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Stop Margin is based on OAT / FLEX Temp?

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Stop Margin is based on OAT / FLEX Temp?

Old 14th Jan 2019, 15:42
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Stop Margin is based on OAT / FLEX Temp?

I was flying on an ordinary day and calculated the TO Data, using LPC Software provided by my company.

The stop margin calculated was 29 meters with the flex temp of 60.
I advised my captain if he wanted to take off with a lower flex temp of 59, as to achieve higher stop margin.
He refused, and without me knowing his reason I inserted the TO Data with the Flex temp of 60 in the FMGC.

A few mins into cruising he brought up why he doesn't want to take off with lower flex. I understand one of the reason was for engine life in the long term and I couldn't agree more with him.

But his statements that is still stuck in my head is about the stop margin...

He said the stop margin WILL ONLY be 29m ONLY IF the OUTSIDE AIR TEMPERATURE is 60 Degrees Celcius (As the FLEX TEMP is 60 Degrees). Of course I did not argue with him, but I kinda disagreed with him on that.

So from his explanation, this is his point of view....
What he's saying is the stop margin result of 29M will only be achieved IF the ACTUAL OUTSIDE AIR TEMPERATURE is MATCHED WITH the FLEX TEMP.

OAT at that time was 30 Celcius

I'm not sure if he's correct. However, this is my simple explanation of stop margin, correct me if I'm wrong...

The Stop margin (in my case, it was 29M) is the remaining distance of when the takeoff is rejected at the speed of V1 until it comes to a full stop before it reaches the end of the runway. Again, that is the simplest form of explanation that I can come up with.

Therefore, it brings me to the conclusion that the OUTSIDE AIR TEMPERATURE DOES NOT HAVE TO BE THE SAME AS THE FLEX TEMP, in order to achieve that STOP MARGIN.

While as my captain thinks that the result of the stop margin computed, is based on THE OUTSIDE AIR TEMPERATURE HAVE TO BE THE SAME AS THE FLEX TEMP that we insert on our FMGC.

I'm a new first officer and I'm still learning as I'm pretty sure most of you guys do here, so please enlighten me on which conclusion is right...
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Old 14th Jan 2019, 21:37
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Hang in there and keep asking questions. One of the best ways to learn what's what.

Your colleague is observing the fact that the sums are based on the higher temperature and should be reasonable if that temperature existed. However, if the actual temperature is lower, you are going to get to the reject point quicker (ie less roll) than for the higher temperature case.

End result is that, for the reject, the actual numbers will be somewhat shorter than the assumed temperature calculation numbers and your pad will be greater than the assumed temperature calculation indicates.

As the ambient temperature gets closer to the assumed temperature, this difference will reduce progressively.

I'd be a bit wary about believing distances to the metre. A bit of conservative rubbery-ness probably isn't a bad idea.
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Old 14th Jan 2019, 21:46
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Easy to check... Calculate/look up performance for the case when OAT equals your previously selected assumed temperature. Assumed temperature is for thrust selection, the calculated stop margin is the difference between the ASDA and ASDR with actual conditions! In short: Be very careful with somebody else's stale ''knowledge', nod politely and look it up 😉 You're correct!
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Old 14th Jan 2019, 23:54
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Your accelerate stop distance will be less than calculated because your TAS will be less at the actual temperature than the TAS assumed at your FLEX temperature. The real figure will be about 3% less for every 10 degrees difference in temp.

Eg.
Calculated ASD with a flex of 60 gives 2500m
actual temp is 10 degrees

ASD is reduced by 3% x (60-10)/10 = 15% (so 375m)

actual ASD is 2500-375 = 2125m
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Old 15th Jan 2019, 00:00
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At 60C 100kt indicated is 108 TAS
At 30C 100kt indicated is 103 TAS

As above.

Last edited by FE Hoppy; 15th Jan 2019 at 00:01. Reason: EGP beat me
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Old 15th Jan 2019, 01:38
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Originally Posted by john_tullamarine View Post
Hang in there and keep asking questions. One of the best ways to learn what's what.

End result is that, for the reject, the actual numbers will be somewhat shorter than the assumed temperature calculation numbers and your pad will be greater than the assumed temperature calculation indicates.
What you're saying is my colleague statement is correct?

However STBYRUD is saying that my statement is correct...

