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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 5th Jan 2023, 05:38
  #61 (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).
The ATM that is applied is valid, so long as the actual temperature doesn't exceed the assumed temperature. The crew are finding the limit case for the weight they are at, therefore as far as runway performance goes, they are hot to trot. However.... the V speeds should be recalculated if a significant change occurs. At least one airline gets around that by using the V speeds for the limit case... can't say I really like that concept. The company flight ops engineering or performance team can conduct a sensitivity analysis and make a determination as to when they wish the V speeds to be corrected to match actual conditions or not.

...

As to concerns that a pilot may ask for guidance on the subject, I personally am comforted that they would do so. Having an uncertainty in what is necessary is a troubling situation to find yourself in. Performance is bad enough without having some means to get a better comprehension than an airline and national licensing system may achieve. I do performance flight testing in R&D, and I end up asking questions also...

Boeing's performance manual, the Performance Engineers Manual, or Jet Transport Performance methods, should be mandatory reading for anyone with a Part 25 rating... so should the USNTPS FTM-108 Fixed Wing Performance and FAA AC 25.7D, Flight Test Guide for Certification of Transport Category Airplanes. Airbus's performance is a dark art...


JTs comment is on point, having a cognitive awareness of the situation saves a lot of heartache, like the bingle of the B742F at Halifax NS. The more we go towards a computer solution, the greater the chance of a gross error leaving bits of tin around the end of the paddock. Having a conceptual understanding of what the numbers means will assist in recognition when things are not quite as expected. Recall, that to be airworthy, the aircraft must conform to its TCDS as a part of its requirement , and the other is that it must be safe to fly, whatever that may mean. The TCDS includes the basis of certification that includes the AFM which has the legal TO performance requirements. Much performance is guidance material only and not certified, but TO performance is not subject to negotiation.
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Old 6th Jan 2023, 05:24
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On a different point.

My operator hasnít allowed us to update to iPadOs 16 yet. Are there other operators with this?
If so, anybody got any idea what the holdup is?

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Old 6th Jan 2023, 11:29
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Originally Posted by dogtired
On a different point.

My operator hasnít allowed us to update to iPadOs 16 yet. Are there other operators with this?
If so, anybody got any idea what the holdup is?
Usually itís the authors / owners of the software suites that need to approve the upgrade. They arenít usually very fast.
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Old 6th Jan 2023, 19:55
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Originally Posted by fdr
The more we go towards a computer solution, the greater the chance of a gross error leaving bits of tin around the end of the paddock.
Why? History has proven completely opposite is correct. We think we can do better. Usually we canít. Computers can be tested on the ground under pressure and they will handle complexity in a consistent way. Humans need simplicity, and even then won't survive under pressure.

Last edited by BraceBrace; 6th Jan 2023 at 20:12.
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Old 7th Jan 2023, 05:06
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Originally Posted by dogtired
On a different point.

My operator hasnít allowed us to update to iPadOs 16 yet. Are there other operators with this?
If so, anybody got any idea what the holdup is?
itís approved with my mob so not sure what the holdup is with yours.
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Old 7th Jan 2023, 08:24
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I checked on EFB OAT 20 į flex56į. With change of OAT by 5į to 25į there was no difference in speeds nor flex. Change of QNH by 4hP didn't change them either.

Last edited by vilas; 7th Jan 2023 at 09:40.
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Old 7th Jan 2023, 10:32
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iOS 16.1.1 was finally cleared for use in mid November. There were some holdups with compatibility.
You might have some other managed apps on there that are incompatible. .
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Old 9th Jan 2023, 14:08
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Originally Posted by WhatShortage
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...
Do you mean because of adding more OAT means nothing because you're assuming the maximum already?

As far as I am concerned and some have mentioned already, beyond a certain temperature flex will keep going down due to performance limitations and before that point it won't matter if it goes down.

There's no stupid question and certainly this is not.
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Old 19th Jan 2023, 10:22
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Change of temperature changes your performance. Always do another one, or do the initial performance anticipating this increase in temperature. Too little thrust is never good.
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Old 19th Jan 2023, 23:06
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Originally Posted by 737pilotguy
Change of temperature changes your performance. Always do another one, or do the initial performance anticipating this increase in temperature. Too little thrust is never good.
You might want to read the thread first.
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Old 7th Jul 2023, 04:00
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Originally Posted by john_tullamarine
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.
well, you don’t even need computers for that, paper manuals can do it too. Ours are translated into American, and so I have to deal with limitations like max landing weight of 145,505lbs. Pretty sure there’s a reason AB rounds all of those weights to a number ending with a few zeros……
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Old 7th Jul 2023, 06:44
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Are companies still using RTOW charts?

Last airline I flew with printed a set of performance data for different runways and conditions. The crew requested actual performance numbers via ACARS from the performance service provider and compared those with the paper numbers. So long as the actual temperature and pressure didn't deviate by a certain tolerance from the paper numbers the plane was good to go.

