TOP of descent, and descent monitoring. (A320)

[WhySoTough]

Bare with the newbie please guys, I've got another question.
Is the right way to calculate TOD "altitude to lose multiplied by 3 and add 10" miles for deceleration?
If I find that multiplying my current distance times 3 plus 10 to be easier, I could also be doing that I assume?

Every how many miles do you cross check this?

Yesterday on my very first flight of line training, the captain decided to use open descent instead of managed(can anyone think why)? Yes we were in Nav.
He asked me to keep telling him if he was high or low and to use the alt to use times 3 technique. Personally it confuses me, but it was the first time for me doing this.

Any input, advice, examples, tips are welcome.

Thanks in advance!

[felixthecat]

Maybe he was doing it so that you had to monitor the descent and correct it. In VNAV (I'm Boeing) equating to the managed mode it calculates it for you and keeps you on the path (generally…with some exceptions)

You often don't know exactly how far you have to run so the 3x height plus 10 is a great way to monitor your descent, and work it into your expected distance to run….remember that the arrival you have in the FMC is generally giving you way more track miles that you will actually do due to vectors etc etc.

Remember to take into account the head/tail wind and correct for it also…

You should monitor the descent all the way down, I do a cross check about every 5-10000ft and more often as I get closer to the airfield and the ground. Don't just rely on the automatics…rubbish in rubbish out…..

[sevenstrokeroll]

back before FADEC and FMS we managed our descents quite well with the following idea. Mind you we didn't even have a DME ground speed readout...just DME mileage.

Let's say you are at FL310 with a descent clearance to FL200

11(,000) feet to lose, so 11 (thousand) times 3= 33 miles (nautical) to lose the altitude...but no smooth level out or option to decelerate if there is a clearance or requirement to do so. So, ADD 10 nautical miles as a "FUDGE" factor...this in case you get an increase in groundspeed due to unknown changes in wind.

so, 33 plus 10 = 43 miles before any required crossing fix, start your descent.

AH, but what rate of descent?

multiply your groundspeed ( in knots) by 5....let's say you are doing 500 knots
in this case 500 knots times 5 =2500 (this in feet per minute). So, start your descent at 2500 fpm and you should be fine.

Its not alot of heavy lifting.

You can use your watch/clock to make a caluclation of groundspeed based on changes in DME readout.

During the course of the descent, recalculate and see if you are still going to make the crossing restriction.

AND, remember, it is very UNLIKELY that you will be doing more than 700 knots groundspeed...so if you don't even know your groundspeed, you can use 700 times 5 =3500fpm in case. you will be early in your descent, but that's only fuel!

I always get a kick out of how we now have to make the machine happen instead of other way 'round. Just before takeoff, I watched one super duper FMS equipped wonder jet get a re route from ATC. It took the crew 10 minutes to load the re route. In prehistoric times we would have taken off and headed to first fix (VOR) and just kept up with it, like sight reading a piece of music.

And one more thing...I just wonder if sucking birds into a FADEC equipped engine compared to the same engine NON FADED would have the same result... a FADEC commanded shutdown or reduced thrust. Still think if Sully was flying a DC9 with JT8d's and sucked birds he would have made his destination on time.

[BOAC]

Originally Posted by wst

If I find that multiplying my current distance times 3 plus 10 to be easier, I could also be doing that I assume?

- ?? What are you trying to work out there?

Descent will take just over 20 minutes to 3000', give or take, which is 1/3 of an hour, thus allow 1/3 of the wind ie 20 kts allow 6-7 miles. In ssr's and my day it was 3 miles per 10 kts. Same cat....

Ball park 'check points':

Around 20,000ft at 70 track miles to run, and 10,000 at 30 (at 250kts)

the captain decided to use open descent instead of managed(can anyone think why)?

- yes! Stick with what your TC was getting you to do - watching an FMC managed descent will teach you very little!

[Lord Spandex Masher]

Select FPA, twiddle to 3.0 degrees and Bob's your mother's brother.

Oops. There I go sounding like somebody who like a colour close to pink!

Is the Airbus even that advanced?

[WhySoTough]

Thank you all very the quick, informative responses.

Felix,

Thanks for the reply.
Makes sense. If you had a headwind, would u multiply alt x 3 and add 5 maybe?

Sevenstrokerroll,

Thanks. I understand better now.
Though now I have more questions.
Firstly if the ATC gave me an initial clearance to descent from FL 310 to FL 200, wouldn't it be better to still do the 31 x 3 plus 10, since I know I'm most probably going to be given further descent clearance?
If ATC were to tell me to be at Fl200 by a certain point, i would definitely start my descent and use the 43 miles in this case.
Right idea?

