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Derate vs ATM

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Derate vs ATM

Old 13th May 2018, 01:01
  #21 (permalink)  
 
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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
Yes I understand, that makes sense.
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Old 13th May 2018, 02:49
  #22 (permalink)  
 
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If you have paid for the software request a refund :-)
The ASDR are always longer than the Engine inop go distance (unbalanced field)
> Look at all the other figures, they confirm it

Last edited by new_era; 13th May 2018 at 03:07. Reason: Need more info about term
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Old 13th May 2018, 03:06
  #23 (permalink)  
 
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Sorry, I am familiar only with terms like TODR, EMDA and EDR... but what is exactly the definition of "Engine inop go distance"?
Thanks
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Old 13th May 2018, 03:13
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Originally Posted by new_era View Post
Sorry, I am familiar only with terms like TODR, EMDA and EDR... but what is exactly the definition of "Engine inop go distance"?
Thanks
aka "accelerate-go": the distance it takes to take off if the engine quits at V1. They worded it kinda clunky because they also list the takeoff distance with all engines operating.
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Old 13th May 2018, 03:20
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Originally Posted by new_era View Post
If you have paid for the software request a refund :-)
The ASDR are always longer than the Engine inop go distance (unbalanced field)
> Look at all the other figures, they confirm it
not necessarily so, if you had a clear way the unbalanced accel-go could be longer than the accel-stop.

GF
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Old 13th May 2018, 03:22
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Originally Posted by Vessbot View Post
aka "accelerate-go": the distance it takes to take off if the engine quits at V1. They worded it kinda clunky because they also list the takeoff distance with all engines operating.
Thanks!
There is definitely something not logical with the software (unless the vodka is too strong or I'm getting old)
How come for 93.4% of N1:
24K assume 31 you need 2165 m
26K assume 45 you need 2288 m
26K assume 46 you need 2254 m
and suddenly for full rating it is 3051 m...at that distance you are almost at the end of the runway (3100m)
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Old 13th May 2018, 03:25
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Originally Posted by galaxy flyer View Post


not necessarily so, if you had a clear way the unbalanced accel-go could be longer than the accel-stop.

GF
but here the clearway is 190 m and the stopway 60 m...
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Old 13th May 2018, 04:37
  #28 (permalink)  
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I haven't played with the device in question so my comments can only be generic.

One of the problems with the various first principles computer calculations is that you can get some "strange" results (with our pilot hats on) on occasion, eg with integer temperature increments there can be considerable head scratching variations in the speed schedule with quite small temperature variations. Also, the reduced thrust technique is a little different for EPR and N1 engines so that may introduce some head scratching (I am presuming that you are looking at an N1 thrust setting ?).

These sorts of observations are an artefact of the systems being used and one just has to live with them - a small penalty to pay to get the last few kilos for the takeoff calculation. It used to be a lot easier to see what was going on with the older AFM chart presentations but one had to accept that these were conservative and simplified to make them workable. Nowadays, the mighty bean counter driven dollar on the bottom line is the be all and end all.

Several comments -

(a) while the cited TODR1 delta is a bit eyebrow raising, we would need to know some details of how the program approaches the problem to make much sense of things. That detail should be in your manuals for the box. V1/VR protocols are the main concern I can see, as other posters have cited.

(b) How come we need a longer distance engine inop go with FULL than ATM Not enough information for me to hazard a call on that. However, book distances are very sensitive to V1/VR ratios so that may have a role to play here ?

(c) This is a mistake, sorry. A lower V1 gives a longer, not shorter, accelerate-go distance You might like to revisit that comment ? ASDR will reduce, TOD1 will increase

(d) I don't think V1 as an importance V1 (ie V1/VR ratio) is extremely critical to what distances might fall out of the sums.

(e) 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. Do be careful. There are two separate calculations involved - take off distance and take off run. Best not to mix one with the other.

(f) the all-engine go distance is longer than the inop go distance. Does that make sense to you? Doesn't to me The calculations are different - TOD2 is factored. Whichever case gives the critical distance will be longer on the day.

