Balanced V1 lower than Vmcg?

[StatorVane]

The question asks:

If the value of a balanced V1 is found to be lower than Vmcg then which of the following is correct?

A. Correcting this will increase the ASDR and decrease the 1 eng inop TODR
B. The take off is permitted
C. The one engine out take off distance will become greater than the ASDR
D. Vmcg will be lowered to V1.

The correct answer is B, apparently! So if this is true do we now have a paradox where the aircraft could be beyond V1 (committed) but is not controllable on the ground? We cannot abort but we cannot continue either, assuming we have yet to attain Vr. :eek:

Would it not be the case that we would increase V1 which would increase our ASDR but decrease our TODR which is option A?

[truckflyer]

The definition of V1, at least one part of it, is that you can continue the take off at V1 with one engine inoperative.

However if you have not reached Vmcg, you will not have aerodynamic control when you loose an engine at Vef.

Per definition of Vmcg it must be Lower or equal to V1, hence V1 can not be lower than Vmcg.

So B can not be correct. You would have to lower Vmcg, I would assume this could be done by using reduced thrust setting on take off, which will give you a smaller yawing moment if you lost an engine during take off if not you would have to increase Vmcg if possible, or reduce the TOM, if TOM is reduced to the Vmcg limited TOM.

By the way your assumption of A can't be correct either, remember V1 speed, you can take off within the TODA or stop within the ASDA/ASDR - so V1 will be the highest speed you can do either. It will give you the best of both worlds.

If your V1 is lower than Vmcg, you will end up outside the runway in a big ball of fire, as you try to take off without any directional control!

[StatorVane]

This is what I thought mate. It's obviously got mis-logged.

Cheers.

[truckflyer]

Yes, there are always some errors in these questions, but at least it shows you are paying attention.

[StatorVane]

I also see what you are saying about "A". Thanks.

[SloppyJoe]

I believe the only way this would be permitted is if it is balanced due to reduced thrust already. By using TOGA you could reach VMCG earlier with more runway remaining for the go or stop option. The only other way is to reduce weight (disregarding weather).

[whiskey1]

Are you sure answer B does not say:
The take off is NOT permitted?

W1

[mutt]

Answer can be A. Nothing said that you were field length limited, therefore you can increase V1=VMCG, we do it all the time on contaminated runways.

or reduce the TOM, if TOM is reduced to the Vmcg limited TOM.
V1 is already LOWER than VMCG, therefore reducing takeoff MASS will have no affect.

Mutt

[truckflyer]

Mutt, what utter nonsense

Reducing TOM will not have any effect!!!!! :ugh: :ugh: :ugh:

First decrease TOM, and V1 will increase, I needs to reach Vmcg at least.

It says Balanced Field, this means TODA = ASDA, the whole point of a balanced field is to give you the best chance either way,

A balanced field takeoff is a condition where the accelerate-stop distance required (ASDR) is equal to the takeoff distance required (TODR) for the aircraft weight, engine thrust, aircraft configuration and runway condition

If A was correct, how could increasing V1 reduce the TODR?

Vr will still happen at the same speed, providing conditions are the same!

Reducing your weight would let you accelerate faster, ergo you would be able to reach a higher V1, less weight will also give you less inertia, so you can stop faster!

[mutt]

Mutt, what utter nonsense Are you sure?

First decrease TOM, and V1 will increase, I needs to reach Vmcg at least. Decrease weight and V1 will increase? Interesting concept, so you are saying that at MTOW you will have a lower V1 than at a lower weight?

It says Balanced Field, this means TODA = ASDA True, but it doesnt mean that its the complete runway length, and it doesnt mean field length limited!

If A was correct, how could increasing V1 reduce the TODR?
Vr will still happen at the same speed, providing conditions are the same!
How many engines are operating prior to VEF?



Mutt

[truckflyer]

First lets confirm, V1 must be increased to reach Vmcg!

Information given is that it is a balanced field calculation, so we can't assume it is an unlimited runway, it means V stop/go is a predetermined point/speed for this specific runway.

In the case that V1 is LESS than Vmcg, conditions will be that it is an unbalanced field, that's the whole point of of having the definition Balanced field, it is to make calculations easier, to determine the MAXIMUM speed, that you can reach, and still decide to stop or to continue!

The conditions you are mentioning would make it an unbalanced field.

TODR = ASDR = BALANCED FIELD
I agree that conditions would be that V1 could be increased, ASDR will increase, TODR will decrease, Field required would be Increased - however that would make it an unbalanced field!

Answer A describes an unbalanced field take off!