What I found more concerning was the tiny margin on some of those calculations. Some calculations had less than 100m runway remaining. That's not much room for error.
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Old 7th Jul 2023, 07:15
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Originally Posted by BraceBrace
Why? History has proven completely opposite is correct. We think we can do better. Usually we canít. Computers can be tested on the ground under pressure and they will handle complexity in a consistent way. Humans need simplicity, and even then won't survive under pressure.
Charts and tables give a rough order of merit to a calculation, computer programs like calculators, smartphone apps etc give numbers, which are perfect when no error occurred in the data entry. Human nature is to err, and some software packages had processes that occur that were not within the crew awareness, and can result in catastrophically erroneous output. Anyone using a calculator should have a rough idea of what the aircraft will do, what the speeds are likely to be and same for thrust setting etc. On occasions, you can also get paradoxical outcomes for surprising reasons.

Poster child was the B747-200F at Halifax, worth a read of the accident report.


A comment previously in this thread by a pruner, was related to N1 vs temperature, and the statement made needs to be qualified.
As temperature rises, N1 increases to achieve rated thrust UP UNTIL CRITICAL TEMPERATURE, thereafter N1 will decrease, and the engine will no longer make the rated thrust, it is limited to the thrust for the temperature, (you can do the simple maths on density ratio to work that reduction in thrust achieved out).

Depending on your software, caution may be needed with environmental temperatures below CT, and applying an assumed temperature above CT. If the temperature is extremely cold, the N1 to achieve the full rated thrust may be say, 88% N1, but a temp around say, CT+10C may be give the hot day limit weight you are trying to determine the limit for, and that may have a higher N1 than the actual temp full thrust N1 case. If you use say 98% N1 with an engine that achieves full thrust at 88%, you are in fact over boosting the engine, and not by a little bit, that is a biggly baddie.

Another set of issues arise when the computer analysis uses a balanced field solution for the output. This one can be a real pain, makes it good to have the standard charts on hand. Say you are taking off on 25L at VVTS, and you use half the runway to get to 35', you might be surprised that there is an obstacle limit weight that comes into play, when the nearest tall buildings are about 10 miles off the centerline, and no close in obstacles actually exist. Landing from the other direction, 07R, there is an ILS, and there is no building that you have to fly through to land, or power line or tree to dodge, but there is a small obstacle, within the splay, well past the end of the TODA, and that becomes the obstacle limit problem. The fact that you will be at 35' 1/2 way down the runway, and will have an extra mile or more to go to get to an obstacle that insults the OCS is discounted, and a restriction applies. Insanely, cutting back the runway length with this particular software, which should give a new supplemental takeoff distance and OCS to the object eventually did achieve that, so cutting back the revised software package would remove the obstacle, and increase the departure weight.

IMHO, and coming from a performance flight test background, around 5% of all takeoff charts that I get to analyse have oddities that arise, items where the answer that is given doesn't match reality. It is always the case that these anomalies are more limiting than the true condition that would be achieved by doing a full analysis.

Computer output quality matches the input and the calculation algorithms.

With the plethora of what is called information but which devolves into white noise during most preflights, little gems like runway length reductions etc are a major cause of non compliance. A takeoff using the wrong runway length will give the operator hours of fun working out how such a thing can happen.
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Old 7th Jul 2023, 08:02
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Interesting as always, fdr. Thank you. I have not come across critical temperature of a jet engine before wrt performance calculations. Where would I be able to find what it is for a Trent 1000 and is it variable or fixed?
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Old 7th Jul 2023, 09:15
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Originally Posted by wondering
Are companies still using RTOW charts?

Last airline I flew with printed a set of performance data for different runways and conditions. The crew requested actual performance numbers via ACARS from the performance service provider and compared those with the paper numbers. So long as the actual temperature and pressure didn't deviate by a certain tolerance from the paper numbers the plane was good to go.

What I found more concerning was the tiny margin on some of those calculations. Some calculations had less than 100m runway remaining. That's not much room for error.
I routinely see stop margins of 1m. Provided the difference between flex and actual temps is high it's not a problem.
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Old 7th Jul 2023, 18:50
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Originally Posted by BleedingOn
Interesting as always, fdr. Thank you. I have not come across critical temperature of a jet engine before wrt performance calculations. Where would I be able to find what it is for a Trent 1000 and is it variable or fixed?
Engines are generally rated for full thrust at "ISA+X" - where X is somewhere between 10 and 25 degrees C. Since ISA drops with altitude, the flat rate temperature will be less at altitude than at Sea Level. Above the flat rate temp, the max rating will decrease as a rate that gives a roughly constant EGT as the OAT increases.
It is also common that the flat rate for Takeoff - max five minutes (sometimes 10 minutes) is higher than for continuous operation - e.g. Takeoff might be IAS plus 20, but Max Con might be ISA plus 10.
I'm afraid I don't know what the numbers are for the Trent 1000 - your operations types should know (and it should be in the flight manual).
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Old 8th Jul 2023, 04:56
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how many accidents, or near-accidents, in Airbus history, have occurred because of a change in air temperature.
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Old 8th Jul 2023, 18:54
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Originally Posted by 321XLR
how many accidents, or near-accidents, in Airbus history, have occurred because of a change in air temperature.
Airbus has warning if Flex goes below OAT. And in eight seconds if you don't set TOGA it sets automatically.
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