flex take off tempreture ?

[flame_bringer]

Alright i know whats a flex take off which is a take off with a reduced thrust in case if full thrust is not needed ( to extend engine life or w.e ) .
But whats the deal with the tempreture ? why are we inserting flex tempreture in the MCDU if we intend to perform a flex take off ? does it have any relationship to the ambient tempreture ?

[Piltdown Man]

The amount of thrust an engine produces is directly proportional to the density of the air. As the temperature increases, the density reduces. So if you don't need all of the thrust that the engine can produce, you get it to produce less thrust by telling it a lie - you tell it that that outside temperature is greater than it actually is. Then when you select take off thrust, it will now produce less thrust that it normally would. And the deal with ambient temperature is that there is normally a maximum amount you can flex, ie. difference between ambient and flex temperature.

PM

[flame_bringer]

so the more the flex tempreture we enter in the MCDU the less thrust it produces ?what if i enter the ambient tempreture ?will TO,GA thrust will be achieved irrespective of the ambient tempreture value ?
And what happens if i dont enter a flex tempreture when im programming the MCDU ?can i still achieve flex take off by putting the thrust levers in the FLX,MCT detent ?

[john_tullamarine]

so the more the flex tempreture we enter in the MCDU the less thrust it produces ?

indeed

what if i enter the ambient tempreture ?

the engine will produce whatever is the limiting (maximum) thrust for the day

will TOGA thrust will be achieved irrespective of the ambient tempreture value ?

if you command TOGA, you will get whatever the computer is programmed to give for TOGA and that will include consideration of OAT

The amount of thrust an engine produces is directly proportional to the density of the air.

For turbine engines, the parameter of interest is OAT rather than density due to hot section temperature limits. For pistons, density becomes more of interest.

And the deal with ambient temperature is that there is normally a maximum amount you can flex, ie. difference between ambient and flex temperature.

Not quite .. the limit is the maximum thrust difference (rather than the maximum temperature difference) which is related to the temperature difference.

There are numerous threads on flexing so it may be useful for you to do a search.

[flame_bringer]

Im still confused
perhaps i'd understand if i get an answer to this question.
And what happens if i dont enter a flex tempreture when im programming the MCDU ?can i still achieve flex take off by putting the thrust levers in the FLX,MCT detent ?
i mean whats the use of the FLX,MCT detent :S considering what you guys said , we enter a flex tempreture then we set the thrust levers to the TO,GA detent and it will produce a less maximum thrust :S so when do we use the FLX,MCT detent then ?

[john_tullamarine]

by putting the thrust levers in the FLX,MCT detent

That would be specific to the Type. It might help if you nominate the specific Type so that someone competent on it can offer a comment.

[Torque2]

Simply put:

On the airbus if you put the thrust levers in the TOGA detent you will get TOGA thrust no matter what you have inserted as a flex temperature...thats why it is the TOGA detent.

If you put the levers in the FLEX detent with a valid flex temperature in the MCDU, the fadecs will supply the thrust relevant to that temperature but with some built in safeguards of a minimum % (can't remember exactly what it is) of max thrust.

If you put the levers in FLEX detent and haven't inserted a flex teperature then you will get a message telling you something like 'flex temp not set' then you must select TOGA as the fadec has no reference for a reduced power setting.

In the case of single engine, with the levers in the FLEX detent, in order to get MCT you must move the levers out of the detent towards TOGA briefly and then return to FLEX/MCT, this will then schedule MCT. :ok: