View Full Version : descent question again

4th May 2003, 00:50
Few days ago,i'v have a suddenly changed arrival:the ATC changed the procedure,and we lost about 25 nm,so the captain make a quickly VS change about 1500fpm,i feel that is a miracle.
Does there have some method that can make the caculation very fast or some just by experience.Could you teach me someone,thanks a lot.

4th May 2003, 12:32
When I and Checkers sober up we'll post a response. It will be based on the x times profiles. ie height (knock off the zeroes) times the distance to run.
I'll explain it later.

5th May 2003, 02:47
Your normal descent rate would be about 2000 fpm/planned descent speed.If ATC put you 'closer',extended speed brake/spoiler,increase speed to highest legal(A320 disconneCt the Auto pilot to get'best' rate-full spoiler).If still high or have to reduce speed -dump the 'gear..!!

In this configuration one can get as much as 6000 fpm descent,when back on profile auto pilot on, clean up as required.:bored:

5th May 2003, 10:28

It will depend on the type of aircraft you are flying....but for the A340/330 I use the following:

Top of Descent: 4 x Altitude (making an allowance for airfield elevation)

High Speed Descent ( >250kts): 3 x distance to go - 4000ft
eg: with 50 track miles to run I should be at 3x50 = 15000ft - 4000ft = 11000ft

Normal Descent: (<250kts): 3 x distance to go - 2000ft

Of course in China you'll have to convert to metric at some point!!

There are many far more complicated formula around but I find this works well and and gives you 10Nm to slow up for the approach.

5th May 2003, 17:34
Wot? reddo and Checkers sober? Quick! Someone notify the press! :p

I suspect that you're flying something like a Kingair, anyewo. I fly a B200 Super Kingair, so the following might be helpful to you. For a standard profile, I use a "3 times" profile until within 5 NM, if on a visual approach with no ATC restrictions, or until GP intercept if in cloud. The way this works is that I work out the total amount of height to lose, to reach the runway, divide by 1000, then multiply by 3. This gives me the distance that I will use in the descent, as in the following example.

Descent from FL 240 to 1000 feet means I must descend thru a total of 23,000 feet.
Now divide 23,000 by 1000 = 23
Now multiply 23 by 3 = 69.

I now know that my descent requires 69 NM. But, as I want to make final configuration change at 5 NM, I add that on, so 69 + 5 = 74 NM. To achieve my "3 times" profile, I initially set up a ROD of 1200 FPM and allow the speed to build up to 220 KIAS.

If I then have to adjust my descent profile due to an ATC requirement that reduces the track distance available for my descent, I'll reduce power initially to 400 ft/lbs, increase ROD to 2,000 FPM and bleed the speed back to 170 KIAS, if icing is likely to be a problem, or 140 KIAS otherwise.

This gives me time to work out what my actual descent gradient needs to be, at the reduced track distance. For this, using your example and mine, I need 69 NM for the descent, but only have 44 NM available (your 25 NM short-cut). I must now lose 23,000 feet in 44 NM, which equates to about 500 FT/NM, which is close enough to 1500 FPM at 180 KTS G/S.

I then roughly figure my TAS and the wind and, mostly, it works out for my speed of 140 knots, so I can reduce my ROD from 2,000 FPM to 1500 FPM. If I must maintain 170 KIAS due to the possibility of icing, I'll hold that speed and my 2000 FPM ROD until I'm out of icing conditions - subject to any other ATC restrictions. In either case, at the reduced speed, I don't need to consider the 5 NM buffer for final configuration changes.

The operating environment plays a part in all of this. I can usually get away with a lot of very rough calculations because all my operations are in tropical areas. In other parts of the world, where the weather can be a more serious consideration, it would pay you to think a little more carefully about the above calculations.

After you've flown a few hundred hours in command and been given a few tricky clearances, you really DO get a feel for what you need to do by way of descent performance. This is especially true if you're very familiar with the place you're going to! The really good thing about a Kingair is that it's flexible enough to meet most challenges.

5th May 2003, 22:41
Thanks a lot upstairs.

Chimbu chuckles
7th May 2003, 00:09

There are a few different ways and in the end they all work...it's merely personal preference- some favour more complicated formula to give it all an air of mystery.

Very simply in most aircraft,

3 x height to lose gives you distance (Track miles) to lose it + add a buffer for deceleration...say 5nm to slow down to 250 Kts at 3000m and another 5nm to slow down to initial configuration speed by the Initial Approach Fix.

Big winds aloft will require allowances to but even God doesn't have a rule of thumb for those:D

If all of a sudden you get track shortening just increase ROD by a 'suitable' amount - experience being the best guide- until you regain the original 3x profile plus buffers as appropriate.

You must keep track of the profile in your head every 1000' or so to pick trends early - before variations from your 'ideal' become so great that massive rates of descent are required - lowering gear early/ speed brake, etc or at the other extreme flying level spooled up and burning extra fuel. The later is not dangerous but is not terribly professional either - not the sign of a Commander who is on top of his craft.

If you are being radar vectored to the FAF or some other point keep the same running tally in your head of approximate track miles to run and compare it to your ideal profile...once again experience and practice will make this easy - familiarity with particular aerodromes/ATC will give you a good idea of how much you will be vectored on average & therefore how much to pad your descent point to allow for it.

