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New Takeoff performance with a change of air temperature

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New Takeoff performance with a change of air temperature

Old 31st Dec 2022, 14:00
  #41 (permalink)  
 
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Can we keep it simple and state that the answer to the question is: not required (not sure how you would get an increase of 5C in the time frame between perfo calculation and actual takeoff). If you have time available, you're not doing any harm by recalculating. Just use common sense. Can we agree we don't fly on the limits, but we fly safely around in a world protected by safety margins?

If the question is about "optimum" performance, that's not a question to be discussed on the flightdeck by pilots. Because my initial question would be "optimum what?". Assumed temps were introduced for maintenance purposes. So what are we optimizing? (don't worry, I'm NOT looking for an answer here)

What's the point of a 1kts Vr increment, if 99% of the pilots don't call out the rotate at the exact speed? The world is human. Let's keep it human. Not robotic, nor autistic.
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Old 31st Dec 2022, 15:57
  #42 (permalink)  
 
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Originally Posted by tdracer
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).
I probably didn’t express myself correctly, and I would definitely hesitate to question your expertise.
But I have always read it as: Assumed/FLEX will set the power for that temperature. If the actual OAT is lower, as it should be, you will have an additional margin. If the OAT rises, that margin will decrease, but up to OAT reaching FLEX your FLEX power setting should be safe to use. You say that there would be no extra margin because the calculation takes OAT into account, and if OAT rises you would have to do a recalculation. I obviously understand that you are correct if there is no extra margin, so will go back to my books.
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Old 1st Jan 2023, 05:33
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Originally Posted by hans brinker
I probably didnt express myself correctly, and I would definitely hesitate to question your expertise.
But I have always read it as: Assumed/FLEX will set the power for that temperature. If the actual OAT is lower, as it should be, you will have an additional margin. If the OAT rises, that margin will decrease, but up to OAT reaching FLEX your FLEX power setting should be safe to use. You say that there would be no extra margin because the calculation takes OAT into account, and if OAT rises you would have to do a recalculation. I obviously understand that you are correct if there is no extra margin, so will go back to my books.
Books tell you nothing more than what I have produced already. OAT and wind intersection gives you MTOW which will decrease if OAT goes up. But many times it's beyond structurally limited wt. When you want flex you enter wind find ETOW then on the left you get flex. There's no OAT column. Which is checked later against flex. No performance document tells you to recalculate performance with OAT rise.
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Old 1st Jan 2023, 07:54
  #44 (permalink)  
 
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I guess by now we are all on the same page when it comes to temperatrue.

What about a change in QNH? In the past manuals there was a flow chart like the one posted by Vilas in #36 under the Performance chapter that say up to 2 hpa drop there's no need to recalculate our performance but that has since been removed since the introduction of Flysmart.

In theory, any reduction in pressure reduces aircraft performance so we recalculate for any pressure drop. Is that what everyone out there is doing?
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Old 1st Jan 2023, 08:40
  #45 (permalink)  

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Never saw any such rule to ignore changes. Also makes no sense, the manufacturer provides a method how to calculate what needs to be found. Not a cookbook how to bend the input data.

What the operator writes in their manual are different RoE.

There are some embedded allowances and getting picky about 0.25 hPa is unnecessary. Namely so as there are several Pandora's boxes - who supplies the operator with obstacle data and how often those are updated is an example one not in a plain sight.

A severe practical problem lies where to draw the line how much of wrong is acceptable. Now and then something leaks that sounds rather silly, and while you can appreciate the effort there cannot be any points for trying.

Happy new year, safe skies and jolly arrivals!
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Old 1st Jan 2023, 21:02
  #46 (permalink)  
 
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We recalculate if the data we used changes by more than 2hPa, 2 degrees C, any increase in TW, 5 kts HW.

Using Boeing Performance app.
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Old 2nd Jan 2023, 04:04
  #47 (permalink)  

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Paraphrasing the PPRuNe's all time classic: Tell me, please, which airline you fly for so that I can apply with them in the future!

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Old 2nd Jan 2023, 08:31
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Originally Posted by ScepticalOptomist
We recalculate if the data we used changes by more than 2hPa, 2 degrees C, any increase in TW, 5 kts HW.

