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Descent Calculations A320

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Descent Calculations A320

Old 22nd Apr 2013, 13:10
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Descent Calculations A320

Hi everyone!

I am doing my supis as of now and one thing just keeps me boggling is the computation of the descent profile.I understand how to calculate the TOD ie.your FL multiplied 3+10 nm for deceleration +/- one third of HW or TW. 1)Beyond this how do I check whether I am high or low on profile?
2)The distance calculated by the above formula should be checked against the VOR dist on ND or on the MCDU.
3)there are some other calculation if ATC is giving you a straight in approach or vectoring you around.How to cater for these calculations?
4)When calculating descent profile do we take the airport elevation or the platform altitude?
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Old 22nd Apr 2013, 13:23
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Good Morning Stalwart2010

Airbus has a great VNav system.

Just verify the STAR altitude constraints are in your Nav page are correct. insert your descent winds and 10 miles before your descent point (with ATC blessings of course) push nut pull the altitude selection and you are in managed descent. The reason for 10 miles is less noticeable change in engine sound, a 1,000 foot a minute descent and when the profile is met power is reduced to flight idle and all being equal you stay in the profile window.

Hope that helps
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Old 23rd Apr 2013, 13:28
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To put it bluntly, I find the vertical deviation calculated by the FMGC useless for the later part of your approach.

If it is your companies policy or preferred technique to extend the runway centerline once on vectors, the vertical deviation again is useless since it calculates a profile from your current position directly towards the CF without taking in to account the realistic amount of track miles, which you as a pilot can guesstimate with the help of TCAS, or even better, ATC might just tell you. Often your TCAS gives you a good clue as to how many people are ahead of you on finals, and if there is nobody ahead of you most controllers will give you at least a 7 or 8 mile final, if not 10.

Also, most people activate the approach once they start using selected speed, if ATC wants, say 220KTS. With the approach activated the vertical deviation is again useless since it works out a descent at green dot speed, if you're still doing 220 or 240, or whatever, then in open descent you're going down a little bit quicker then at green dot.

The FMGC also works out your descent profile with only half the headwind you're experiencing and double the tailwind, it also assumes the engine anti-ice is switched on and assumes a thrust value for that, which equally is more often then not the case.

The Airbus profiles are definately safe and conservative, but I find it a total waste and poor airmanship to fly a descent with thrust on, which is what you'll often get when you go down at, or a mile or two, before the descent arrow, hence again I find most people will fly past the descent arrow to force the autothrust to idle. Next time you fly go 10 / 15 miles past the arrow and pull open descent, you'll be on profile again around 20000ft, which proves my point.

What I find works best is track miles to run x 3 + 10% (1NM = 330ft on a 3 degree slope) + airfield elevation to get a mental model of a 3 degree slope, then you add whatever you feel comfortable with for your deceleration etc, I would say 5-10 miles depending on how heavy you are and which Airbus you're flying.

This has served me well on the 319/320/330/340 and 380.

Last edited by PGA; 23rd Apr 2013 at 15:20.
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Old 23rd Apr 2013, 14:33
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Just like you, I am also a newbie and have found this technique EXTREMELY useful.
I put in the GS intercept point, or on NPA the FAF on the MCDU prog page to give me the distance.

I use the altitude to lose x 3 + 10
I add or subtract some more depending on headwind or tailwind.
I calculate from platform altitude since that's the point I have on the prog page.

This way you will either end up spot on, or slightly low, which as newbies is safer than ending up high.
I have found that with more flights, consistency, calculation gets better, I take into account traffic, etc.
Even then sometimes I am not certain and I will ask the captain what he thinks.
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Old 23rd Apr 2013, 20:05
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I'm not on the Airbus, but on the Boeing, but assuming basic aerodynamics are not much different, here are some pointers I've picked up. They may or may not be applicable to you.

1. The VNAV/computer path is only as good as the info you put into it. It is only correct when you actually fly what's in the box, which is practically never.

2. If you are in doubt if you are high or low, then you are high.

3. There are only two ways to unfu*k number 2: go fast and clean (like a brick), or slow down, dirty up (like a leaf). The first one is better if you are far out, the latter is better if you are close in.

4. If you need to eyeball it when being vectored on downwind, use the range-to-altitude arc to plan to be abeam the field at twice your platform altitude if at normal descent speed, or add up to 2000 feet if you have already been slowed down.

5. If you are number x in sequence, plan 1500' pr plane ahead. So if you're number 6, about 9000' isn't too far off the mark. Use a lower figure (like 1000' pr plane ahead) for denser airspace.

Good luck!
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Old 24th Apr 2013, 19:25
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Never ever ever trust the Airbus VNAV.

You will bust a crossing faster than the FAA can take your ticket.
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Old 24th Apr 2013, 19:29
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Standard 3:1

Add 1nm for every 10 KTS TW

Add 1nm for every 10 KTS speed to lose approaching a speed restriction.