So I'm confused now...
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Old 15th Jan 2019, 09:12
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So I'm confused now...
No need to be.
John Tullamarine, Old Smokey, EGP Flyer, FE Hoppy are all correct.
Discussed here:
Performance - Additional stop margin at RTO due to TAS effect ?
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Old 15th Jan 2019, 11:23
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STBYRUD, thats plain wrong.

Ondrayy, your captain is perfectly right and you have been undertrained, which is what computerized performance made possible. Not your fault.

The whole of FLEX or Assumed Temperature Thrust is based on iterations using TOGA with ever increasing OAT, until you hit the limit. And then AST = TOGA (OATlim -1). Point here being the entire aerodynamic / accelerating / stopping model is calculated under the assumed temperatures. Sometimes even higher than the A/C certified limit for the actual operation.

Google for TAT Effect ASDA or similar, your thirst will be satisfied. . Three direction markers and one warning for you:

  • The 3% quoted by EGPFlyer is also found in a UK CAA approved Ops Manual of a very large airline with a deeply knowledgeable performance department. It can be quickly verified if you have access to the full Performace Engineers Package SW.
  • F = -a*m
  • Ek = 1/2 * m * v * v
  • The calculation model of the deceleration phase for RTO does not consider the effect of (wet) paint and rubber deposits towards the far runway end.

Last edited by FlightDetent; 16th Jan 2019 at 10:25.
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Old 15th Jan 2019, 16:41
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Two decades as a "professional" pilot, and the last 5 in an aircraft that has flex (A320), and embarrassed to admit that I have never heard/thought/seen of this subject...
I always assumed(!) it would take the actual temperature for the speed and distance calculation, keep reducing engine power till ASDR/TODR hit ASDA/TODA and calculate an assumed temp to give me that required power, will definitely read up on the way flex is calculated on the bus.
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Old 15th Jan 2019, 19:56
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The method is far less elaborate, based on the (only available) TOGA certification data. You could do AST Method with Tu-134 if you had the N1 tables.

A very helpful Boeing presentation attached.

The additional margin when ASDA-ASD difference shows 0 grows from the two main discrepancies
- V1 expressed in IAS is reached geometrically earlier due to TAS (= GS assuming zero wind) being lower
- The kinetic energy at the start of braking is significantly less in reality compared to the mathematical model used to calculate the perf due to the V squared.

That's it. Mass is the same and Thrust is made equal by setting the reduced N1/EPR.

For a typical 65 t, on a 2700 m RWY with 45 FLEX in 15C air
- you have only 85ish% of energy to kill
- deceleration begins about 80 meters sooner.
compared to the calculated.

The full autopsy would show another (small) benefit from the difference in air density, which is squarely beyond my ken.
Attached Files

Last edited by FlightDetent; 16th Jan 2019 at 10:27. Reason: Corrected an overstatement..
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Old 16th Jan 2019, 08:24
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The full autopsy would show another (small) benefit from the difference in air density
Full autopsy here. See 27.10 (page 540 onwards)
http://www.internationalflightresour...ods%20copy.pdf
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Old 16th Jan 2019, 10:28
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.. and a very useful book, indeed. Good to see that Walt gets a credit in this one. A thoroughly fine fellow.
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Old 16th Jan 2019, 23:49
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The only tricky bit I think is finding the correct N1 (EPR) as it requires a density correction which I have found is not cross engine compatible so you need some specific engine data to correct. But otherwise, pretend it's 60C in all respects.
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Old 17th Jan 2019, 02:29
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Originally Posted by FlightDetent View Post
STBYRUD, thats plain wrong.

Ondrayy, your captain is perfectly right and you have been undertrained, which is what computerized performance made possible. Not your fault.

The whole of FLEX or Assumed Temperature Thrust is based on iterations using TOGA with ever increasing OAT, until you hit the limit. And then AST = TOGA (OATlim -1). Point here being the entire aerodynamic / accelerating / stopping model is calculated under the assumed temperatures. Sometimes even higher than the A/C certified limit for the actual operation.