Using Boeing Performance app.
Why with variation of 2hPa when chart gives you a block of 10hPa?
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Old 2nd Jan 2023, 12:22
  #49 (permalink)  
 
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Because in many companies the Boeing performance application is the primary tool. Hardly anybody uses tables anymore. It's not because the table works in 10hPa blocks, it is "the" way. It is "a" way to calculate.

Last edited by BraceBrace; 2nd Jan 2023 at 12:34.
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Old 2nd Jan 2023, 13:48
  #50 (permalink)  
 
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Originally Posted by BraceBrace
Because in many companies the Boeing performance application is the primary tool. Hardly anybody uses tables anymore. It's not because the table works in 10hPa blocks, it is "the" way. It is "a" way to calculate.
I was just trying to show it doesn't make a difference. Even in Airbus it's computerised and as I said in my first reply it hides the process and just gives magic numbers. The process shows you that rise in OAT or change in QNH doesn't compromise anything.
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Old 2nd Jan 2023, 18:29
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The rise in temperature is connected to the assumed, and therefore there is a performance margin. If it turned out to be a performance limiting takeoff, the question would not be asked, it would be visible as the temperature would rise above the assumed temperature. With a different QNH this is not that visible. The 10 is an arbitrary number. You cannot simply ignore.

I understand and agree on your reasoning in performance cases with large assumed temperatures. But there are also many takeoffs done close to the limit performance. And basic rules have to cover all situations.
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Old 3rd Jan 2023, 03:20
  #52 (permalink)  
 
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Originally Posted by BraceBrace
The rise in temperature is connected to the assumed, and therefore there is a performance margin. If it turned out to be a performance limiting takeoff, the question would not be asked, it would be visible as the temperature would rise above the assumed temperature. With a different QNH this is not that visible. The 10 is an arbitrary number. You cannot simply ignore.

I understand and agree on your reasoning in performance cases with large assumed temperatures. But there are also many takeoffs done close to the limit performance. And basic rules have to cover all situations.
Previously Airbus had limitation on flex temp being higher than OAT by 6 or 9C to avoid a situation where during taxiing the OAT could rise above flex. But Airbus removed that after it carried out a mod by which if that situation arises then then on setting flex pilot is warned to set TOGA and if not done TOGA is set automatically. As far as QNH is concerned it's corrected in blocks of 10hPs and not interpolated so it's not that critical

Last edited by vilas; 3rd Jan 2023 at 04:07.
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Old 3rd Jan 2023, 07:08
  #53 (permalink)  

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Entirely different opinion on that, both accounts.
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Old 3rd Jan 2023, 07:51
  #54 (permalink)  
 
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Originally Posted by vilas
Why with variation of 2hPa when chart gives you a block of 10hPa?
Its just the policy we have. I assume its more to do with whatever optimisations theyre favouring.
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Old 3rd Jan 2023, 07:53
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Originally Posted by FlightDetent
Paraphrasing the PPRuNe's all time classic: Tell me, please, which airline you fly for so that I can apply with them in the future!

Hey FD, trust me, some of things we do is just to prove that re-inventing the wheel, and making it worse, is possible..
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Old 3rd Jan 2023, 09:24
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IMPRECISE ACCURACY

This thread has shown some clear differences in attitude to routine ops. The OP outlined the scenario of a FLEX departure and a subsequent small rise in actual OAT used initially. Many of the posters have outlined the inherent "buffers" which are already used in the calculation ( esp. Villas ) Since the common use of EFBs for calculations it seems that many people get hung up on spurious accuracy which these will show. By that I mean : How accurate is the loadsheet really - assumed weights plus look at the amount of hand luggage regularly carried. How accurate is the fuel on board actually - sure the gauges read to 10 kgs but what is their accuracy levels really.? How accurate has the runway line-up been done - did you definately not cut the corner on line up to get away quickly? On hot days - how much warmer is the air above the runway versus the ATIS value used? Is the wind component accurate ( sure it is factored but what if zero wind was used)

The buffers are build into the software for good reason - do not assume that implied extra accuracy is there just because the EFB is used and if minor changes produce critical situations then perhaps FLEX is inappropriate?