As a general rule I always like to be a little high on profile in these cases...it nearly always works out better that way.

If you are being vectored around to the far side of the airfield for an appoach think of it as a big DME ARC and by estimating an 'average' DME distance you can estimate the track miles via the 1 in 60 rule- i.e. Initial track inbound 270 and being vectored to runway 06 via south of the field. That's 150 degrees - if they start vectoring you at say 20nm East of the field you might work on an average of 15nm away from the field the whole way around. Track miles in that vectoring would be about 36nm...approximately 12000' 'allowance'. From the same inbound track around to runway 36, average distance from field 10nm would be 1/6th of 90 degrees - 15 track miles/ 5000' altitude allowance

It's just practice...even if it's not your sector have it all worked out in your head where YOU would descend and keep a track all the way down. Pay attention to trends/habits of the various ATC at your regular destinations and you will pick up constant 'habits' of the local controllers and be able to allow for that in your descent planning...it's fun to outsmart their, completely innocent, attempts to spoil your smooth, professional descents :D

Remember always think in track miles to go to the distance/altitude point that is the end of your enroute descent and the beginning of final descent, generally about 10 DME/2500'. Always plan backwards from that point to your TOPD and allow as per above.


Cruising FL 390. Inbound via 240 radial to a 10nm arc to runway 36 ILS/DME. Requirement to be at 210kts/Flap 10 at FAF (2500'/ 8 DME) 39-2.5 = 36.5 x 3 = 110 +10 (decel) = 120 track miles before FAF. Track miles in DME arc = 10 so still air TOPD = 110 DME.

If at 60 DME you get a vector direct FAF? If on profile you'll be at about 20000'.

20-2.5= 17.5 x 3 = 52 + 10 decel and + 8 because the FAF where you want to be at 2500'/210kts is before the field.

You need 70 and you only have about 52 to the FAF...only 42 allowing for deceleration.

Average Ground speed 300kts = 5nm/min which = 8+ minutes to lose 17500' which means 2200'/min + ROD required.

Or another way approximately 2.5nm/1000'. That's a 2.5x profile as opposed to your 'ideal' 3x ! 5nm/min devided by 2.5nm/1000' = 2200'/min ROD at LEAST required to regain profile AT the FAF.

Normal 3x descent profile at 330kts might yield a ROD of 1900'/min so to arrive AT the FAF configured for final approach you need to increase ROD by 300 odd FPM...increase by 1000'/min and you will regain your original profile at perhaps 10 or 15 nm before FAF and then you can resume normal profile ROD and arrive at the FAF well rested and not stressed:D

With practice and experience the VNAV function in the FMS will be;

1/. Treated with the caution bordering on contempt it deserves,
2/. Double checked for silly errors that can creep in when programming
3/. Happily ignored and not missed when the inevitable vectoring or late runway change turns the VNAV calculated TOPD into pure fiction.

I know it sounds hard but with lots of practice the brain can do all that last minute replaning in not much longer than it takes to turn onto the new heading direct to the FAF.


7th May 2003, 00:37
Chimbu chuckles

Thanks for the info on calculating track miles for DME arcs - never heard of that before.

Unfortunately I haven't yet developed the spare brain capacity to apply it!

8th May 2003, 00:28
Like the guys say:

Height-to-be-lost (in thousands) x 3 (+ a couple NM buffer) = distance from TOD to fix. ROD = 5 x GS. Works perfectly every time.

From 25.000' to 5.000' = 20.000' to be lost. Start descend some 60NM out. Buffer of 2-5NM optional, but recommended if you need to be on target speed when reaching the desired altitude.

If your GS is 300kt, use 1500'/min ROD. Needs to be re-calculated as GS increases in descend, and the wind component is likely to change.

10th May 2003, 19:27

Another way to do this is; if you are comfortable using the FMC, (if you are flying a Boeing with FMC), just redo your arrival properly, then look at your descend page, look at your FPA, V/B and V/S. If your FPA is less than your V/B, increase your vertical speed to slightly below your V/S (e.g V/S 2349, use the vert speed to 2400fpm) This will result in your FPA matching or bettering your V/B. With that situation,you will be able to capture your path. Keep monitoring the V/S...

Hope that helps...

Pilot Pete
11th May 2003, 06:09

I think the whole idea is to have a backup method in your head which confirms what the FMC is telling you, or tells you when it has screwed up the VNAV profile due to several updates to the LNAV track.



ps simplest method has to be 3xheight to lose (in thousands) plus 1nm for every 10kts airspeed you have to lose down to your 'centre fix speed'. Works 99/100. Just put a fix in on the field whilst the other pilot is madly updating LNAV track every time your heading changes and guesstimate the track miles to the field. You are only trying to get in the ballpark to end up at about 15nm to run with the speed and height under control.

11th May 2003, 07:56
Can't agree with you any less there, Pilot Pete. I for one use this method as an absolute backup:

Passing 20000 ft, shud hv 63 miles to run
15000 ft 50 nm
10000 ft 35 nm
5000 ft 20 nm at 250kt

If for instance at 50 nm to run I am at 16500 ft, I am 1500 ft high.

At any other level, I just add or subtract 3 miles per thousand feet. I wont be too far off...