Also,
If I was using the V/S, I don't think it would be a good idea to use speed breaks. I could be wrong here, please do tell.
Basically don't use speed breaks, and just increase V/S if I am above my profile?


BOAC,

What I meant was, instead of altitude to lose x 3 plus 10.
Example if I'm at 20,000 feet. I should be approx 70 miles for a nice constant 3 degree descent angle.
Instead of that I'd see my track miles, for this example I will use 70 miles.
So 70 x 3 plus 10. I should be at 22,000 feet.

The captain said this was another way to calculate. Though it gives me slightly different numbers.

[flapsupdown]

WhySoTough,

Just wanted to share some info with you - hopefully will help answer some of your questions.

Using speedbrake in V/S: there is nothing wrong with doing so as long as you are aware of what the autopilot mode is trying to achieve. In V/S the autopilot is maintaining a selected rate of descent. As such, using speedbrakes will only reduce your airspeed, it will NOT change the rate of descent.

When you are talking about your descent - whether it is ALT x 3 or Dist x 3, you have to remember that all this means nothing without taking into account your speed. If you are below the "profile" (within reason) but your speed is high, or you are above the "profile" (within reason) but your speed is low, its not really worth getting back on profile immediately. I guess what I am trying to say is that you need to be thinking about this with your airspeed in mind.

[Lord Spandex Masher]

In V/S the autopilot is maintaining a selected rate of descent. As such, using speedbrakes will only reduce your airspeed, it will NOT change the rate of descent.

But, it will change your angle of descent.

[sevenstrokeroll]

why so tough:

indeed if ATC is predictable in a total way, make the calculation down to 10,000
feet above touchdown/airport...things change at 10 to the 250 below ten rules in many places.

now...I would LOVE to tell you that you will get an uninterrupted, perfect descent to touchdown. I"ve had that ONCE in 30 years of flying. And be sure to rev up your engines at least once to make sure they are spooled up....or ready for action, or whatever you guys say now a days...don't get caught like that BA 777 guy in London. Not that I am blaming him per se.

It is always cool to NOT need speed BRAKES (not breaks) but if you need them, use them.

There are lots of reasons to stay high...but there are good ones to get down early.


I know here in the good old USA, if you get below FL230, you get into the lower sectors and can sometimes get DIRECT! Indeed, one day in my DC9, our pressurization was out and we flew 5 legs at 10,000'....including Montreal To Baltimore. Well, we thought we would use more fuel...but because we were the ONLY plane at 10,000 feet that day, we got DIRECT everywhere...on time, under fuel burn.

Those guys that blindly stay high are not always right (transoceanic legs aside)

good luck, happy flying....and remember its YOUR LICENSE not the computer's!

[Microburst2002]

t

he captain decided to use open descent instead of managed(can anyone think why)?

there is many a reason

WST

Sorry for asking, but is the 320 your first airliner?
Disregard any advice from a training captain that tells you to keep always DES. Many do it. Disregard it.

The third dimension is the whole point of flying, and being a pilot is all about the third dimension. You must be capable of knowing if you are high or low at any time without the need of a magenta donut.

In the first place, the computer will only give you good profile information when the airplane lateral fpln is going to be made good. And if all data is inserted correctly, and if planned speeds are going to be maintained as planned.

Many many many pilots do this:
they fly in OP DES when instructed to fly a much higher speed, then when recapturing the profile, the donut, they manage descent again, but keeping the high speed. In fact they are high, because they have extra energy due to the extra speed. They are blindly following the computer.

You must know your required distance to land as a result of your altitude and speed. The rule of thumb FLx3 +10 is quite good, but can be refined. Some airplanes sink like stones, others glide like sailplanes.

During descent, altitude is what you know. Distance to go is not always known, so work parting from your altitude. Then decide which distance you think you will fly to touchdown, then find if you are high or low. Only if the FPLN is updated so that you have in it what you will actually fly you can trust the donut. So many times it is not the case! You have to look at your ND and visually see the likely distance to go. Many times is radically different. If you are a donut follower you will never master the third dimension. If you don't you can barely call yourself a pilot.

Take every oportunity to fly in OP DES until you master the way your airplane descends, then when you use DES, just monitor its consistency.

[felixthecat]

If you had a headwind, would u multiply alt x 3 and add 5 maybe?

Its not quite as easy as that. You may +/- 5 or more it depends on the winds. Thats the important thing about monitoring your descent so you can adjust it as you go.

If its a headwind you will need to take away miles and a tailwind add them not the other way around as you quote. A headwind will increase your air distance and a tailwind decrease it.