Computing 101: accurate data in - accurate data out.
less than accurate data in - dubious result out
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Old 3rd Jan 2023, 10:00
  #57 (permalink)  

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Originally Posted by ScepticalOptomist
Hey FD, trust me, some of things we do is just to prove that re-inventing the wheel, and making it worse, is possible..
Oh, well. .... even better: A familiar environment!

Adding to say that I can see the truth in the last vilas' post but the explanation would be based on a very different set of arguments in my class.

Meikleour On the level this discussion rose up to, we don't say the EFB SW has inherent buffers. There are some inherent buffers in the algorithm and mathematics behind it, and the SW does not introduce any more margin to it, if anything the contrary is true. Don't wish to rehash what's been said, just pointing out that beyond a specific moment the terminology requires accuracy too.

Whether or not the pilots strive to achieve numerical precision, when and why is that a void effort is perhaps best debated with clear distinction. I suppose people who know how to apply the results safely in daily ops and can zoom out their attention might find the mathematics irrelevant to discuss at all, however, please understand the noise thereafter makes listening to the fine tunes very difficult for the likes of A.W. As well for the original OP, who got laughed at first yet no explanation was provided until 2 years later. Well, except for being told effectively his attention to that particular detail was superfluous.


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Old 3rd Jan 2023, 10:45
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Originally Posted by Meikleour
IMPRECISE ACCURACY

This thread has shown some clear differences in attitude to routine ops. The OP outlined the scenario of a FLEX departure and a subsequent small rise in actual OAT used initially. Many of the posters have outlined the inherent "buffers" which are already used in the calculation ( esp. Villas ) Since the common use of EFBs for calculations it seems that many people get hung up on spurious accuracy which these will show. By that I mean : How accurate is the loadsheet really - assumed weights plus look at the amount of hand luggage regularly carried. How accurate is the fuel on board actually - sure the gauges read to 10 kgs but what is their accuracy levels really.? How accurate has the runway line-up been done - did you definately not cut the corner on line up to get away quickly? On hot days - how much warmer is the air above the runway versus the ATIS value used? Is the wind component accurate ( sure it is factored but what if zero wind was used)

The buffers are build into the software for good reason - do not assume that implied extra accuracy is there just because the EFB is used and if minor changes produce critical situations then perhaps FLEX is inappropriate?

Computing 101: accurate data in - accurate data out.
less than accurate data in - dubious result out
In more succinct terms - measuring with a micrometer and cutting with an axe.
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Old 3rd Jan 2023, 20:11
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measuring with a micrometer and cutting with an axe.

This has been a major problem since we obtained general access to digital computers spitting out answers to a precision of a squillion decimals but still maintaining a reasonably realistic accuracy of real world figures. Aircraft weight and cg is my favourite - I see numbers to a precision of, say, 2 or 3 decimals where the accuracy is still, maybe, a quarter inch or worse (on a good day).

Especially with younger folk, GIGO is a real hazard across the board.
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Old 4th Jan 2023, 09:25
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Originally Posted by FlightDetent
Oh, well. .... even better: A familiar environment!

Adding to say that I can see the truth in the last vilas' post but the explanation would be based on a very different set of arguments in my class.
Truth...?

Lesson nr 1 I was taught starting left seat upgrade: the only truth there is in aviation is the truth of numbers, and the ability to explain those numbers black on white if something happens and questions are posed. Fixed numbers are ie limitations. Offices don't care, you do what you want if all goes well. But if you venture into the world of limitations and decide to "neglect", you give somebody a hammer to slam if something ended up not so well. If you want to "taste" it, have some SAFA inspections. They will kindly tell you the importance of 1kg if it’s written on a paper somewhere.

The question was about temperature, that's pretty visible because assumed is a temperature. Other influences... as long as I'm sure I'm not busting a limitation, fine. But when it’s not clear I prefer to set my ego aside and do a quick recalculation. No harm done. Calculate, store, get on with life.

Ps: we have no rules on the subject. Stay out of the office is the main goal (and those people do like clean safa reports to...)

Last edited by BraceBrace; 4th Jan 2023 at 12:30.
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