If you are not field limited, your V1 will increase with mass.
But if you discover that you are field limited, and can't reach your required V1 what would you do?
Reduce mass, this will let you accelerate faster, reach the speed you require earlier, also a lighter aircraft will be able to stop faster, less inertia when braking!
So reducing mass gives you the chance to increase your V1 within the limits, this is one alternative.

[StatorVane]

Are there two ways to look at this? To be fair, the question talks only of a balanced V1 and does not mention a balanced "field" so I could not assume that we were field length limited.

Could we have situation where the field is not balanced but the V1 is balanced? Or is there only one definition of balanced V1 and that is when the TODA = ASDA? I thought that a balanced V1 could be manufactured and would be TODR = ASDR (ASDA). But I see that if this were the case I could still not increase my V1 to match Vmcg since this would push my ASDR possibly beyond the geometry of the stopway?

My thinking is of a lightly loaded aircraft who's TODR = ASDR (therefore balanced V1). There is no mention though of the ASDA and if I said the ASDA is sufficiently large then I could do two things: 1) reject from a higher V1 thus reducing my TODR but increasing ASDA (no longer balanced V1 but pointless) or 2) increase the weight which would increase TODR and ASDR keeping a balanced V1. I could do this up until the point that ASDA is reached. All along through this there is no mention of the TODA being equal to ASDA so this is not a balanced field. The question didn't say I could not unbalance the V1 or at least that is my interpretation.

My thinking with "A" was that I could use the option (if I had it) of pushing up my V1 which would increase my ASDR, admittedly, but would decrease my TODR. I can also see the other logic though.

My brain is fried so if this reads as utter rubbish I'll pick it up tomorrow and correct it! :\

[EK4457]

My GBP 0.02;

Answer A.

If by 'correcting it' they mean increasing V1 to be >= Vmcg.

As previously mentioned, whilst balanced field does mean ASDA=TODA, it does not necessarily mean that you are field length limited. Nor does the question state or imply you are FLL either.

With this in mind, if you increase V1, you will increase your ASDR. If you then loose an engine at an increased V1, you have spent longer accelerating on 2 engines. This will reduce your TODR. Hence answer A.

If you 'correct it' by reducing Vmcg by reducing thrust then I *think* A still applies. My head hurts though.

On a side note, there seems to be a bit of chat about the effect of TOW on V1. I seem to remember that this depends on whether you are FLL on ASDA or TODA.

V1 increases with TOW if you are TODA limited as you need to accelerate on 2 engines for longer if you are heavier.

V1 decreases as TOW increases if you are ADSA limited as you now need longer to stop.

Disclaimer

This is all off the top of my head with no access to my Bristol stuff. I could be very wrong.

EK

[EK4457]

Oh, just noticed another thing that has confused me for a while.

I always thought balanced field meant ASDA=TODA.

However, some on this thread are saying that it is ASDR=TODR. I personally find hard to get my head around the actual distances REQUIRED being the same.

It is also being said that by increasing ASDR and decreasing TODR it is now not balanced as they are not equal. Therefore A can't be correct.

I would say it is still balanced as the calculations are still based on the AVAILABLE distances being equal.

Again, NOT 100% on this.

Edited to say that just found that both scenarios can be called 'balanced' and mean different things.

So I give up......

Would be nice if KW or the other resident bookworms could help us out?

EK

[keith williams]

The question as stated in the JAR CQB was not wuite as it is listed in the OP in this thread.

The CQB question was as follows:

If the value of the balanced V1 is found to be lower than VMCG, which of the following is correct?

a. The ASDR will become greater than the one engine out take-off distance.
b. The take-off is not permitted.
c. The one engine out take-off distance will become greater than the ASDR.
d. The VMCG will be lowered to V1.

The JAR "correct answer" was option b but this has been appealed many times. The problem was not helped by the fact that the CAA examiners did not really understand the arguments.

My comments below were written for the consumption of students, so they are a bit laborious for some of the more expert members who have alreday contributed to this thread.


V1 is the take-off decision speed. If an engine fails before V1 then the take-off must be aborted. If an engine fails after V1 the take-off must be continued. V1 must therefore be selected such that it is possible to complete the take-off within the distance available, or to stop with the distance available from the point at which V1 is reached. If the runway is wet the friction between the tyres and the runway will be reduced. This will reduce the effectiveness of the braking system, thereby increasing the distance required to stop from V1. This in turn will reduce the maximum allowable value of V1.

The distances required to complete the take-off or abort are determined by the mass of the aircraft and the value of V1. V1 is said to be balanced when the two distances TODR and ASDR are equal. An aircraft is said to be field limited when one or both of the distances available are equal to the distances required (TODR = TODA or ASDR = ASDA). Under these circumstances there is no spare distance available.