Sit down and think, learn some gates for your aircraft type i.e. at 5000 at FL100 at FL200 at FL 300

Learn the aircraft and work out your own profiles then you will be able to use managed profiles and know if they are giving you the right information or not. Be conservative, especially as a new guy….don't get high and fast ( easily done when chasing a profile ) low and slow is just costing you a little fuel but buys you thinking time as a new guy. The profiles will get better and better as you get more used to it. After a while you will just "know" if your high….

Happy Flighting

[Microburst2002]

In my models (CFM, old with extra drag) FLx3 + 10 is the basic for a GW of 60 T, 300 kt, 250 at 10,000 ft. Below 10,000, use FLx3 + 6. As you build hours, it should be sufficient with that, but you can refine it a lot, when you get used to do quick mental calculations.

For very heavy, I increase the distance a 10%. For very light I increase a 10%, for super light, a 20%.

For wind, add or substract a 5% per each average 20 kt component. You can calculate that with the vertical wind profile in your CFP, and then monitor during descent.

For a higher speed, such as 330 knots, reduce a 10%. For slow speed, such as 270, increase 10%.

What I do is add up all corrections and apply to the basic FLx3, then add the 10 NM for deceleration, 8 if light. Below 10,000 add only 6.

Seems complicated but I do it real quick thanks to practice. I am absolutely not worried ever about 3rd dimension when I fly. It gives me a lot of peace of mind when I fly for that reason. The FMGC never gets me in trouble.

During the descent, monitor your gradient. That will tell you if your calculations were accurate enough. Do the following:

fpm / GS in NM = fpnm

GS in NM is easy. Look at your GS, then remember this table:

240 kt is 4 NM/min
300 kt is 5 NM/min
360 kt is 6
420 kt is 7
480 kt is 8
540 kt is 9

If you are making 2,100 fpm at a 420 kt GS, your gradient is 300 fpnm. 330 fpm is FLx3, so your gradient is now a 10% shallower than that. You should be doing 2,300 or so to be on the FLx3. If your gradient was 400 fpnm then your angle is a 20% steeper. Keep monitoring since gradient varies constantly but tends to oscillate about an average value. That will tell you if you will tend to get high or low. GS and fpm will change constantly. Soon you will know if the actual gradient was too much or too little. The different of that gradient with the gradient in still air (using TAS instead of GS) will give you a hint on what is the effect of the wind.

If you see you are high when still at high level, it is very easy to recover. The change in energy by increasing speed for a while means you will dive and loose a lot of height due to the acceleration only. Shortly after being again on profile, reduce back to your target speed. During deceleration your rate reduces, yes, but the net effect is an energy loss, exactly what you need. This process is energy wasting, like using speedbrakes, but gives you less flight time in exchange.

If you are low, just select V/S -1000 ft until you are again on profile.

A perfectly calculated OP DES will make the airplane descend at constant speed, while the magenta donut will go up and down, but it will come to you, instead of you going to it.

At 10,000 ft and 250 kt you should have 38 miles to go.

As you get closer to the field, calculations are easier because the effect of GW or wind are less, in terms of feet. The basic FLx3 + 10 will nearly always be good enough below 5,000 ft.

With time you will learn by heart:

5,000 ft, 21 miles (5x3 + 6)
4,000 ft, 18
3,000 ft, 15
(if you are at 250 of course. add 1 mile per extra 10 kt which is conservative enough. only half if you are light and/or with a headwind)

When you have green dot, FLx3 plus 3 is good enough.

When S, FLx3 is good enough.

These ones are particularly helpful during approach, specially when being vectored.

During approach, if your speed is significantly different than that of the magenta profile, this profile will be in significant error. Typically if you are flying at 200 kt but the profile is for 250, you might think you are high with the donut below when you are actually low...

It may seem too much maths, but is always the same, you will get used to it with practice and it will give you peace of mind in the 3rd dimension, which means a lot of spare mind for so many other tasks.

As a note, for TOD calculation from high cruising flight level, this is accurate enough if at TOD you don't pull OP DES right away but instead select V/S -fpm for 330 fpnm (still air) and then let the airplane accelerate to target IAS while maintaining target mach. Otherwise the acceleration will be a dive, very steep if the airplane is light and the target speed is high. This would give very high rates, reduce the time to descend to field altitude and increase the average cabin rate That is why the FMGC does a similar thing, the repressurization segment.