(g) Boeing says FMC gives speed for a balanced field performance BFL is useful for manual calculations which need to be done in a hurry. For the computer case where the takeoff is to be optimised on a first principles basis, BFL (as a policy) makes no sense ? As to what the aircraft FMC might be able to do will depend on how it is designed and how the operation is conducted.

(h) 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! I don't think this post makes much sense at all ?

(i) The ASDR are always longer than the Engine inop go distance (unbalanced field) While I can't comment for the particular aircraft, the statement generically is incorrect. Run V1/VR down and ASDR decreases while TOD1 increases. Which is the greater and where the two might cross will be Type and runway dependent.

(j) There is definitely something not logical with the software Not necessarily the case. It all depends on how the program logic is set up. Each calculation will be from first principles and, depending on what the logic might be, there might be little or no relationship between one calculation case and an adjacent set of data calculations.

(k) but here the clearway is 190 m and the stopway 60 m.. If the calculation is optimised, that might well give you a significant difference to the BFL simplified calculation
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Old 13th May 2018, 05:15
  #29 (permalink)  
 
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Originally Posted by john_tullamarine View Post
I haven't played with the device in question so my comments can only be generic.

(i) The ASDR are always longer than the Engine inop go distance (unbalanced field) While I can't comment for the particular aircraft, the statement generically is incorrect. Run V1/VR down and ASDR decreases while TOD1 increases. Which is the greater and where the two might cross will be Type and runway dependent.
My bad I mean ASDA not ASDR should always longer than EDR (As I understood Engine inop go distance required stated here is the EDR)

As for the 3051 m from the software, I am sorry but I cannot trust it untill I get a clear explanation.

PAG, can you check please the Engine inop go distance for full rated (no assume) with 26K? Just to have an idea.
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Old 13th May 2018, 05:19
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Originally Posted by john_tullamarine View Post
(h) 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! I don't think this post makes much sense at all ?
I was just describing something that isn't necessarily obvious, i.e. that if the objective is to become airborne in the shortest possible distance you don't want to gain _any_ additional speed after vr, because that additional speed only serves to make you cover more distance on the ground post vr. So if for some reason you were tasked to achieve the minimum ground run, a theoretical optimal thrust scheduling would be max thrust until vr, then quickly pulled back to just enough to maintain VR through rotation.

Like I said it was pointless 'what if' stuff that would probably turn out differently in reality due to reaction times and thrust advance/decay timing.

I was just not immediately seeing how an AEO go-distance could sensibly be longer than a OEI go-distance, v1/vr being identical explains it according to the above.
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Old 13th May 2018, 05:31
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And I think the conclusion is that it needs to be determined if the 3 distances are calculated using min, max or policy v1, and the same for the second set. The first set suggest that min v1 is used...

Last edited by Lascaille; 13th May 2018 at 05:41.
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Old 13th May 2018, 05:43
  #32 (permalink)  
 
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Originally Posted by PAG View Post
New_era
Full 26K is 98.9%
Engine inop go 2625M
asdr 1931M
All engine go 1829M
Disregard PAG, I got it!
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Old 13th May 2018, 07:55
  #33 (permalink)  
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ASDA not ASDR should always longer than EDR (As I understood Engine inop go distance required stated here is the EDR)

TODA is fixed for the runway and is applicable both for the AEO and OEI TODR calculations. Which of these is limiting will depend on the aircraft, runway and ambient conditions. TODA will include any clearway up to the maximum declared by the airport authority and usable for the aircraft. What ASDA is declared will, likewise, depend on the runway design and the airport authority's declaration. Whether TODA is less, equal to, or more than ASDA is not fixed but specific to any particular runway. Although it's a question which I don't think I've ever fussed over too much, I suspect that, for most cases, ASDA will be less than TODA. Your statement is not quite correct, I fear.

As for the 3051 m from the software, I am sorry but I cannot trust it until I get a clear explanation.

Now, you might have some difficulty doing that. Suggest you run it via your CP, who should then run it by your ops eng folk, who, if they can't provide an answer, should run it back via the OEM .. which might, perhaps, give them an answer.

if the objective is to become airborne in the shortest possible distance you don't want to gain _any_ additional speed after vr,

However, the objective is not to do that. Rather the objective is to replicate, as closely as the pilot is able, what the AFM says is the required takeoff technique. I really think that you should revisit the certification logic lacking in your statement.

