3 degree descent planning, Time vs. Distance

[Chadflies]

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

[Banana Joe]

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.

[MD83FO]

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.

[TheEdge]

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)

[easymxp]

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 it’s fine or need to be adjusted (speedbrakes).

[fdr]

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.

[FlightDetent]

@chadfliesDistance-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.

[411A NG]

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?

[FullWings]

Sounds like the HS121...

[ironbutt57]

I think most pilots can do a pretty decent plan...

[misd-agin]

VNAV. With updated winds. Trust but verify. Intervene if necessary.

It’s smarter than we are.

[Vessbot]

I think most pilots can do a pretty decent plan...
But some are only winging it

[Samten]

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...

[FlightDetent]

Oh Lord, brimstone we deserve.

[Capn Bloggs]

My head was hurting pretty badly, but now it's even worse!! :bored:

[jmmoric]

But some are only winging it

What is wrong with winging it if it works?

[misd-agin]

What is wrong with winging it if it works?

Years ago watching an FO’s descent planning. It wasn’t working. We’d be level 30 nm prior to a fix, etc. It was slower and burned more fuel. I’m watching and trying to figure out which method he’s using but after several flights I couldn’t figure it out. So I asked. He said “I use various methods for whatever works at the time.” To which I replied “none of them are working so you should pick one technique and figure out how to make it work.” Mind you this was in a plane with VNAV that he didn’t use!

[CanadianAirbusPilot]

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).

[CaptainMongo]

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.

[Vessbot]

What is wrong with winging it if it works?
Because that would be indecent :}

[jmmoric]

Years....

Fair enough... it does sound like bad situational awareness to me though... should be easily corrected "on the way down" with a reduction in descend rate, at least saving a bit of face.

[Officer Kite]

Altitude to descend x 3 gives distance to descend on a -3 deg path

Ground speed x 5 gives ROD necessary to stay on the -3 deg path

Say you need to descend 30,000ft and your GS is 400kts. So it's 30 x 3 = 90 miles needed

400 x 5 = -2000fpm ROD required to descend within the 90 miles.

Of course the altitude will need to have the necessary QNH adjustments to it for the calculated distance to be perfect.

I'm not so clever so complicated formula don't work, keeping it simple has so far!

[FlightDetent]

For small Airbus (when the VNAV calc works as designed):
- Honeywell: VNAV + 700 ft
- Thales: VNAV + 1700 ft
assuming IDLE factor 0.

[AerocatS2A]

Altitude to descend x 3 gives distance to descend on a -3 deg path

Ground speed x 5 gives ROD necessary to stay on the -3 deg path

Say you need to descend 30,000ft and your GS is 400kts. So it's 30 x 3 = 90 miles needed

400 x 5 = -2000fpm ROD required to descend within the 90 miles.

Of course the altitude will need to have the necessary QNH adjustments to it for the calculated distance to be perfect.

I'm not so clever so complicated formula don't work, keeping it simple has so far!

There’s little point doing QNH adjustments when your basic formula is so coarse.

[felixthecat]

Altitude to descend x 3 gives distance to descend on a -3 deg path

Ground speed x 5 gives ROD necessary to stay on the -3 deg path

Say you need to descend 30,000ft and your GS is 400kts. So it's 30 x 3 = 90 miles needed

400 x 5 = -2000fpm ROD required to descend within the 90 miles.

Of course the altitude will need to have the necessary QNH adjustments to it for the calculated distance to be perfect.

I'm not so clever so complicated formula don't work, keeping it simple has so far!


Sounds complicated enough to me when I’m heading down with much else to do. 3x FL plus 20 and keep reviewing. Plus 10 when back to 250Kts. Worked well for me for the last 25+ years

[easymxp]

Sounds complicated enough to me when I’m heading down with much else to do. 3x FL plus 20 and keep reviewing. Plus 10 when back to 250Kts. Worked well for me for the last 25+ years

Agree, on 32x family I go for 3xFL normally plus 10 and 5 when at 250. But if very light I can accept less margins...

[Officer Kite]

Sounds complicated enough to me when I’m heading down with much else to do. 3x FL plus 20 and keep reviewing. Plus 10 when back to 250Kts. Worked well for me for the last 25+ years

I think we've more or less said the same thing bar ROD calculation being GS x 5 (as a rule of thumb)

How do you calculate the ROD as a matter of interest?

[sonicbum]

Oldie but a goodie Airbus ALAR (http://picma.info/sites/default/files/Documents/Background/Airbus%20ALAR%20brief%202000.pdf)
Section 4 is a must read.

