THR IDLE / OP DES TOD Calculation

[go_soaring]

G'day jet drivers,

I'm interested in the finer points in calculating the TOD for a Thrust Idle, Open Descent - with spoil up at 1000'AGL.

So far, I know the basics based on a 0.78/300kt descent:
3 times height to lose, then;
add 5nm to slow down, then;
add 1.5nm for every 10kts of tailwind or,
subtract 1.0nm for every 10kts of headwind, then;
add or subtract 1.0nm for ever tonne above or below 60.0t LW.
Then monitor on the way down, adjust as required (using speed or brakes)

What are the calculations for:
Changes in QNH from datum of 1013
Changes in SPD from datum 0.78/300kts
Others?


go_soaring! instead

[remoak]

This sounds like trying to find the hardest possible way to perform a simple task!

You can't be a real pilot, if you were you would know that the computer calculates all that stuff for you and just tells you when to start down... :}

If you haven't got a computer-driven aircraft, just use the First Officer... :ok:

[Joker 10]

altitude in 1000 * 3 = miles to run simple really

[NoseGear]

Oh mate, why not make it hard for yourself and do Sudoku on the descent as well!?:rolleyes::E

There are 2 easy ways that I use, 4 x height for a gross error check for TOD, then use that down to FL150, its accurate enough. From that point use 3 x height, plus 1 track mile per 10knots of slowdown to 150kts, ie from 300 to 150 is 15 track miles, 250 to 150 is 10miles etc.....Too easy.

Or, multiply your track miles to run by 3, and subtract either 3000 feet from the total from 300kts, 2000feet from 250, and 1000 feet from 200 to Green Dot for the slowdown...easy.

[Metro man]

A lot depends on the airport and runway you are going to. If you are going straight in on a runway aligned with your track you need to be very accurate to get down and reduce speed correctly.

Different story if you need to do a 180 turn for final. ATC will play a big part as well possibly holding you high, giving you vectors all over the place for seperation or calling for an early speed reduction when you had counted on diving the height off.

Asking for track miles will give a good idea of how the descent is progressing. Judgment and experience come in as well, if you are 5000' at 15 miles doing 250kts speed brakes or early gear extension will be required. Same conditions but green dot speed will only require flap and gear at the normal times.

Each approach and landing is different, that's why we are paid the mega salaries.;)

[chimbu warrior]

Maybe it is just me, but it seems the answer might be a bit easier to find if you gave some clue as to the aircraft type involved.

[rodney rude]

Chimbu

He did! Thrust Idle / Open descent = Airbus

[donpizmeov]

THR IDLE /OP DES (same as flight level change in the boeing, SPEED / DES is the same as VNAV) makes it a bus, and the .78 would make it a minibus i would think so a 320?

Now Metro if you can land one from 15nm 5000' and 250kts straight in your a far better driver than I. Unless these minibuses slow and go down a lot better than their larger cousins. A 330/340 would need to be 20nm min to make that work, most would stick to 25nm.

I do it like nosegear but use the 4 times table to 10000 and then swap to the 3 times table with adjustment for speed, if at 250kts would add 5nm if 210kts and flap1 would add 3nm, if 180kts flap 2 would stay on the 3 times profile.

The Bus FMC uses a level segment to slow down, so to fly a continuous descent approach its profile is not accurate in the later parts of the descent.

[nomorecatering]

Thrpttle idle/open descent.....WTF.:sad::rolleyes::ugh::ouch::sad:

Is there a closed descent, or a half open half closed descent. Or maybe an almost closed ....but just a touch open descent. Like why do they have t make it so effing complicated. It seems these days one needs a phd in compters and the ability to type at 80 words per minute just to taxi the buggers.

God forbid, you have 13 modes for descent, god knows how many for climb and cruise...........But can it make a decent coffee......see not so smart afterall.:p

Does anyone know where to buy a copy of Avionics for dummies.:ooh:

[Metro man]

4 Descent modes on the A320

DES - means the aircraft will follow the plan ie speeds and heights at various way points will followed.

