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.)

The stranger: My post was a comment on the lack of performance training that seems to be the norm these days with new pilot training. It was not a criticism of the OP but rather a reflection on the poor training that he/she seems to have had. I am surprised that you do not seem to think that that scant level of understanding should be considered all right. FlightDetent sums it up.

I definitely don't think my company manuals are awesome, but they do specify exactly when I have to redo the performance numbers. 10 deg temp, 10 kts wind, 0.1" pressure (US) aso. Surprised that is not standard...

Do you know what assumed temperature is? When you run the performance it gives you a Flex/assumed temperature. That temperature is the highest temperature you can safely take off and meet all the required distances/gradients, and that number is used by the fadec instead of the OAT to set N1/EPR. So unless it is hotter outside than your flex number you can take off. The actual OAT doesn't matter, as long as it is below the Flex number.

Sorry, but that is not only wrong, it's dangerously wrong. If actual OAT is 25, and ATM comes up with 75 for thrust setting, that simply means that the thrust (typically N1) for 75 can still be used safely.
If the actual OAT goes up, the aircraft performance goes down (hotter air = less lift at the same airspeed). IF you don't redo your ATM calcualation for the new OAT, you're thrust setting will be too low for that OAT. Plus, if you're talking N1 thrust setting, 100% N1 at 50 degrees creates significantly less thrust than the same physical N1 at 25 (that old "square root theta" thing - thrust is constant at corrected N1, not physical N1).

Just as a matter of interest, I've run the numbers at Wellington, NZ with 25ºC vs 30ºC (yeah ok, maybe not entirely realistic for the windy city, but it does get warm sometimes), all of the speeds, thrust, take-off margin remained the same but the EO acceleration altitude changed. I don't think it is quite correct to just say that performance at one temperature will be identical to performance at another temperature assuming it remains below flex. The system seems to be using ambient temperature to calculate aerodynamic performance, and that does change.

tdracer hans brinker

I just checked my company manuals, and for a FLEX takeoff (Assumed temperature in Boeing speak), as long as the OAT is less than the flex temperature, we don't need new numbers.

Let's not mix up

A) tolerance of balanced-risk to accept performance numbers calculated from an expired report

with

B) validity of AST result for varying OAT.

From what I've seen, the OAT does not enter the AST algorithm as an input value (simplified).

AerocatS2A: I am rather convinced what you see is temperature correction (delta ISA) on the geometrical ACC ALT, the altimetry error goes both ways and a tweaked SW suite can play with hot temps too.

With your last sentence, at face value, I could not disagree more.

If assumed temperature is 75° and OAT is 25°, the takeoff performance calculated is actually better than what would be at 75°OAT because the true airspeed at 75°OAT will be much higher. So some rise in OAT is a non issue.

I think you’re correct, the maths checks out.

Thanks for the feedback, relieved to hear.

Funny fact: if the paper RTOW are smartly configured before exporting, an all live-iteration EFB will not bring much tangible benefit but this is one.

At least Airbus OEM prints the (single!) eoacc alt already inclusive of the required ALT correction to match the coldest temperature displayed on the chart.

Which carries a double penalty against the Max TOW for hot temps. A) the plane is made to climb aerodynamically higher, up over the alt+correction; B) the plane also climbs geometrically higher due isoplanes expansion.

​​​​​​When making a FLEX data departure under the assumed higher temperature, the above translates into a double performance margin inherent in the result.



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As always, a good discussion: but don’t shoot the messenger.

The two key points worth emphasising:

1. The lack of proper training.
2. The lack of required documentation (when/whether to recompute).

The OP was unfortunate to be flamed for lack of knowledge - but how do you achieve that in today’s world?

Nope, #22 did not show a lack of knowledge. Came here 2 years later, guns blazing with the opposite of what the correct knowledge is, shaming around.

Both your points are solved with operators who are thorough. It's reading the regulations and AFM alone, without the needed expertise, which could lead a young gun astray. Always has been and no need to ask how we know.

Reading the thread again, #11 has the key issue unanswered. He did not get different speeds after selecting another flap for the modified temperature. The calculation threw him a different OPT flap as a result after adjusting the OAT while still well below the flex and, god knows, even the corner point. That should not be happening, right?