I was just not immediately seeing how an AEO go-distance could sensibly be longer than a OEI go-distance, v1/vr being identical explains it according to the above.

Not so. For that case, the OEI distance would be greater than the raw AEO distance due to the OEI thrust rundown. However, the AFM-scheduled AEO distance very likely will be greater due to the AEO TOD factor incorporated in the scheduled numbers. This often is the case where OEI performance is reasonble - think lower weights, lower pressure heights and temperatures.

The first set suggest that min v1 is used...

I wouldn't have thought so. The OEI/AEO TODR split doesn't appear to support that view.
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Old 13th May 2018, 08:50
  #34 (permalink)  
 
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Originally Posted by john_tullamarine View Post

... Whether TODA is less, equal to, or more than ASDA is not fixed but specific to any particular runway. .
TODA = TORA + Clearway
ASDA = TORA + Stopway
A Stopway is always a Clearway but the reverse is not always true (in case the Clearway is a water or just a ricefield,...) So technically ASDA cannot exceed TODA and if the Clearway couldn't be used as Stopway ASDA = TORA
What do you think about that?
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Old 13th May 2018, 09:22
  #35 (permalink)  
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What do you think about that?

One needs to exercise caution in quoting generic relationships. In general, ASDA will not exceed TODA. However, when protected areas are taken into account for the more testing runways, sometimes things don't quite fit the generic expectations - hence the need for declared distances so that folks don't get led astray by the length of the seal .. which may not be the material length permitted for use ....

If you be interested in reading further on this, the various airports standards are the place to go. Note - I am not, in any way, an airports design specialist - that is the province of the airports design civil engineers and the airports regulatory folks.
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Old 13th May 2018, 15:01
  #36 (permalink)  
 
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Guys, I have 2 questions please if anyone can shed light on them.
1- When we check the takeoff distance from the fcom Perfo Disptch10.1, which of these 3 distances we get (engine inop go or asdr or all engine)?

2- If not using the software, how can we know these distances and for what use (because for perfo calculation either runway analysis or fcom is enough)
Thanks!
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Old 13th May 2018, 15:35
  #37 (permalink)  
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as usual, this topic starts to diverge in every direction, which is fine. I sent a request to a Boeing TRI and to the performance department of my airline, I will come back to this topic with an answer in the coming week.

thank you, everyone, for having given a positive input on this topic
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Old 13th May 2018, 17:34
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Originally Posted by PAG View Post
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

Pilots.... !! Extra Information... Any how.
I tried the same experiment with on various fields (ATM/No Atm with and without Derate/No Derate).

Your reference to de-rate was not required.

>>In the case of ATM or FULL the V2 remains the same.
>>In ATM the v1 is higher
>>In FULL the v1 is Lower

In any case Vef (considered ) would be higher than v1 therefore in the case of ATM i would need lesser runway to accelerate from vef to v2 where as I would need more runway to accelerate from vef to v2 in FULL

ATM v2-v1 < FULL v2-v1

Also in the case of an ATM the speeds are based upon a higher density altitude there fore the actual performance achieved by the aircraft is better than that calculated
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Old 13th May 2018, 19:33
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Is anyone else confused by the lawyer speak to not use this data for runway selection? Isn't the purpose of this exercise to find out if an aircraft can takeoff from a given runway under given conditions?
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Old 14th May 2018, 10:44
  #40 (permalink)  
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Bucks_raj, that's a very good point but you're saying that only 2 kt difference would give approximately 900m more, that's way too much from my point of view. What I think is that ENGINE INOP GO DISTANCE displayed for FULL 24K is based on the lowest V1 >>> 94 kt, in this case, 46 kt difference, giving 900m more, makes more sense.

Regarding ATM, temperature, density, performance or whatever, the end of the story is that when you press TOGA, engines accelerate to N1%, ATM gives a lower N1 than FULL, so less acceleration. ALL ENGINES GO and ASDR prove this, FULL gives better performance.
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