OPEN DES - Power back, aircraft drops. Same as C152

VS - Feet per minute rate of descent. Usefull if instructed to descend early by ATC, give them 500' per minute

FPA - Flight Path Angle, fly a 3 degree descent for an instrument approach.

If you want complication look up "mode reversions";)

Now Metro if you can land one from 15nm 5000' and 250kts straight in your a far better driver than I.

I wouldn't fancy doing it straight in, but from a 90 degree angle autopilot out and as much speed brake as VLS would allow followed by an early gear extension could be do able depending on weight and headwind.

Must remember to note the exact figures next time ATC give us a short cut.:)

[go_soaring]

All good answers, but not quite what I'm looking for. If there's any ex-Ansett drivers reading, and if you can remember the calculations, then I'd love to hear from you!

Those who have answered, cheers. Obviously in the end, it's just an aircraft, nothing hard, just fly it! One of the problems with the Airbus (probably all jets for that matter!), is that there seems to be many different ways to fly the approach for example. 20nm @ 250kts, manage speed and pull for THR IDLE / OP DES will get you to 1000' before getting the jets to spoil up. You can also drive it in at 250kts, level it up, manage speed, wait for the G/S intercept and hey presto, same result. You can leave the A320 in managed mode, and it'll do a beautiful job if you've put all the correct constraints (ALT's and SPD's) in (albeit slightly inefficient). You can...

Though the question I was asking was for the finer calculations in calculating a TOD point (obviously the QNH one is taking it a bit far, but if there is something for it, then why not arm yourself with the knowledge?). That's all. Not interested in re-fining the profile once on DES, or how to fly the approach. I understand that this changes daily, ATC, more/less wind than estimated, error in calculations, etc.


go_soaring! instead

[VH-ABC]

Never heard of spoil up... but have heard of spool up. Is spoil pretense for spool? Or the other way around...?

[Angle of Attack]

Well I prefer to disconnect autothrottle and idle the ba$tard then just fly the gates, forget autopilot modes they are just secondary. Any real pilot can pretty much tell the profile using the DME and distance, jeez it is not rocket science. But asking for details will depend on aircraft type so I cant tell you about the computer Airbus, I prefer to use my head!:bored:

[KABOY]

A rough estimate without scientific formulae will be better than a managed descent anytime. Then there is the speedbrake, which will always correct any high energy descent!.

Better lay down now, feel exhausted with all this theory.:eek:

[NoseGear]

go soaring, maybe if you posted exactly what your after you'll get your answer. You asked for TOD calcs and then proceeded to give all kinds of ways to scratch an itch, but then say it's not what your after.

There is no problem with an Airbus in descent, it does a fine job of it, but your the pilot, so you need to have a way of backing up and checking what it's doing. All the Airbus family, as I'm sure the Boeing, will all do Managed approaches with constraints very well. The constraints are not "put in" but are part of the database which you must check, there is certainly nothing "inefficient" about that. What is inefficient is QNH adjustments at TOD while 140NM out:rolleyes: Or, "driving it in level"..now that's inefficient in a jet. Your TOD calc is the least of your worries really, refining the descent is the trick, with due regard to winds, ATC, traffic, track shortening etc etc.

I personally have never heard of QNH corrections to TOD, after all, your in Standard, so how that could affect your TOD I have no idea. But then, if your doing your proper descent calcs, you'll catch that at Transition. Quite apart from it can't be more than a few hundred feet.:suspect:

Maybe just let the prooners know what how you want to "arm" yourself with said knowledge might help. :ok:

Nosey

[go_soaring]

Cheers NoseGear, I was starting to think that's what I'll have to do. Will give it some thought overnight and try again tomorrow...


go_soaring! instead

[Tempo]

Keep it simple....

TOPD = 3 times altitude + 10nm

On descent:

3 times track nm = Altitude (i.e 100nm = 30,000ft).
From 300 to 250kts I use 1000ft and from 250 to flaps up I use another 1000ft.

Therefore, at 100nm doing 300kts I want to be 30,000ft - 1000 - 1000 = passing 28000ft. At 30nm 250kts I want to be 9000 - 1000 = 8000ft.

