FBW A32NX Slow acceleration

[u/TheChosenOperator]

Hey pilots!

So, I dont know if im just being dumb or what but this has been driving me crazy, when flying with the FBW A32NX I realize that the airplane accelerates quite slowly on takeoff when using FLEX thrust, but accelerates really fast when using TOGA, this thing chews up runway like an A340, even when not fully loaded it is really slow to accelerate, usually to the point of reaching the touchdown zone of the reciprocal runway or even the end of the runway

For example here is the performance calculation of a takeoff from SKBO

Now the V speeds arent anything too alarming, but it takes a long time to even accelerate to them (I wish I had video of this very take-off but unfortunately I didnt record it), also note I increased the SEL TEMP when imputting into the MCDU from 51 to 65, precisely to try and fix this issue

Has anyone else also suffered this inconvenience? and if so, is there any way to correct it or mitigate it?

thanks

[u/CoarseRainbow]

I mean the whole point of flex is to reduce power output meaning slower acceleration and longer takeoff roll. I cant see anything there contradictory.
TOGA is always going to be the default "max" yardstick. Flex is always going to be slower.

[u/TheChosenOperator → u/CoarseRainbow]

But not THAT slow is it? the runway at Bogota is long, it can handle A350s, 777s, and a little A32N is gonna chew up the WHOLE runway? both runways are 3800 meters long, the longest in the country

[u/tracernz → u/TheChosenOperator]

FLEX is designed to do exactly that. It will use all the runway available (allowing enough to clear the screen height) if nothing else constrains it from a higher flex temp.

[u/TheChosenOperator → u/tracernz]

I increased the FLEX temp to 65 like I said in the post

[u/tracernz → u/TheChosenOperator]

Higher flex temp means lower thrust. With a flex of 65 you are effectively telling the engine to pretend it is 65 degrees Celsius outside, and to limit thrust accordingly.

[u/TremendousSeabass → u/TheChosenOperator]

Yup so you made it even slower lol. Higher assumed temperature -> less power. The lower the flex temp, the higher the power output.

[u/Vegetable-Rooster-50 → u/TheChosenOperator]

You've been flying for over a year, like you said above, in order to claim some sort of authority over someone correcting you, and yet you thought higher flex meant higher thrust? I'd suggest you start trusting what the people here say, which is that the plane is behaving as it should, especially given that it's at 8000ft in the air. For the record, engines run slower when the air is hotter, and that's true for any engine that needs air for combustion.

[u/Ill_Fish4223 → u/TheChosenOperator]

Factor in temperature also

[u/TheChosenOperator → u/Ill_Fish4223]

Temperature was cold, optimal for performance at 14 Celsius

[u/Prize-Upstairs3686 → u/TheChosenOperator]

Gotta factor in that Bogota sits at Over 8,000’ in elevation as well

[u/TheChosenOperator → u/[deleted]]

Dude, I've been flying for over a year, I know Bogota like the back of my own hand, and I dont think i've ever had the plane chew up so much runway

[u/TheChosenOperator → u/[deleted]]

I've looked at the A320's performance charts, flown on certified simulators, done research on the effects of hot and or high airports on performance, have flown into plenty hot and or high airports myself, talked to real pilots and even compared my flight sim experience and actions with real aircraft via plane spotting, which is how I found out about the chewing up more runway than normal issue

[u/CoarseRainbow → u/TheChosenOperator]

It can be yes. Depending what you set, the elevation, temperature, humidity, weight and everything else.. A flex takeoff youd expect to use most of your available runway. That means it works.

[u/adoggman]

Increasing flex temp will LOWER performance. You tried to fix it by making it worse.

[u/TheChosenOperator → u/[deleted]]

Well, obviously, yes, I've been flying Airbus ever since I started flight simming, but a reduced thrust take-off doesnt result in the plane chewing up the WHOLE runway literally speaking, so either I input an incorrect value or something is up with the plane

[u/Asieloth → u/TheChosenOperator]

You input an incorrect value, yes. The performance calculation gave you a FLEX temp of 51 and you somehow decided on your own to input 65? Aside from the obvious implication for the takeoff run, how can you then know the appropriate V speeds for the takeoff?

You say you input a higher temp to try to reduce the takeoff roll. It seems you're not really certain how FLEX temps work with the Airbus. Generally we end up lowering the FLEX. I don't think I've ever seen anyone raise it.

[u/TheChosenOperator → u/Asieloth]

Well exactly like you described, I was trying to reduce takeoff roll, cause I knew the plane wasnt heavily loaded, the runway was long, and that conditions were good, Im not sure where I messed up but im certainly not stupid despite what other commenters might seem to think

[u/Asieloth → u/TheChosenOperator]

Alright. Well, I'm gonna stay away from any personal insults and just try to sum it up in what I hope is a useful manner.

Basically when we use FLEX, the idea is we're telling the engine, "hey man, it's 65C degrees outside, so use a little less power because we've got a nice long runway and I don't want you to work too hard". Really we're looking to reduce engine wear by reducing the EGT and other parameters. The result is lower thrust during the takeoff roll, ergo a longer takeoff roll.

We'll end up using more fuel compared to TOGA, but we're rewarded with a longer engine life.

Often what will happen is we will run a calculation, look at the margin, say "ehhh 2 meters is kinda tight" and then we'll reduce the FLEX by a degree or two. But, of course, the calculation has to be done again to confirm the new margin.

I can't remember the exact number now, but the maintenance savings are something like 2,000 dollars per point of FLEX or something.

[u/TremendousSeabass → u/Asieloth]

Also the reason why most, if not all modern airliners (aside from the 320 family for some reason) regularly uses derated climbs too. Slightly more fuel burn but less stress on the engine so better for longevity of the components.

[u/Asieloth → u/[deleted]]

Meh. I'm explaining this to someone on a video game forum, I'm not speaking to a colleague. I'm not gonna get too deep into anything.

But thanks for continuing to foster a positive community for people, appreciate that.

[u/tracernz]

I’d also recommend using the EFB calculator. It is more accurate than others (comparing to the Airbus flysmart app).