How do you calculate a derated take-off?

[NickN]

Can one of the resident pilots give a brief outline of what is used in order to calculate what percentage of N1 is used for a derated take-off. I'd like know for my own curiosity.

[Brenden S]

On a 21 degree day they say that its 41 degree's which then will give you your de-rated take off power. Basically you put it in the little box and it works it out for you.

[NickN]

Damn that little box, I was hoping for some sort of equation or methodology which could be calculated by a human being.

[Bill S]

Damn that little box, I was hoping for some sort of equation or methodology which could be calculated by a human being.

All that stuff is pretty complex as it involves looking at the aircraft performance over a large range of temperatures and also how the local terrain affects the flight path, and also if there's any additional restrictions.
So it's not something you do in a couple of minutes in the cockpit, it's worked out by a professional team and their results are compilated into a book with a bunch of tables & numbers that the pilots read from to get the performance data.

[Russell D]

In my search to find out what a derated takeoff actually is (you learn something new everyday) I stumbled across the following:

http://www.pilotosdeiberia.com/images/tecn/airbus_sfo/24derated/mcdu1.gif

and

http://www.pilotosdeiberia.com/images/tecn/airbus_sfo/24derated/mcdu2.gif

From thew website http://www.pilotosdeiberia.com/areatec/airbus_sfo/24derated.htm

[Hugh Jarse]

Nick, you probably know that the idea of a de-rate (or FLEX) takeoff is to ensure at least the climb performance of CAO 20.7.1b.

There are a couple of ways to do it, one being the "assumed temperature" method. What you do is get your current BRW and look in your charts for a corresponding maximum temperature that you can take off from that particular airport. Some of the variables are runway, wind and flap setting. Usually the operator has charts for runway and flap, with wind components tabulated on each chart.

Depending on the aircraft type, the methodology varies slightly. In some instances you simply slide up the wind column to your actual BRW and use the figure there, whereas with others you have a maximum temp/thrust combination and/or have to refer to an extra table (as in the E-Jet) to come up with a figure.

Normally only takes a few seconds to work out. As always, there are operational limitations associated with de-rate takeoffs, which vary from type to type.

Hope this helps.:)

[Chris Roope]

A little more background to the process, info based on a GE CF6 on a 767/744 etc.

As temperature increases the density of air decreases, so the amount of thrust an engine can produce decreases. The engine is able to produce its maximum takeoff thrust up to 30 degrees celcius (by spinning faster as temperature increases to produce the same thrust). Above this temperature the amount of thrust it can produce decreases.
Takeoff charts for determining takeoff speeds/thrust take into account this decrease in maximum thrust above 30C (which results in a reducing performance limited maximum takeoff weight as the temperature increases over 30C). When calculating an assumed temperature takeoff you simply look on the chart to determine, for your current weight, what the maximum temperature you could theoretically takeoff at is (the takeoff charts go up to temperatures of around 70C for this theoretical calculation).
This temperature is then entered into the FMC or a table in the performance book to determine what thrust setting at your current temperature corresponds to the maximum thrust available at the theoretical assumed temperature. And that is the thrust used for takeoff! (Well not quite... you actually subtract 6C off the assumed temperature as a 'pad' so as not to put yourself in a limiting situation).

[Matt D]

This chart is a couple of years old but was actively used by a certain (non-Australian) airline for Rwy25 at Sydney (B744 RR Flap20). It shows many of the factors mentioned above and ultimately showed how much weight could be lifted off the runway under certain conditions. The assumed temp (derate) figures are included on the right.