Gates:
10000ft, 250kts, 40nm
8000ft, 250kts, 30nm
5000ft, 250kts, 20nm

Obviously serious tailwinds/headwinds make a difference. I use 1000ft/20kts of wind.

Keep it simple.

[go_soaring]

What I'm trying to figure out is. We're sitting in cruise @ FL350 into a sea level aerodrome. The broken arrow (Airbus's calculated descent point) is indicating a managed descent @ 117nm from destination.

350 * 3 = 105nm
105 + 5nm to slow down = 110nm
I've predicted 20kts T/W on descent, so 110nm + (2 * 1.5) = 113nm
The a/c weighs 58t on LW, 113nm - 2nm = 111nm

So if I wanted pull for THR IDLE / OP DES, and keep it that way to the ground (all things being equal!), then the TOD point would be 111nm or 6nm past the Airbus A320's broken arrow.

What I want to know is.. if ATC says, 0.76/280kts. What do I then do to the 111nm calculated descent point. In this case, it's a slower descent, so I must (surely!) add (X)nm per every say 0.01/010kts for example.

The QNH thing is pedantic, but if there's a calculation out there for it, why not factor it in and try and nail it!


All the usual stuff of working around gates, and 3* profiles + whatever still stand, but I'm just trying to find that TOD point where I can pull for THR IDLE and start my descent. Obviously I'll be monitoring everything, if I notice we get lower, I'll adjust accordingly; if we get higher, I'll adjust accordingly. All I want is that initial TOD point, the rest I can figure out myself, adjust, use my experience, local knowledge, etc, to get to the goal: 1000' spool up!


go_soaring! instead

[SixDemonBag]

Don't get too hung up on descent points. Wind changes and atc will impact your profile more than the Baro ever will.

Pity you don't fly a classic 74... Just descend 20 mind after the f/e opens reserve 1&4!

[Capn Bloggs]

One of the problems with the Airbus (probably all jets for that matter!)
I cannot believe a modern jet is so bad at a simple task. There are other jets that do do the right thing in Auto. :rolleyes:

[waren9]

Sounds to me Go_soaring that you've already got it sorted in your own mind.

Re


The QNH thing is pedantic, but if there's a calculation out there for it, why not factor it in


As I think you already know, there is. Give the aircraft all the data then use DES. Airbus use some very clever software to get the aircraft down the most efficient way. If you think you can do it better on the back of a fag packet then we will have to agree to disagree.

Remember managed modes will use the FM generated speeds and profiles that give a best fit for the conditions/constraints and the cost index the operator asked for. Selecting anything else is a departure from that.

Just when you get that sorted, they'll change the IDLE/PERF factors and you'll have to come up with something else again.

This question in the Tech Log forum will reach a far wider audience.

:rolleyes:

[Mr. Hat]

ATC and offsider allowing just keep adjusting the speed to ensure the taps stay closed. Can be nearly impossible in todays airspace (speed up slow down max speed min speed).

[Artisan]

go soaring,

Add 1Nm per 10Hpa above standard (1013), subtract 1Nm per 10HPa below standard.

E.G. You are cruising at FL400 on standard (1013). QNH at destination is 1033Hpa. A 1HPa pressure change is equivalent to 30 feet (approximately). So you actually have 40,000ft + 30x20 = 40,600 ft to lose, assuming the airport is at sea level. Hence you need an additional 2NM to lose the additional 600ft, based on a nominal 300ft/NM descent.

When checking the profile on the way down, use 3x(Flight Level/10)+ 10% TailWind + Decel((280 to 250 =3Nm) + (250 to 200 = 5Nm)) + QNH; down to transition level. Then 3x(Altitude/1000) + 10%TW + Decel (8 as above); down to 10,000ft. Then 3x(ALT/1000) + 10%TW + Decel (5); down to 200kts.

[Artificial Horizon]

I have been using 3x Altitude + 10 nm for a tailwind and - 10 nm for a headwind. Keep track of it the whole way down. If your are 250kts or less and 10000' or less at 30d them you are in the slot. If you are high you are o.k. if you are slow and vice versa.