VMCG is the minimum speed at which it is possible to maintain control following the failure of the critical engine during the take-off run. If the failure occurs before VMCG then the aircraft will go out of control and run off the side of the runway. For this reason V1 must never be less than VMCG. If pre take-off performance calculations reveal that V1 is less than VMCG, then V1 must be increased to equal VMCG.

But this increased V1 will increase the distance required to stop the aircraft if the take-off is aborted. It is not therefore possible to increase V1 if the aircraft is field limited. So if V1 is less than VMCG and the aircraft is already field limited, the take-off is not permitted (option b).

It should however be noted that this question does not specify a field limited condition. Under these circumstances it may be possible to increase V1. This will increase the accelerate stop distance and decrease the take-off distance. This action might result in a situation in which the ASDR is greater than the one engine out take-off distance (option a).

A further possibility might be to carry out a reduced thrust take-off. This would reduce VMCG and might possibly make it less than V1 (option d). But VMCG cannot be reduced arbitrarily simply to make it less than V1.

Overall this is a very badly worded question, which has caused problems for many years.

[mutt]

Reduce mass, this will let you accelerate faster, reach the speed you require earlier, also a lighter aircraft will be able to stop faster, less inertia when braking! interesting, but as the weight is decreased the balanced V1 will decrease, but what happens to your VMCG? Does this change with weight ?

Keith, thanks for clarifying that the original question was written incorrectly :)

Mutt

[StatorVane]

Thanks guys. Just for the sake of sanity though I did write the question verbatim. I don't want people to think this is my error because I sat staring at this for some time before I put it on here. I did indeed come across the other version of it, the one that Keith refers to, some questions later.

**I went back to treble check. it does say "take off is permitted". Happy to link to a screenshot if that doesn't breach any copyright or pprune policies!

Nonetheless we all agree take off is not permitted under these conditions.**

[truckflyer]

Yes thanks Keith for the wise words on that.

Out of interest where did you find this question? I assume it was some QB of some kind.

Mutt - you still want me to "nail" me on that weight issue!

I do believe from what I read, Vmcg does not change with weight, Vmcg can only change because of the density - pressure, temperature etc.

Low pressure altitude, high density, low temperature etc. will make Vmcg higher, however V1 is effected by weight.

I have some book somewhere, which states that fact, can't get the exact quote now, but it was something like if you are Field Limited, V1 will decrease with increased mass (of course there will be a limit, where you will have to start reducing more mass, to get a better V1), and if you not field limited V1 will increase with weight.

This does make logical sense, as an example your MTOW is 60T, if you are field limited and can only take off with 57T, what will you do if during your calculations you find out that your V1 is below Vmcg?
You have to find some way to increase V1, because Vmcg will not change unless the weather changes or you use reduced thrust, which might put you within limits.
However to use reduced thrust, you might also need to reduce the mass, but disregard this for one moment, just on the V1 issue. To change V1, you need it to reach a higher V1 within a shorter distance, the aircraft must accelerate faster, and reach that point earlier, if it has less mass. Now as it is faster, it will also require a longer stop distance, you will have this because it reached V1 faster and earlier as you took some weight of it.
However to be able to stop within these constraints, you might need to reduce the mass more, as increasing V1 GO to Vmcg is not enough, if V1 STOP is to high, for it to be a balanced V1, the mass must be reduced to make sure you can stop or go at your V1, and the main thing to give you this would be reduction of mass.

If you are not field limited, it does not matter, you increase your acceleration distance, you can increase your V1, as long as you can Go or Stop, and if not field limited that will not be an issue, in that case V1 will be increased with mass, as you will need a higher Vr with increased mass to take off. And no field limit, of course you want to be able to make that V1 has high as possible, to give you best options in case of an engine failure!

V1 must be less or equal to Vr. Ideally you would like that V1 was equal to Vr, as in the worst case you could still decide to stop, however for this to happen your runway can't be field limited due to your TOW. If it is, the main way to adapt is reduce the weight!

[StatorVane]

This question was one from the Oxford ATPL Performance CD. At the end of the lesson you can run through questions on that topic. I don't know how representative they are of the actual exam but I find them useful at sorting what I know from what I don't.

[keith williams]

Statorvane

I'm not suggesting that you have misquoted the question. It is quite possible, indeed quite likely that the examiners have taken a defective question and "improved" it to make an equally defective question. They have done this many times in the past.

If we take the question as stated in your original post

If the value of a balanced V1 is found to be lower than Vmcg then which of the following is correct?

