Originally Posted by
eng1170
Good Evening,
I was wondering if someone would be kind enough to either direct me to some good simple explanation somewhere (internet sites, manuals etc) or explain it in relatively simple terms (I'm not crew, but a Licensed Eng!!)
Rgds,
Eng
(Disclaimer: This is - intentionally - a "simple terms" explanation in an effort to explain the purpose behind "flex". Also, I am only going to explain the "temperature" component, not the other values. As is always the case, refer to your SOP, FCOM and performance charts for accurate information.)
In simple terms, “Flex” is an assumed temperate used to prolong engine life.
On a given day, and a given temperature, you have to determine the thrust requirement that your aircraft needs to achieve a specified climb requirement from a given runway.
Now, if you can achieve that required amount of climb on a much warmer day, then obviously the amount of engine power (output) required -with the hotter temperature - would be less. (We are talking about the power output, or the WORK that the engine has to do)
So, you make the take-off calculations based on the much hotter temperature. If, in fact, you can make the required climb performance with those assumed higher temperatures, you now have a “flex” condition.
You tell the FADEC (MCDU Entry) that the temperature is higher (e.g. 58 degrees) than the actual temperature. Then, the engine power output becomes that which would reflect a 58 degree day, thus less engine power output is developed, but the required amount for the assumed hotter temperature is developed.
In the old days, we would have simply pushed the throttles less in the direction of the full thrust point – basically.
Again, I've only explained the temperature component, as you asked for a simple explaination of the matter. Also, there are times when you can not use "Flex" and must use "Full-Thrust".
That’s as simple as it gets. Sorry!