Never had any issues in the last 6 years.

[waren9]

Artisan


A 1HPa pressure change is equivalent to 30 feet (approximately).


I learned that in PPL Met too.

Send Airbus an email for us mate because they seem to think it makes more difference than that. Have you seen how much the profile changes when you change QNH in the PERF page?

[go_soaring]

Thanks Artisan, exactly what I was looking for! Now to figure out the change of speed for descent, and I'll have what I'm looking for :)


go_soaring! instead

[Artisan]

Waren9

Yes I have and 1Nm/10HPa works well in practise.

Go Soaring

For 320Kts descent try 2 x Height + 10% + QNH Corr + Wind Corr + Decel, for the portion of the descent that will be flown at 320Kts. Use 3 x Height for the portion of descent below 250kts.

E.G. FL400; 2x40= 80 + 10% = 88NM.

[chimbu warrior]

Did some research and found this ;

TOPD (nm) = (FL x 3) + 10 + 2% per 10k of tailwind.

:cool:

[John Citizen]

(3 x altitude) + 10

plus the following corrections :

- 3nm/10 knots above econ descent speed (high speed = descend steeper)
+ 4nm/10 knots below econ descent speed (low speed = shallow descent)
(econ descend speed is normally about 290)

+/- 3nm per 10 knots tail/head wind (average wind for descent or perhaps the wind at the level 1/2 way down)

+/- 1 nm for each tonne above/below 60t

- 1 nm for each 333' of airport elevation

+/- 1 nm for each 11 hPa above/below standard

+/- 1 nm for every 3 degress above/below ISA temperature

Example :
FL350
descend at 280
average tailwind = 20 knots
62.0 tonne
QNH = 1024
ISA + 15

I came up with a descent point of 133 nm

Now to monitor on the way down the formula "(3 x level) + 10" does not work as you did not use this formula to calculate the descent point to start with.

You need to calculate a different descent factor from the original 3 x profile.

To do this, subtract 10 nm from 133 for the slow down (10 nm to slow down 100 knots). Now you get 123 nm. Divide this distance (123) by the initial cruise level (FL 350) and this now gives you 3.5.

So to calculate if you are on profile on the way down, just multiply your altitude by this factor (3.5) and add 10. This will ensure you have a continuous linear descent on the way down from your initial descent point.

If you multiply your initial cruise level (FL350) by this factor and add 10, you will get the 133 nm again !!

Just in case anyone asks or wants to criticise me, no I do not do these calculations on every flight. I just let the computer work it out.

[Lookleft]

The only segment of the descent that is really important is 20 track miles 250 knots. Manage the speed and set V/S to -800'/min. Everything else is subordinant to that requirement. The ToD calculations can't take into account ATC speed restrictions imposed at 15000', airspace restrictions (9000' at 30 miles) and last minute runway changes. You can craft a beautifully calculated ToD only to discover that at 22 trackmiles you are 7000' at 280kts!

[go_soaring]

Thanks John Citizen for that post, exactly the response I was hoping to get from here. Took a couple pages worth of replies, but we got there in the end!

Loved your last two sentances, got to cover all bases on here..


go_soaring! instead

[ForkTailedDrKiller]

(3 x altitude) + 10
plus the following corrections :
- 3nm/10 knots above econ descent speed (high speed = descend steeper)
+ 4nm/10 knots below econ descent speed (low speed = shallow descent)
(econ descend speed is normally about 290)
+/- 3nm per 10 knots tail/head wind (average wind for descent or perhaps the wind at the level 1/2 way down)
+/- 1 nm for each tonne above/below 60t
- 1 nm for each 333' of airport elevation
+/- 1 nm for each 11 hPa above/below standard
+/- 1 nm for every 3 degress above/below ISA temperature

Ohhh, my head hurts! :confused:

I knew there was a reason I didn't become an airline pilot!

As for sticking your arm up a cow's bum? Ahhhhh YES! There's not much mental arithmetic involved in that! :E

Dr :8