opinions on how to calculate this, personally I prefer the time method vs. distance but wanted to post and see what other opinions are as to which method works best.. currently Iím of the opinion either is fine.

For time: ETA / altitude needed to be lost.

(IE: if you at 30,000 ft and need to meet a crossing restriction at 10,000ft with a 1000fpm decent - you need to start your decent 20mins prior)

For distance: change in altitude x 3

(IE: you are at 14,000 and need to get to 6000ft, giving you 8000, start your decent 24nmís back)

Both can factor a 3 degree decent profile if that is something you want to consider. 1/2 ground speed, add a zero.

Both in my opinion work, tho I constantly hear the argument that distance is better, without any reason as to why.

Does anyone have some deeper knowledge base that can provide a rationale? Personally I find time an easier variable to adjust and calculate, but thatís just my preference.

blue skies and tailwinds,

chad
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Old 8th Sep 2019, 20:00
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Because it's easier to multiply than to divide. You could also determine your TOD by dividing your FL by 3 i.e. FL330/3=110nm. The difference with the multiplication method is the deceleration segment.

I started a thread not so long ago asking how many pilots on the 737 multiply the altitude to lose to FL100 by 2.5 rather than 3, to account for the steeper descent gradient at high altitude and high speed. The answer to that was, nobody.

Keep. It. Simple.
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Old 9th Sep 2019, 02:15
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altitude times 3 has always worked, even watching the airbus fmgc descend doing pretty much the same thing.

descending at a fixed vertical speed will result in varying true airspeed throughout the descent giving you inconsistent time.

fixed V/S works well on slow unpressurized planes but on jets its about the economy of idle thrust from top of descent all the way down.
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Old 9th Sep 2019, 09:52
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Originally Posted by Chadflies View Post
So I’ve had differing opinions on how to calculate this, personally I prefer the time method vs. distance but wanted to post and see what other opinions are as to which method works best.. currently I’m of the opinion either is fine.

For time: ETA / altitude needed to be lost.

(IE: if you at 30,000 ft and need to meet a crossing restriction at 10,000ft with a 1000fpm decent - you need to start your decent 20mins prior)

For distance: change in altitude x 3

(IE: you are at 14,000 and need to get to 6000ft, giving you 8000, start your decent 24nm’s back)

Both can factor a 3 degree decent profile if that is something you want to consider. 1/2 ground speed, add a zero.

Both in my opinion work, tho I constantly hear the argument that distance is better, without any reason as to why.

Does anyone have some deeper knowledge base that can provide a rationale? Personally I find time an easier variable to adjust and calculate, but that’s just my preference.

blue skies and tailwinds,

chad
What i do i also add to the calculated distance, the difference between my current IAS and and average speed of 200kts, (i.e. if before the descent i am flying at an indicated speed of 270kts, i add to the calculated distance additional 7 NM), and to conclude I also add or subtract the wind factor (10kts/1NM) depending whether i have tail or headwind.

Doing this i am usually really precise when compared with the TOD on the MCDU.

Example:

FL330, IAS:270Kts, Wind negligible

My calculated TOD will be : (33*3) + (27-20) = roughly106 NM, to be compared with the distance to treshold in the PROG page (Airbus world)
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Old 9th Sep 2019, 11:19
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I normally keep it simple, just do 3xFL and add deceleration, wind, weight factor. Basically by looking at the distance to go I can decide if itís fine or need to be adjusted (speedbrakes).
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Old 9th Sep 2019, 14:05
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Whatever works best in your system.

B744, 757 767 777 787, VNAV does an adequate job but it is only as good as the wind profile that is entered into the box. WInd change is not linear in most instances, so any assumption of that condition will result in less than optimal path establishment. It is interesting to watch crews fly a VNAV path in an altering gradient, as the energy state of the aircraft is paradoxical to the crews actions; e.g., in path, an headwind that is greater than planned will result in a higher speed than anticipated, and heads off towards a "drag required" message, and putting out the boards, whereas the energy state of the aircraft is now low... resetting the path by going direct to the current waypoint, or by reentering descent speed etc will give the opposite result, waking up the throttles..

Airbus VNAV is disappointing in it's limited scope.

Using a multiple times your height sounds great, if you know your distance, and you are going to do the maths. Most nav systems will provide time to a WPT and as the desired outcome is what is the correct vertical path, which is dy/dx, but is flown as a rate of descent, dy/dt, then time seems a logical measure to apply. 5 minutes to a waypoint, x feet to lose... An assumption of X times distance is only valid when you have the correct value for X which changes every wind case (and different weight if a constant speed profile is intended to be flown).

