When to de-rate?

[United744]

Hi,

Apologies if this is a dumb question or should be obvious, but given the lack of performance data, what is the best way to determine when (and how much) to de-rate takeoff/climb?

[Jeroen Hoppenbrouwers]

You need very specific information about your intended runway, and the weather. Normally there is no way you can really do a de-rate unless you perform significant calculations, or use a pre-made table from somebody who did those calculations for that runway ahead of time.


Hoppie

[nicolas]

I will ask my dispatcher ;-)
PFPX
Nic

[Vnav]

In real world, the answer is: when your takeoff performance software says so. You have options that allow you to let the software go for the optimum - ie the lowest - derate available + assumed temperature according to company policy. Using weight books, one probably would have to check the available derates and see what he gets.

Now, if you have no performance software nor weight books at home    then use the FPPM or the FCOM performance dispatch. Generally, on the 747, you are mostly limited by the runway length (other cases can happen - obstacle, brake energy...- but are more complex to calculate). You can  then use excel or an equivalent software to build a simple spreadsheet calculating the max allowable takeoff weight (based on runway length and current conditions) and the corresponding assumed temperature. You can do it with derate if you have the data in the books, that's just more work. It will take a bit of time but if you don't want to buy PFPX or TOPCAT and still get something sensible, this is the way.

Climb derate is automatically selected by the FMC depending on the reduced takeoff thrust but I can imagine that some airlines may have more specific policies.

[Michael Benson]

Hi,

We've got a very early version of our performance calculator:

https://www.dropbox.com/s/87ebha438f2yngq/PSXPerf.rar?dl=0

Notes

- Only works RB211 Engines
- Only works with flap 20
- Only works with TO thrust
(The only options BA works with)
- At the moment PSX works with GA EPR rather than TO EPR so in order to get the rightish EPR you need to use the Hardy TO/TO1/TO2 and temp to get a similar takeoff EPR on the perf pages.
- At the moment only takes field and climb limit into consideration.  V speed wise its only dry runway and minimum control.

It's not even version 1 yet so while feedback is welcome don't expect miracles!

[Will]

Hello Triple7, thanks for the contribution to the community.

1. Does it work with a Mac?

2. Any chance you'll allow users to choose units (kg, tons, lbs, etc.)?

Thanks!

[Michael Benson]

Slight bug - if you enter a TOW that isn't a multiple of 10 (200/330/250/etc) it'll crash.

I've fixed it and also added the ability to work out max TOW for a runway/ambient conditions.  I shall upload when I get home from talking to planes to tonight.

Will I should be able to add those things.  I've never coded anything before this so may just take a few days.  It doesn't work on Mac and I've got no idea where I'd start!

[Michael Benson]

Just to keep it a little neater on the forum updates to the perf calculator will be here