For instance, for a 464 TAS indicated (approx 480 kt, that is 8 NM/min), I will gradually select a V/S of 2,700 fpm (less if req to avoid MMO exceedance). IAS will increase gradually. MACH will command thrust to maintain mach number. As IAS increases for the constant mach, the target is ahead of actual speed and thrust is thus much higher than idle, but after a while you'll be reaching target IAS, SPEED will be in command of thrust, now, and it will reduce to idle, then you can pull OP DES.

If captains allow you to do that, of course...

[Ollie Onion]

Gidday,

I have found you are best just to keep it simple. I personally use 3 x altitude + 10 nm for calculating T.O.D. From there on in just a basic 3 x table to monitor the descent.

So from 35,000 I would plan on TOD being approx (3x35)+10 = 115nm to run. Once I start down you just keep a running total going in the background.

So:

So at 20,000 ft if you have less than 60nm to run then you need to increase your rate of descent.

At 10,000ft you should have no less than 30nm to run and aim to be at 250kts.

From there aim to be at G/S intercept at Flap 1 S speed.

[felixthecat]

I agree with Onion….KISS principle works best…Keep It Simple Stupid…...

[Microburst2002]

It may be simple, but 30 miles at 10,000 with 250 you are about 2,000 ft high. That is pretty much at that stage...

So are you at 20,000 ft with only 60 nm to go, but maybe not if you are light or there is a noticeable HW, or the airplane is very "draggy".

I keep it simple, but within reason.

Try to pull OP DES exactly ar the FMGS calculated TOD, at FL390, GW 49 T with an AUTO speed of .80/330 knots. You will get Veeeery low, even if the FMGS has into account GW, because of the lack of repressurization segment when in OP DES.

Or try to use FLx3 plus 10 in that day. Tou will see that your TOD is over 20 miles before FMGS' TOD.

Durin approach, it is actually simple: FLx3 + decc, decc being 6 at 250, 3 at green dot, nil at S or at 180.

[Natstrackalpha]

what a cool thread

Quote:
In V/S the autopilot is maintaining a selected rate of descent. As such, using speedbrakes will only reduce your airspeed, it will NOT change the rate of descent.
But, it will change your angle of descent.

Magic, well I think so . .

My theory is for the Thick, like me, and, being thick I need building block solutions to move forward: based largely on the above (most excellent) posts.

3x alt = dist (no 10 added)
sw from Mach to spd and start down at that spd (intially, for a bit)
monitored by me/her or us selected spd all the way down -
based on 3 times table only (no 10)
280 generally, say, then 250 by 10,000
We put us `by 12 miles` spd 210
by 10 miles gives +/-3000`
spd 180 by 8 miles about 2400`
160 by 6 miles and into landing configiurations - 3 flaps down by here
at 4.5 miles will have gear down and be selecting FULL and managed spd or going around
Even if being vectored with no height, I find this a cool system needing only a quick looky see calc to determine track miles (and not dist to TD)- Big Notwithstanding MSA, ground to hit/buildings OCL etcetera.

ADDED LATER ON: By the way, the/my above profile, in Selected, is almost the same profile, (speeds too !) as if it were all done in Managed.
. . . and a - ive, what I don`t like about V/S is that it does not honour Alt constraints. . . as we all know . . . and, per se` I don`t like Open Descent much either, not that I have problems, it is just that, well, to be ahead of the aircraft is fundemental for this a/c, if you are in V/S . . .

[Microburst2002]

at 5,000 ft at 250 kt, FLx3 is 15 nm

You are high, very high, because now you are too close. You will have to take positive actions, maybe a not very stabilized approach if wind and weight are inconvenient.

FLx3 + 6 is 21, you are confortably on profile. mile more mile less, but 6 miles difference is too much.

[macdo]

Talk about over complicating things, the answer is yes! But most people just do Managed Descent at the down arrow!

[Microburst2002]

And many of them screw descents very frequently because of that...

[macdo]

I disagree, IMHO, so long as you have adequately programmed the FMGC for the anticipated approach, I find that the FMGS calculated down arrow is almost always correct. Some local knowledge for early ATC descents, short cuts, speed control will help as they can be programmed in, as will the having the capacity and spatial awareness to see how the FMGC might need modifying as you progress to destination, but this is all part of the job. A good example would be Larnaca in Cyprus rwy 22, where you can easily let the FMGC fly from top of drop on the STAR to short final on the ILS with no intervention. In twelve years of flying the bus for a major carrier I have very rarely seen anyone not use the down arrow and it is certainly what our excellent trainers would expect unless there was a good case for 'going non-standard'. The small bus is very flexible and usually produces a reasonable stab at the down arrow, even if you forget to put the winds in! I'm a bit puzzled by your comment that you see "plenty of people screwing up their descents", if that is happening, there must be a problem with the way these people are being taught to do them.