View Full Version : Airbus Flex formula

22nd Jul 2004, 15:42
Could any bus experts or drivers please tell me the formula or a formula used to calculate FLEX temps. I have always wondered how it is done.

Thanks :cool:

22nd Jul 2004, 15:49
** warning - I am no performance guru **

Airbus flex is he same as Boeing's Assumed temperature.

Basically you take the take off mass and the runway details, and work out the maximum temperature at which you could lift that mass off the runway. You then 'tell' the FMC/FMGS that that is the OAT.

The computers then schedule the fuel flow as if that is the temperature, reducing thrust and saving wear on the engine etc.

The resultant thrust can not be less than the climb thrust, which I think is 75% of certified thrust.

In my company, you either go into a table specific to the runway, flap setting and wind component and then look for the flex temp or send off an ACARS request with the details for a more sophisticated program to do the work and send the answer back.

Does this help?

22nd Jul 2004, 16:07
Ok I kind of get how it is calculated by that, what kind of runway data is required?

Are we talking elevation above MSL etc?

(temp at MSL 15oC so say 09L at EGLL is 75ft so 0.02x 75 + 15 + max temp?)

How exactly is maximum temperature found..is there a set formula to find this?


22nd Jul 2004, 16:51
You need runway length, slope, elevation, clearway, stopway and obstacle data. You produce a normal takeoff chart up to say 69C. When the aircraft weight is known, enter the chart to find the flex temperature. Legally you cannot flex more than 25% of the rating in use. The FMS will protect you for this.


23rd Jul 2004, 09:09

please tell me the formula or a formula used to calculate FLEX temps May I suggest a Perf A course to learn the basics of Performance. Only then will you be able to see where you need to start.

On the day we look it up in a book, or get it via a computer. It depends on many variables, all applied in their own formulae (TORA limited? 2nd segment climb? using increased V2? what Flap setting? Obstacles?).


Old Smokey
25th Jul 2004, 13:25
There is no formula, 'per se', for calculating Flex Temp (or Assumed Temp if you're Boeing).

For each runway direction that the aircraft operator will use, Airbus, Boeing, or the performance engineering provider will conduct an airport analysis, considering Runway characteristics (Length, Stopway, Clearway, Slope, etc.), obstacles, acceleration altitudes, Pressure Height, Temperature, for each Takeoff Flap setting and for a standard series of wind components.

So far, in this this much simplified paragraph, the performance engineer would have had to refer to thousands of formulas, and tens of thousands of data extraction points from the Airplane Performance Manual or AFM.

From this runway analysis (discretely different from all other runways even if their physical characteristics seem to be the same), the performance engineer produces RTOWs (Regulated Takeoff Weights), being the limiting weights in a Temperature / Wind Component matrix.

Thus, for a particular runway, the pilot can extract from the RTOW, the performance limited takeoff weight for a data pair of Temperature and Wind.

If, on a given day, the Actual Takeoff Weight (ATOW) is below the limiting weight (RTOW), the pilot may set the thrust to that which would be delivered at the temperature at which the ATOW equals the RTOW, for example,

ATOW is 200.0 T, OAT is 20C

RTOW at 20C is 250.0 T, I am well under the performance limit.

RTOW at 42C is 200.0 T, if the temperature increases to 42C I can still go, or ALTERNATIVELY, if I only use the thrust that the engine would produce at 42C I can still go, and use much less thrust in doing so. Therefor I will 'Flex' my Airbus engines to 42C, or tell my Boeing engines that they may 'Assume' the temperature to be 42C.

This has been long-winded. It was deliberately long-winded, but even so, fell far short of establishing the extremely complex business of performance engineering when it comes to Airport Analysis. There are no formulae, the process requires countless applications of formulae and application of performance data. Even when some formulae do work, a 'break point' will always jump up to cut off your formula when you least expect it.

Pilots (I'm one of 'em, but also a performance engineer), love to reduce complexities to simple and quick formulae. 99% of the time they will work within practical limits, the other 1% of the time, they can kill you.

27th Jul 2004, 08:25
Built into all serious computers is a line of code that calculates "sigma" the relative air density. If you want to do "sigma" manually try http://wahiduddin.net/calc/calc_da_rh.htm for very good on-line calculators. Next you need a glance at TOSTA, the takeoff safety training aid provided by Airbus Flight Operations Support & Line Assistance. (Issue 2 -11/2001?) Inside the computer also will be look-up tables of effective thrust plotted against ram drag and lift coefficient plotted against V LOF. The first guess of flex will be based on (Square of Weight)/(sigma x wing area x lift coefft x effective thrust).Then the computer will check its legality within certain bounds. The Boeing does the same, it's physics after all. The better airline POH's require you to look at Rel H or dew point, the less well thought out ones ignore it. Ah well!