New Takeoff performance with a change of air temperature
 

Let’s say your max Flex temperature is ISA plus 60. ( sea level, standard atmosphere, max flex 75 degrees)

Let´s say we get a Takeoff performance of 75 degrees flex at an OAT of 20 degrees. You taxi out to the runway and notice the temperature has increased by 5 degrees. Do you have to do a new performance calculation? I just calculated some random figures and it seems the Takeoff performance figures don’t change at all if the temperature goes up and you have a very high flex temperature. Any explanation?

With a 75 degree flex, you’re assuming the OAT is 75 degrees. If the actual temperature goes from 25 to 30, you’ll still be well inside the performance envelope that you’ve calculated. No need for change.

How do you know that you are still good without doing the calculation again. You could be field length limited. i don’t understand how you can possibly know? What about when youchange the temp from 20 to 35 degrees? Is it still good, where is the limit?

Does anyone else find the OP's question seriously worrying?

Using Flex reduces the take-off power of the engines by telling the computers that the OAT is higher than it actually is. So you have told the engines that the OAT is 75 deg, as long as the OAT does not exceed 75, you will be OK (but prob outside the operating limits for the aircraft).

If I do that question to a captain while on line training that flight would've been a nightmare and probably a big redflag for not knowing tref trefmax etc...

Ah, in your case the limit is 75 degrees. You've calculated numbers that say at your planned weight the aircraft will meet all the TO and climb performance requirements up to an OAT of 75 degrees. Since the OAT is less than 75 the aircraft will actually perform a bit better than it would on a 75 degree day.

Because the TO speeds are based on an assumed temperature of whatever your flex is. At least for Airbus (see getting to grips with performance), and I’m sure it’s the same for other manufacturers as well.
So if your flex is 75°, your indicated TO speeds assume a much higher ground speed.

I’m no performance engineer, so please correct me if I’m wrong: With a flex of 75, your performance calculation assumes max engine thrust (TOGA) at 75° and takes into account ground speed at 75°. So any increase in OAT, up to your flex temperature shouldn’t change your performance. The only difference is that the lower your OAT is, the more safety margin you have due to the lower ground speeds and less distance used. An OAT of 75° should give you TOGA with the same performance figures. Although an OAT of 75° is probably above the aircraft certification, so you might not be able to calculate this.

And the only way to be sure is to try a calculation with 35° yourself. Or even higher temperatures.

For info: The company I work for prohibits the use of flex if the flex temp is less than OAT+5°. I believe it’s because the temperature sensor at the engine might measure a different temperature than your OAT sensor and thus your TO performance might be invalid. TOGA is required in that case.

On flat-rated engines where N1 sets power (GE, CFM), the OAT affects the N1 when using the assumed temperature method. This is because N1 is not constant below the corner point temperature. See attached graphic. On engines where EPR sets power, the OAT below the corner point temperature does not affect EPR when using the assumed temperature method.
https://cimg6.ibsrv.net/gimg/pprune....87b0f2831d.png

Thanks for the inputs so far.

i just did a calculation for a Takeoff which seems to be obstacle limited with an OAT of 9 degrees.

Results:

V1 136
VR 158
V2 166

Flaps 1+F and Flex 46



As soon as I select 10 degrees OAT I get a different result.

V1 133
VR 152
V2 159

Flaps 2 and still Flex 46. This takeoff is not obstacle limited anymore.

So, you can not just blindly trust that your performance will not change with a change in temperature?

WhiteKnight,

This is a very valid question. I subscribe to the school of thought that you do not have to recalculate your performance. The changes that you provided in your calculation is possibly some form of optimisation and does not mean that your original calculation is invalid. I suggest you run the same calculation but hard tune the Flap 1+F in the configuration. I would be very surprised if it produces red numbers, or any changes at all.

You can put literally any temperature you like in the computer and it will produce the same result bar a very few circumstances where optimisation changes the configuration. The only difference is the performance limit weight whose only real value is of a cross check function with your colleague(or if it is limiting).

As long as OAT is below flex -1deg, you are good to go. At OAT = FLEX -1 you need TOGA and at OAT=FLEX you will be over the limit.

The company I flew for a 2º limit on temperature change before you had to redo the figures.

I don't think the temperature change is driving the different v speeds in this example.

EFB hides the process and just gives you the magic figure. Go through a RTOW chart. In flex calculation what role does the OAT have? You go for ETOW and not temperature and for the calculated Flex temp to be valid it has to be higher than OAT that's all(and Tref). By how much is not a condition. Takeoff speeds change even with surface wind. Previously there was a restriction of flex had to be higher by some 7 or 9 degrees to cater for OAT increase during taxi. But now the modified ECAM software gives a warning during TO and automatically sets TOGA thrust if not done in ten seconds or so.
WhiteKnight your question is a doubt but certainly not some Eureka. Flex temperature has been around and certified for decades. All angles have been looked into.

White Knight - The issue there is that you have out different flap settings. Don't be fooled that the Flex was the same - that's coincidence given the runway criteria.

Do the same calculations on your EFB, but force the flap at say Flap 1 on both temps. You will find the speeds stay the same.

​​​​​​.. Absolutely.. Now everybody knows..
🤔🤔🤔

Fly safe,
B-757

Hey don't go tell everyone. That’s how we training captains find out our trainee is completely clueless. Flex 55, OAT increase, and they run the computation again...

Used to have 2 days (4 + 4 hrs each) of take-off performance course. Numbers, charts, kinematics and thermodynamics. Today, it is 2 hrs, the test included. Still called Performance but the content is EFB SW handling. Useful, but not the same thing.

Does your airline teach to understand the AST method algorithm calculates EVERYTHING under the assumed temp, not only the thurst reduction? Can your training captains explain, what is the rounding-up gremlin that will give you (on a very long runway) higher flex from an intersection opposed to a full length? :E

Don't blame the one asking...

No, not at all.

Should he know it? Probably, but at least he is trying to find out and not hiding the lack of knowledge.

With a higher OAT, your N1 will increase, but you still have exactly the same available thrust, anywhere in the range from calculated OAT to assumed temperature.
We don’t calculate or set an N1 value. We calculate a flex temp and enter that in the FMGS/FMS. The FADEC (Airbus) will set your N1 and your thrust is whatever you expect it to be at the assumed temperature.

Vilas explained it perfectly: TO performance doesn’t change with OAT on paper TO performance charts when using a flex temp, so why would it change with an electronic calculation?
Yes, you might get different speeds and stop margins, because who knows what’s going on behind all the calculations and the software is designed to optimise all of this, but your initial calculation is valid for any OAT up to the assumed temperature.
(At my previous company, selecting TOGA gave us lower speeds than flex. However, if you use your speeds from the flex calculation and use TOGA, your performance is still valid. This was an interesting situation in case of reported wind shear. Some Captains(!) absolutely wanted to have TOGA in the calculation, unknowingly reducing the safety margin in case of wind shear, due to the lower speeds.)