eng1170

Good Evening,

Just looking for some info from Airbus crews. I completed my A319/320/321 CRS course earlier this year and we kind of stuck to the "engineery" bit of the throttle system/engine controls/ECAM etc, only really mentioning "FLX/MCT" as a throttle detent. I haven't had much hands on with the aircraft yet hence the following request.

I've had a read through the back of the 320 performance manual which had a brief single page on flex temp and a couple of charts but to be perfectly honest it didn't really help much in my understanding of what/how it all goes together.

My (possibly wrong!) understanding is that flex temp is a crew selection made into the FMGC/FMS based on aircraft weight and environmental conditions - yes/no?! What I don'treally understand is how do you derive what the flex temp setting is and what does this give you in terms of N1/N2

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!!)

Any help in learning something new and a little bit more in depth would be appreciated, thank's in advance.

Rgds,

Eng

Carnage Matey!

Correct.

You generally don't derive it yourself, you look it up in the aircraft performance tables like you do with the take off speeds. I seem to remember that when you've entered a FLX temp the upper ECAM display will show the target derated EPR/N1 which you will compare against the achieved EPR/N1 to check the aircraft is producing the required thrust. We generally didn't/don't monitor N2 apart from checking it looks like it's in roughly the right place.

captain_jeeves

(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!

DesiPilot

There are a few factors that can effect the Flex temperature. If the calculated Flex is 67 degrees from performance chart using your TOW, we take in factors if the runway conditions are wet (usually reduces the flex temperature), also the QNH plays a part if QNH is anything but 1013 it changes the flex temprature as well. So the end result may not be 67 but it could be lower (if wet and low QNH).
Now there are certain limitations when you can or cannot flex. I'll be more than happy to type it out for you if you need them.

FlapsOne

Have a look at FCOM 2.2.14.

eng1170

Excellent replies thank's all - feel free to add further!

Would I therefore be corrct in saying a lower flex temp = higher thrust output and vice versa?

Thank's again, the pieces are starting to fall into place!

Eng

captain_jeeves

In simple terms, yes. More appropriately, the lower the flex, the more power REQUIRED for the given conditions.



If it helps, all things equal and standard (QRH, OAT, Etc) if you "hypothetically" set flex to your rated temp, you would in fact have full-thrust. Again, this is just a generalization.


Guys, remember, this question asked for a general overview. Let's not get too carried away with the FCOM and Performance Chart specifics.

eng1170

Thank's again Captain Jeeves, starting to get to grips with the theory now. I've heard the term on many occasions but it's not a function we as engineers deal with alot, engine running for us requires us to manually make the appropriate power settings.

At the moment I'm keen to "get to grips" with the Airbus as there's an awful lot of different theories/practices compared to those I'm used to on the 737 Classic's.

Loads to learn!!

Thank's again for your help,

Eng

captain_jeeves

You're very welcome. I came from Wide-ass Boeings myself, but have been flying the Bus for about 10 years. If I can help with anything, feel free to PM me.

Cheers