Google for TAT Effect ASDA or similar, your thirst will be satisfied. . Three direction markers and one warning for you:

  • The 3% quoted by EGPFlyer is also found in a UK CAA approved Ops Manual of a very large airline with a deeply knowledgeable performance department. It can be quickly verified if you have access to the full Performace Engineers Package SW.
  • F = -a*m
  • Ek = 1/2 * m * v * v
  • The calculation model of the deceleration phase for RTO does not consider the effect of (wet) paint and rubber deposits towards the far runway end.
Ok, so to sum it up. The Stop Margin calculated by the Take Off Data Computer (LPC-NG).
Is saying that
let's say....
A flex of 60 and the OAT of 20Degrees gives a stop margin of 100m, and that stop margin of 100m will only be achieved EXACTLY at 100m, IF THE OAT IS 60 Degrees Celcius??

However, since the OAT is 20 Degrees the stop margin number is increased, HENCE reduces the ASD.
Lets say the takeoff is rejected at V1 exactly (or a few knots just before V1), the actual stop margin is lesser than that of "100m" SINCE THE ACTUAL OAT is 20 degrees not 60 Degrees.

Thanks for the new formula, so here is my calculation based on what you guys just explained...

Calculated ASD with a flex of 60 by the LPC-NG on a runway length of 2000m gives 1900m (since the stop margin is 100m)
Actual OAT is 20 Degrees

ASD is reduced by 3% x (60-20)/10 = 12% of 1900m = 228m

Actual ASD is 1900 - 228 = 1672m ASD with the OAT of 20 degrees and flex of 60

Therefore the New Stop Margin with actual OAT of 20 and flex of 60 is 100m + 228m = 328m

I hope I'm right about this as it takes quite sometimes to get my head right about this complicated topic
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Old 17th Jan 2019, 09:00
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EGPFlyer is correct. The speeds you get displayed are Ind Air Speeds which are calculated for the out coming FLEX Temp. If you use these speeds now at a lower temperature than FLEX, which is the case as your OAT is lower than FLEX, than the resulting TAS will be lower.

E.G.: Your program assumes a TAS at V1 of 135. But your actual TAS at V1 will be only 130 due to operating in a lower temperature environment than assumed in the program. As you abort Take Off at a lower speed than calculated, you will have an additional stop margin.
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Old 17th Jan 2019, 12:42
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What if the pressure that day is extremely low for example??
Your performance calculations for both actual OAT and assumed OAT will include the same airfield QNH.
Therefore the differences in TAS between actual and assumed temperature calculations will depend on the difference in temperature.
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Old 17th Jan 2019, 13:39
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Originally Posted by Ondraayyy View Post
I hope I'm right about this as it takes quite sometimes to get my head right about this complicated topic
That's exactly how the 3% rule works. And it is not Airbus or EFB software specific.

"the ASD calculated at the FLEX temperature can be reduced by 3% for every 10C temperature difference between the FLEX and the OAT."
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Old 17th Jan 2019, 17:06
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Originally Posted by Ondraayyy View Post
I was flying on an ordinary day and calculated the TO Data, using LPC Software provided by my company.

The stop margin calculated was 29 meters with the flex temp of 60.
I advised my captain if he wanted to take off with a lower flex temp of 59, as to achieve higher stop margin.
He refused, and without me knowing his reason I inserted the TO Data with the Flex temp of 60 in the FMGC.
I disagree with him completely.
As when using FLEX the thrust reduction is made to take advantage of thrust reduction possible using the additional runway length available. The thrust reduction amount is determined by calculating the temperature at which the ASD can be achieved using full length available. The output from LPC shows the distance remaining by reducing thrust and taking longer to reach V1, then to decelerate at same rate to stop. The LPC distance does not differentiate between reaching the point on the runway from which to stop.
OAT does have an impact on stopping performance, this is where the 3% as referred to in above posts comes in.
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Old 17th Jan 2019, 22:16
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I disagree with him completely.

While your choice is respected, it probably is a little unsound technically. Of more concern is your apparent fixation on ASDR as the appropriate limit .. it may be the limiting factor but, often, is not.

The Boeing Training Manual (D6-1420) cited in post #11 is a pretty good reference text (every heavy jet aircraft pilot should save a copy for his/her tech library). Further it doesn't get too bogged down in mathematics and such stuff.

The discussion around page 27-10 is pertinent to your post. Might I suggest that you have a read and contemplation of the text and then reconsider your position on your post's content.
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Old 17th Feb 2019, 22:19
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I thought flex only pertained to engine thrust.
are manufacturers required to punish RTOW further by flexing the speeds also?
just trying to understand.
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