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Derate vs ATM

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Derate vs ATM

Old 12th May 2018, 13:21
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PAG
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Derate vs ATM

Hi Guys,

There is something I don't get right with Take-Off performance. I fly boeings but I guess it's same for every type. When we use Onboard Performance Tool, it gives us a longer ENGINE INOP GO DISTANCE, with FULL DERATE than DERATE AND ASSUMED TEMPERATURE
Here are some figures from OPT to illustrate:
Airport is RAK, RWY10, DRY, 0 WIND, 15C, Q1015, Flaps 5, A/C AUTO, A/I OFF
FULL 24K 94.6% V1 138 VR 139 V2 146
R-24K 31C 93.4% V1 140 VR 140 V2 146

Here is the thing I don't get:
24K MAX TAKE OFF POWER
Engine inop go distance 3051M
ASDR 2069M
all engine go distance 2027M

24K Assumed
Engine inop go distance 2165M
ASDR 2238M
all engine go distance 2210M

How come we need a longer distance engine inop go with FULL than ATM ?

thx for your help

ANSWER:

"With full 24K there is a range of V1 options between v1min and v1max that will satisfy minimum regulatory margins (obstacle clearance, 2nd and 4th segment climb, screen height etc.) The driver for the engine
inop go distance is the v1min, in this case 94kts. This means that aircraft will accelerate to 94kts (relatively quickly) and can continue with a longer engine out ground roll to Vr. On the other hand the driver for the ASD is the V1max.

The V1 selection is impacted by the weight and the thrust. With a lower thrust
set we are no longer presented with a range of V1 speeds but with V1 speed that will just satisfy both the go case and stop case. (balanced field). So in answer to your question the OEIGD 24K is greater than the OEIGD R24K due to lower V1min.

Ryanair’s policy on V1 selection is OPTIMUM. Optimum gives the return on weight when factoring in available clearways and
stopway. For OPT to choose the optimum V1 there needs to be a range of V1 speeds possible "

Last edited by PAG; 18th May 2018 at 10:48. Reason: answer
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Old 12th May 2018, 14:56
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Did it give the same V1 for both cases? If the V1 is lower for the reduced power case, then the accelerate-go distance reduction due to V1 reduction, could outweigh the increase due to thrust decrease.

This is a mistake, sorry. A lower V1 gives a longer, not shorter, accelerate-go distance.

Last edited by Vessbot; 12th May 2018 at 17:52.
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Old 12th May 2018, 15:11
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I wrote V speeds in my first message, 24K V1 is 138, with 24K 31C V1 is 140.
For me, the Engine inop GO distance is between brake released and V2 at 35ft (dry runway), I don't think V1as an importance
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Old 12th May 2018, 16:02
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Derate and assumed temperature are not the same thing of course.

Derate is like having smaller engines, whereas assumed temperature is like having the original sized engines but not at full throttle. Therefore, depending on the figures; your assumed temperature example might result in more power than your derate example.

You quote 24k for each case, but the derate case should use a lower power, so I am slightly confused by your starting figures - possibly my glass of wine is clouding the issue!
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Old 12th May 2018, 16:28
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Take off run is assumed to be with all engines operating until V1, from V1 to V2 with one engine inoperative and as such reduced acceleration.
If you have an engine failure before V1 you abort.
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Old 12th May 2018, 16:46
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so the all engine go distance in the second case is longer than the engine inop distance.

this makes me assume you misread 3165 for 2165, which would make much more sense.
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Old 12th May 2018, 16:58
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I guess your full rated is 26K and now you are using derate 1 (TO-1) which is 24K.
Then full N1 at 15C is 94.6% and reduced N1 with assume temperature of 31C is 93.4%. Some people are confused because of the way you stated things.
Without deep study at first sight, I think the 3051m distance is wrong (maybe mistaken)...2051m is more logical.

Can you please check those distances with full rating (26K) and TO-2 (22K) with the same figures (OAT, Flaps, ...etc)
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Old 12th May 2018, 17:23
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Hi Guys,



I'm afraid my figures are right, I have my Ipad next to me as I answer your suggestions.

Uplinker, I agree for derate but disagree for ATM as it reduces power from the derate, so derate is the smaller engine (24K => 94.6%) and then you ATM from 24k, (R24K 31C => 93.4%),
if ATM was from full power it would be a higher %.

wiedehopf, I confirm what I said, R24K
Engine inop go distance = 2165M
All engine go distance = 2210M

New_era
Full 26K is 98.9%
Engine inop go 2625M
asdr 1931M
All engine go 1829M

Full 22K is not allowed with current conditions
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Old 12th May 2018, 17:28
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PAG try to update the OPT first and then try again. Might change something. 900m difference between the 2 figures is quite interesting.
edit : whats the TODA ?
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Old 12th May 2018, 17:35
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No change at all with the update

here are the figures you want

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Old 12th May 2018, 18:03
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Look at the second example: the all-engine go distance is longer than the inop go distance. Does that make sense to you? Doesn't to me.

With that in mind I'd suspect the calculations.