Well, it did. Not as a function of the AST Method but a good outcome of the Perf SW efforts for an optimum result. Following the proper AST methodology, at first, thou shall calculate (max) PerfLIM TOM for the given OAT (and some other bits which might be OEM specific). This search for PLimTOM(@oat) will also involve some serious magic with V1/V2 ratio (a.k.a improved climb) and clandestine adjustments to ASDA. Even for a minuscule difference of 1 kg rounded up, the result for OAT=9 might give you 1+F but OAT=10 shows flap 2.

And then, the WHOLE performance calc is run again (and again) with ever-increasing fake OAT (even above max allowed environmental envelope) until the value of PLimTOW(max assumed temp) drops down to the actual TOW which is when Tflex is found. Most likely by a present or missing SW feature, the flap setting providing the best PLimTOM(oat) at the first step is locked, and the iteration for max assumed temperature is only done with a single tkof configuration.

The associated N1 for TOGA @ (max flex temp+ELEV+QNH+bleed+anti-ice) is known from the Engine manufacturer. The pilot will feed the chosen Tflex somehow into the engine data pipeline and the engine will later respond with the same N1 which would be the TOGA N1 if it actually was Tflex outside. The sweet part already mentioned is that the thrust provided by N1(flex@oat) is markedly greater than that calculated for N1(toga@t-flex) as the density of air received is higher and the mass of air accelerated air heavier (F=-a*m) eventhough N1(flex@oat) is purposely selected identical to N1(toga@t-flex).

Those impressed by the inherent margin uncovered above should also consider the correlated effect of increased density on airfoil performance. If not completely flabbergasted then, there is a truckful of GS(v1) & distance-to-IAS(v1) happiness to be discussed under the calculated assumed temp v.s. real OAT, nicely perched on top of the whole majestic Ekin(v1) pedestal.

Not satisfied yet? Go fly something else than the venerable B737 and get an extra 1 second of recognition time and an adjusted bite of deceleration.

:O

https://cimg1.ibsrv.net/gimg/pprune....80bf95fd6.jpeg

https://cimg3.ibsrv.net/gimg/pprune....922939fbc4.png
Just follow this and get your answer.

That particular flow defines some of the required compensations between an RTOW chart and reality (whether or not is the chart smartly configured to match the operating conditions) and a couple of validity checks (Vmu, min V2, dry/wet). What I called 'some other bits maybe OEM specific' in the post above.

Needed, but only a small stop on the path to understanding. Just a note for other readers not to get stuck looking into the picture for something not on it.

This thread has kept me awake at night. My take-away so far is that the RTOW flex method and the software calculation have different starting points. The RTOW uses a fixed configuration and therefore a change in, for instance, OAT between the time of planning and the actual take-off will not produce different numbers on a Flex TO provided that OAT remains below TFLEX. The software starts by optimising configuration for the PLTOM at OAT and then runs an iterative process at that configuration to find TFLEX. Thus, with the software, configuration may be changed as OAT changes but with the RTOW method, not. Is that correct?

One other question ... . I know the Airbus EFB software optimises V2 for each take-off under the guise of V2/VS ratio, but do Boeing do the same?

https://cimg7.ibsrv.net/gimg/pprune....347e9f7c24.png
Try this one and where can you fit 5° increase in OAT.

If you put a fixed configuration instead of optimum you won't get different out put.

A.W., agreed.

A change of OAT does not invalidate the AST result, and the calculated numbers remain valid. By necessity of how the figures are determined.

Although RTOW charts may exist with dual configuration printed on them, this does not change a thing.

It is entirely possible a Perfo SW suite could come up with a different configuration due to OAT adhustment, however that is also awailable for manual calculation if you decuded to dive deep enough.

The back office / mainframe or EFB calculations cannot provide any better results over the printed RTOWs. As the RTOWs are in fact a result of the same calculation core (SCAP module they used to call it).

The differences between the RTOW pre-conditins and outside world of the day will require applying corrections which are fixed to the most critical situation and thus overly conservative for 98% of the chart. And combination of corrections makes it a lot worse. Altimeter indication cold temp I chose on purpouse because it is hidden from the reader in the standard layout.

That is when the EFB delivers with pride, QNH of 1013 should not really be -1100 kgs penalty compared to 1013.25.

Second question, absolutely yes. Try a search for 'improved climb'.






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