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, 15°C, Q1015, Flaps 5, A/C AUTO, A/I OFF
FULL 24K 94.6% V1 138 VR 139 V2 146
R-24K 31°C 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 "

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.

I wrote V speeds in my first message, 24K V1 is 138, with 24K 31°C 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

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.

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.

I guess your full rated is 26K and now you are using derate 1 (TO-1) which is 24K.
Then full N1 at 15°C is 94.6% and reduced N1 with assume temperature of 31°C 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)

Hi Guys,
http://i67.tinypic.com/2z7rb10.png


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 31°C => 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

No change at all with the update

here are the figures you want

http://i64.tinypic.com/2lcnq1l.png

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.

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?

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.

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 31°C 93.4% Airport is RAK, RWY10, DRY, 0 WIND, 15°C, 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 45°C 93.4% Airport is RAK, RWY10, DRY, 0 WIND, 14°C, 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, 16°C, Q1030, Flaps 5, A/C AUTO, A/I OFF
R-26K 46°C 93.4%
Engine inop go distance 2254M
ASDR 2331M
all engine go distance 2306M

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 :D

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.

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.

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!

Yes I understand, that makes sense.

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

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.

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)

but here the clearway is 190 m and the stopway 60 m...

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

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.

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.

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...

Disregard PAG, I got it!

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.

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?

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.

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!

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

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

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?

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.