Can you set the system up to produce calculations for the same takeoff n1 but achieved differently?

i.e. calculate for double-derate (22k) n1, calculate for that same n1 achieved through single-derate (24k)+ATM, calculate for that same n1 achieved through ATM alone (i.e. ATM from 26k)

If you can't produce figures for double derate at this location/weight please adjust until you can, for the purpose of this experiment.
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Old 12th May 2018, 18:11
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Hi Pag,
Max TO PWR min V1 is 94 kts.
Assumed Temp min V1 is 140 kts.
Does the software compute the Eng inop go distance assuming you've used min V1?
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Old 12th May 2018, 18:19
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We know that a lower V1 gives a higher accelerate-go distance. (Contrary to what I first wrote in this thread - oops!)

This program puts out min and max V1's, since apparently a range is allowed. Can you explain what "policy V1" is? It seems to me that it's the actual single V1 chosen (from the range) to use for this particular take off.

Well, it would naturally make sense to me that the accelerate-go and accelerate-stop distances would be based on the single V1 chosen. (In that case, the policy V1's are almost the same, which should yield a longer accelerate-go that we expect to see for the reduced thrust, but we don't actually see.)

But instead, what seems to be happening is that the distances are based off the worst-case V1's for each thrust situation, and not the policy V1's. This allows a different V1 to be chosen from the range without having to re-run the calculation. To me, this explains it: for the reduced thrust, min and max are both 140 so there's no range. But the full thrust the min (I.e., worst case for distance) is waay down there at 94, thus jacking the distance up.
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Old 12th May 2018, 18:32
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Thousands of flight are operated every day using this certified app, so I think it's reliable.
Can you please explain again your calculations, what do you mean by double derate ?

Initial conditions for the experiment:
R-24K 31C 93.4% Airport is RAK, RWY10, DRY, 0 WIND, 15C, Q1015, Flaps 5, A/C AUTO, A/I OFF
Engine inop go distance 2165M
ASDR 2238M
all engine go distance 2210M

1- Change

R-26K 45C
93.4% Airport is RAK, RWY10, DRY, 0 WIND, 14C, Q1015, Flaps 5, A/C AUTO, A/I OFF
Engine inop go distance 2288M
ASDR 2366M
all engine go distance 2343M

2-Change
Airport is RAK, RWY10, DRY, 0 WIND, 16C, Q1030, Flaps 5, A/C AUTO, A/I OFF
R-26K 46C 93.4%
Engine inop go distance 2254M
ASDR 2331M
all engine go distance 2306M
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Old 12th May 2018, 18:42
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Vessbot and goldenrivett, that's interesting, I am not sure at a 100% but, in the FTCM, Boeing says FMC gives speed for a balanced field performance, so I guess for the OPT Policy V1 is this speed too, (99% of the time OPT V1 and FMC V1 agree, if not it's not more than 2kt difference)

So what you're saying is that in the OPT, with FULL THRUST, Engine inop go distance is based on minimum V1, giving the longest accelerate distance ? this makes sense, i think that answer my question
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Old 12th May 2018, 19:58
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Originally Posted by PAG View Post
Thousands of flight are operated every day using this certified app, so I think it's reliable.
Can you please explain again your calculations, what do you mean by double derate ?
Sorry, I meant derate to 22k. I was hoping to see three sets of results all for the same n1 but with the n1 achieved through three different means:

​​​​​1. 22k derate but no assumed temp (n1 let's say 94%)
2. 24k derate and assumed temp (n1 also 94%)
3. No derate, assumed temp (n1 also 94%)

With the same n1 the distances should be fairly similar unless different assumptions are being made for one particular condition. This would better show the odd one out.

I still don't really see how the all engine go distance can sensibly be longer than the inop go distance unless a different v1 was used for each calcuation, which seems quite misleading as the results are going to be for one takeoff with one v1 speed.
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Old 12th May 2018, 21:56
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Lascaille, V1 and VR are the same with ATM, so the only difference I can see is that with 2 engines you still accelerate quite a lot, rotate and climb to reach V2, with one engine you accelerate a little bit less, so you fly over a shorter distance
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Old 12th May 2018, 22:03
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Originally Posted by PAG View Post
Lascaille, V1 and VR are the same with ATM, so the only difference I can see is that with 2 engines you still accelerate quite a lot, rotate and climb to reach V2, with one engine you accelerate a little bit less, so you fly over a shorter distance
I'm not following.
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Old 12th May 2018, 22:18
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Not a Boeing guy, but the AEO go distances, the ASDA distances at 24K loom reasonable on their deltas; but the rated power OEI go distance is the outlier—900-ish meters is huge. Their must be discrepancy in the V1 that was used versus the “policy” V1 i.e, it used V1min rather than policy V1. That would explain it—long time accelerating on one to Vr. The 46 knot delta between rated and reduced power min V1 is suspicious, too.

GF
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Old 13th May 2018, 00:45
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Originally Posted by Vessbot View Post
I'm not following.
He means this:

Below v1/vr increased thrust = decreased go distance. Because the slower you accelerate the more time will pass and the more distance will be covered before you reach your target speed.

Above v1/vr, increased thrust = increased go distance. Because having reached your target speed you have to rotate, which takes the same amount of time in both OEI and AEO situations - same target angle, same pitch rate - so more speed at this point increases the distance covered.

So technically the shortest go distance would be achieved by TOGA until vr then chop thrust to give zero acceleration, rotate and climb to 35ft at vr. Then accelerate. Pointless information!

Last edited by Lascaille; 13th May 2018 at 00:56.
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