In extreme cases, time works nicely; In one sporty jet, I known I can get down from FL450 to 10,000' in 2:00, anything else is a choice... (don't recommend that one at night... 30 degrees nose down at MMO/VMO is quite aggressive even when the blue is kept at the top. For a large vertical offset, such as cutting 20 miles off a distance to touchdown leaving 10nm to touch, at 300kts, FL100, well, for that particular type plonking out flaps 40, full brake and gear gives a rate of descent that can be applied to the time to threshold etc... telling the driver if it is going to work or not well before the ground rush comes up. time to target tells you something meaningful and immediate. That is also what you are seeing when you look out the window, your aiming point is the result of your dy/dt, for any given GS, dx/dt, so the basis of the analysis is time.

An efficient descent based on idle from TOD to configuration complete and established at stable condition at an appropriate altitude on final, nowdays is mainly wishful thinking. It doesn't happen in ZGGG, RJAA, UEEE, KLAX, and only the first arrival of the day may do it into EGLL..., it doesn't happen many other places, our airspace is a victim of the success of the industry. Mainly descent efficiency is compromised by STAR design constraints and traffic needs. As such, being off your original desired speed, means that the target path really is already mussed up, and what you have left to work with is time to target... giving needed sink rate.

Whatever works for you is arguably the best method.
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Old 9th Sep 2019, 15:56
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@chadflies
Distance-based methods give you a chance to calculate a short-cut scenario. Estimate two distance-to-go values, easily done just by looking at the ND, and you can get yourself ready for both. I am not sure how that would work with ETAs derived from the active FMS, based on the active wpt sequence and speed assumptions.




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Old 9th Sep 2019, 18:29
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Originally Posted by fdr View Post
Whatever works best in your system.

In extreme cases, time works nicely; In one sporty jet, I known I can get down from FL450 to 10,000' in 2:00, anything else is a choice....
Do you fly a stone?
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Old 9th Sep 2019, 19:47
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Sounds like the HS121...
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Old 9th Sep 2019, 22:45
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I think most pilots can do a pretty decent plan...
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Old 9th Sep 2019, 23:00
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VNAV. With updated winds. Trust but verify. Intervene if necessary.

Itís smarter than we are.
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Old 10th Sep 2019, 00:51
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Originally Posted by ironbutt57 View Post
I think most pilots can do a pretty decent plan...
But some are only winging it
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Old 10th Sep 2019, 06:55
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I'm curious do most of the passenger jets currently used around the world, have metric function? Especially metric alt indication on standby altimeter?
If the answer is "yes", then why do you do all those calculations (*3 +- pressure correction/field elevation)? Why not to set QFE pressure (QNH - field elevation in mb/in Hg) on standby altimeter and get the required value directly (metric alt)? Then all you need is correction for your speed (1000 meters or 10nm for 280 kt) and for the wind...
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Old 10th Sep 2019, 07:16
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Oh Lord, brimstone we deserve.
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Old 10th Sep 2019, 08:22
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My head was hurting pretty badly, but now it's even worse!!
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Old 10th Sep 2019, 11:35
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Originally Posted by Vessbot View Post
But some are only winging it
What is wrong with winging it if it works?
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Old 10th Sep 2019, 11:47
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Originally Posted by jmmoric View Post
What is wrong with winging it if it works?
Years ago watching an FOís descent planning. It wasnít working. Weíd be level 30 nm prior to a fix, etc. It was slower and burned more fuel. Iím watching and trying to figure out which method heís using but after several flights I couldnít figure it out. So I asked. He said ďI use various methods for whatever works at the time.Ē To which I replied ďnone of them are working so you should pick one technique and figure out how to make it work.Ē Mind you this was in a plane with VNAV that he didnít use!

Last edited by misd-agin; 12th Sep 2019 at 14:23.
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Old 10th Sep 2019, 12:08
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In the 320 I find 3:1 with 1 mile added or subtracted for each 10 knots of tail wind or head wind or speed to reduce.

Example. Crossing restriction of 10,000 and 250 kts. Planned transition of 330 kts and starting altitude of 30,000 with a 20 knot headwind all the way down.

60 miles before for altitude lose (20x3) + 7 miles for 330 to 250 - 2 miles for the head wind. 65 miles before my restriction I'll start down. It works bang on, this is done with the trust at idle.

Some times the FMGC is not to be trusted (wrong des speed in the box or a very early Decel pt showing you at green dot over your restriction vs 250 as an example).
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Old 10th Sep 2019, 14:54
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Originally Posted by CanadianAirbusPilot View Post

Some times the FMGC is not to be trusted (wrong des speed in the box or a very early Decel pt showing you at green dot over your restriction vs 250 as an example).

Good point. A quick calculation can verify proper (or improper) FMGC entries in descent or fix crossing speed. It can indicate whether an idle path or geometric path is being used by the FMGC for planning or if FMGC descent winds need updating, etc.

We’re paid to think.



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Old 10th Sep 2019, 20:52
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Originally Posted by jmmoric View Post
What is wrong with winging it if it works?
Because that would be indecent
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