Last edited by junebug172; 24th Apr 2013 at 19:30.
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Old 27th Apr 2013, 15:46
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Airbus VNAV is the biggest crap. Far too conservative. Gets you down low and slow, and very often outside controlled airspace. The aircraft can actually descend steeper than the VNAV calculates. This results in the low and slow profile as it calculates too conservatively. You either end up descending much slower than desired to keep idle thrust otherwise you will need to maintain some thrust during the entire descent to hold the speed and profile. Just crap I say. Definitely not great. And yes I do put all the necessary data in but it just does not work.

Last edited by John Citizen; 27th Apr 2013 at 15:52.
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Old 28th Apr 2013, 18:07
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Cool Newbie City

Please chuck in any comments here: A320

I have not figured out the 10 bit, but have not applied myself to study it yet either - apart from that:-

I use - FLx3(+50ft) = dist to go to t/d, invariably, the surface, not bad if surface equals desired runway. FL350 = 105 miles.

-or- Fl to Fl or alt x 3 again. FL100 desc to 7000 say, (notwithstanding barometric diff between std and QNH - a BIG Notwithstanding, just for this . . example)

So, FL100 to FL70 or 7,000 assuming QNH is 1013.2 today.

3,000 right? 3x3=9 miles. (D9/S250/210=T so 2.16 mins gives about 1153 fpm rod)
quite a hurtle, but see further . .

I aim, to be at spd 210 by 12 track miles (even if the track is vectors and notwithstanding rocks in the way MSAs and the like out to ruin everyones day) using ND at 20 and gradually reducing range rings, you get the drift. . . .

180 by 08 miles

160 by 06 miles -

Also, built into this is my configuration at those speeds.

For example 210 `by` 12 miles 12 miles = 3600` +/-
========180..."" 08 miles = 2400`+/- (f1) (Gear at one dot or...)
========160..."" 06 miles = 1800`+/- (f2)(f3 at160) Gear.
========Managed"" 4.5 miles = - - - - - -(FULL)

Managed being favoured, by many, all the way down.
the mess-ups to the above are ATC, red cells, traffic, Terrain.

I am finding that eventhough we can get . . messed about, these speed gates help to formulate a mental model of the desired spd and profile, given the distances to go which even if they could become opaque with ATCs creative vectoring I can put myself into low level hang out mode and just drive over the approach with a 190 or 180 selected and a F1, and they can keep me up there all day, subject to one or two requirements and needs of course.

A word on ATC, I have nearly always found that ATC not only give you a vector but will always tell you what it is for - i.e., "if you make your heading 235 now that will bring you onto an intercept with LOC 27 - call LOC established . . "
Admittedly, some countries may not offer the same picture profile as others.

P.S. I love the: wait descent until past TOD - for an idle descent, cool.

Also - wpt to a point on app/final watchit! MSA/SSA (Mex comes to mind)
SID or DP to a wpt watchit! MSA/SSA

Naturally, adjustments have to made for a head or tail wind component


Last edited by Natstrackalpha; 29th Apr 2013 at 12:19. Reason: finger trouble.
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Old 29th Apr 2013, 02:45
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Thumbs up

Press the Metric Pb on the FCU
1 feet = 0.30 meters so

just Divide the Metric altitude by 100 and add 10 for deceleration
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Old 29th Apr 2013, 12:30
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Plane boy

Press the Metric Pb on the FCU
1 feet = 0.30 meters so
Thank you Plane Boy. Please assume I am completely thick for this experiment (and you won`t be far wrong . . ).

Am I right in thinking that the decell you are referring too would be straight and level . . . and therefore you are not descending (if level momentarily)?
Adding to this question, can we reduce speed whilst going down hill, apart from gear at the approriate spds and or flaps and or speed brakes . . . ?

Or, (even thicker still) is the "time/period" of decell effecting the RoD because naturally if your speed changes from an intial 280 at ToD say and ends up at 180 kts somewhere on the Initial Approach leg then the 3 deg slant . . .range is going to be altered . . ?

I feel sure I am missing a relevant bit here . . . sorrrreeeee!

Last edited by Natstrackalpha; 29th Apr 2013 at 12:34.
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Old 29th Apr 2013, 12:45
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And you haven't read the bit about "idle-factor". I bet your company has set 0.0
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Old 29th Apr 2013, 13:48
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3 x ALT and start OP DES at your calculated dist, ref the FLT PLN
page. Add in your factors like dist to 210kt etc, expected TW/HW,
flap setting (3 or Full) and GW.

As junebug said NEVER trust that bloody DES mode - it'll screw you
up big time esp when it comes to any combined alt/speed crossing
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Old 29th Apr 2013, 21:15
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Maindude (and all)

-found this PPRuNe link......



So, . . . how about half your groundspeed likein conventional flying and work it out yourself - that way you can keep abreast of the descent profile . . .

what about the semi circle (on the ND) that - if you adjust the RoD the semi circle moves forward or back corresponding to your selected V/S.

Last edited by Natstrackalpha; 29th Apr 2013 at 21:22.
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