THR IDLE / OP DES TOD Calculation

[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.

[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.

[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!

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!

Dr