[misd-agin]

Altitude to descend x 3 gives distance to descend on a -3 deg path

Ground speed x 5 gives ROD necessary to stay on the -3 deg path

Say you need to descend 30,000ft and your GS is 400kts. So it's 30 x 3 = 90 miles needed

400 x 5 = -2000fpm ROD required to descend within the 90 miles.

Of course the altitude will need to have the necessary QNH adjustments to it for the calculated distance to be perfect.

I'm not so clever so complicated formula don't work, keeping it simple has so far!


That's assuming your GS is the same? When does that happen in the descent? What if you have a 120 kt TW at FL410, it's 80 kts at FL250, 40 kts at FL180, and a headwind at FL150?

The box tries to figure that out but it only has the wind 'cuts' in the box. If mother nature is different, which it almost always is to some degree, the computation is off, so guys blame the box. But they'd never be able to compute an idle descent as well as the VNAV can.

In low traffic areas it wasn't uncommon to go to idle at TOD (typically FL410) and be at idle thrust until 1,000'. It used to be 700' until stabilized rules changed. Now it's about 2,000' due to configuration SOP's.
Minor deviations from the path could be corrected by slight increase in descent speed for going high on the VTI (VNAV path) or a slight speed decrease (better glide profile) if you got slightly low on the VTI (VNAV path).

[felixthecat]

I think we've more or less said the same thing bar ROD calculation being GS x 5 (as a rule of thumb)

How do you calculate the ROD as a matter of interest?

I don’t usually need to, idle thrust and x3 +- is a good enough guesstimate. The just keep updating as you go down. If low add thrust, high and getting higher, increase speed or speed brake. KISS

[Officer Kite]

That's assuming your GS is the same? When does that happen in the descent? What if you have a 120 kt TW at FL410, it's 80 kts at FL250, 40 kts at FL180, and a headwind at FL150?



Adjustments during the descent as necessary, multiplying present gs x 5 always gives ~3deg descent profile, it’s never failed me so far. It was actually taught to me by a former MD80 pilot who swore by it during his career.

And Felix, what mode do you descend in then? I thought this was a discussion about VS mode and fast rules for knowing what VS to set to keep a 3deg profile, is that Vnav mode your referring to with a magenta 3 deg vertical guidance indicator on the PFD?

[misd-agin]

The objective for descent planning isn't to achieve a 3 degree descent path. It's to achieve the most efficient descent for fuel and time adjusted for altitude and speed constraints. In the MD80 you had to figure the stuff out. Modern jets, with modern FMC's, are more capable than the overwhelming majority of pilots. I've yet to see anyone grab a calculator and start comparing the descent winds to do the math the FMC does without having to ask it.

The 'rules of thumb' are for the old school flying or non FMC aircraft. They're a good back up, just like the 'trust but verify' aspect of FMC use, but the primary method to achieve efficiency is via the modern tools provided. Updated winds are a key component of the FMC's computing ability.

[misd-agin]

Early MD-80's had a descent planning tool. I forget what it was called. Perf??? It was a box on the center console. Most guys didn't want to go to the effort of getting the better descent planning available from it. Why? Because they had to manually enter the descent winds at various altitudes. With FMC's it's a couple of button selections and the FMC uploads the information automatically.

[MD83FO]

PMS, is used it the entire flight, along with the N1 sync.

[felixthecat]

Adjustments during the descent as necessary, multiplying present gs x 5 always gives ~3deg descent profile, it’s never failed me so far. It was actually taught to me by a former MD80 pilot who swore by it during his career.

And Felix, what mode do you descend in then? I thought this was a discussion about VS mode and fast rules for knowing what VS to set to keep a 3deg profile, is that Vnav mode your referring to with a magenta 3 deg vertical guidance indicator on the PFD?

I am not trying to descend at 3deg. I am trying to descend efficiently and to monitor the FMC profile as a backup....amazingly it isn’t always correct. VNAV Path is great but rubbish in rubbish out, you pick up the errors early and correct them. There are also times when your given short cuts or expect them and you want to be below the path so you use other modes to achieve your self calculated path.

This may sound old school but we still have to think outside the FMC. :)

[misd-agin]

PMS, is used it the entire flight, along with the N1 sync.

Thank you. PMS rings a bell. It's been a long time since I flew it.

[Banana Joe]

Height to lose x 3 + 10 works fine to determine the TOD. Add to it an allowance for tailwind.
During descent I use DTG x 3 and subtract 2000' if at 300 kts and 1000' if at 250 knots. Works well.

As I said earlier, on the 737 the FMC determines the TOD by multiplying the height to lose till 250 knots by 2,5 and then 3.0. Sometimes that can be too tight, and despite having inserted the corrected descent winds it may still need some speedbrake.