A. Correcting this will increase the ASDR and decrease the 1 eng inop TODR
B. The take off is permitted
C. The one engine out take off distance will become greater than the ASDR
D. Vmcg will be lowered to V1.


Option A is correct is we assume that the aircraft was not field limited and we increased V1 to at least equal Vmcg.

Option B is just plain wrong. You cannot legally take-off with V1 less than Vmcg.

Option C will occur only if we go the wrong way and actually decrease V1, but to do so makes no sense.

Option D could be achieved if we were not field limited , and used reduced thrust.

Overall option A is the best answer.

[StatorVane]

Thanks very much Keith. Your description is very clear and to the point.

[mutt]

Mutt, what utter nonsense

Reducing TOM will not have any effect!!!!!

I'm still waiting to discover why my comments were considered utter nonsense.

Mutt

[truckflyer]

And I have explained , TOM will change V1. To keep it within limits you will be able to adjust V1, in this case you want V1 to increase, without going outside your criteria of a balanced V1.
What you explained would have given you an unbalanced V1. I do have a graph on this, but not going to start uploading this now, because havent got time for that just now.:8

[truckflyer]

http://img715.imageshack.us/img715/3533/v1vmcgnew.png

http://img266.imageshack.us/img266/7171/v1vmcgnew2.png

http://img688.imageshack.us/img688/6361/v1vmcgnew3.png

(changed it,was wrong graph before - sorry)

I found some time, here is the graph.

[mutt]

Your graph didnt show up...... but thanks for trying....

I said that V1 is already LOWER than VMCG, therefore reducing takeoff MASS will have no affect. As I see it, your Balanced V1 is less than VMCG, so you have to increase the V1, you have said that you can increase the V1 by decreasing the Takeoff Weight, I really would like to see you explain this, because even in the world of computerized performance software, I have never seen it happen :).

the whole point of a balanced field is to give you the best chance either way, What do you mean by the best chance either way? If this was true, why dont airbus use balanced field as default? Or even Boeing? Are you saying that their methodologies are less safe than Balanced Field?

However to use reduced thrust, For us, reduced thrust is called Flex and it has no impact on the VMCG :)


Mutt

[truckflyer]

So what do they use if they don't use balanced field?

Graph is showing for me.

[galaxy flyer]

Keith

One smallish correction, using a reduced thrust would not help in the case as Vmcg is based on rated thrust. A derate thrust setting would work, however.

GF

[mutt]

We use Optimized, or Improved Climb, or Balanced, or Minimum, or Maximum V1 policies, and we also have to option to adjust the V1/VR ratios. Lots of ways to do the same thing.

If you stick with Balanced V1 when you are not field length limited, you might find that you are climb or obstacle limited, so if you opted to unbalance, you can obtain better second segment gradients than balanced, this in turn leads to better climb limit weights or greater thrust reductions.

And as GF has pointed out, reduced thrust only helps you if you use a fixed derated reduction, not an assumed or flexible temperature reduction.

Mutt

[mutt]

Chart opens on iPhone, is that really supposed to read VMBE?

Mutt

[truckflyer]

Sorry about that, seems I might have uploaded the wrong image, will try again. Even got the text that explains the theory behind this.

If you look at previous post I have now changed the graph there, hope they visible, if not use the links here, and you should also see them.


http://img266.imageshack.us/img266/7171/v1vmcgnew2.th.png (http://imageshack.us/photo/my-images/266/v1vmcgnew2.png/)
http://img715.imageshack.us/img715/3533/v1vmcgnew.th.png (http://imageshack.us/photo/my-images/715/v1vmcgnew.png/)
http://img688.imageshack.us/img688/6361/v1vmcgnew3.th.png (http://imageshack.us/photo/my-images/688/v1vmcgnew3.png/)



[img=http://img266.imageshack.us/img266/7171/v1vmcgnew2.th.png] (http://imageshack.us/photo/my-images/266/v1vmcgnew2.png/)
[img=http://img715.imageshack.us/img715/3533/v1vmcgnew.th.png] (http://imageshack.us/photo/my-images/715/v1vmcgnew.png/)
[img=http://img688.imageshack.us/img688/6361/v1vmcgnew3.th.png] (http://imageshack.us/photo/my-images/688/v1vmcgnew3.png/)

[keith williams]

One smallish correction, using a reduced thrust would not help in the case as Vmcg is based on rated thrust. A derate thrust setting would work, however.



Yes sorry GF, I was being a bit sloppy with the terminology there. Curiously the distinction between reduced/flex thrust and derates has never been emphasised in the JAR syllabus, so the examiners would not have been looking for option d in the original question.

[mutt]

OK, if you are reducing Mass, then using the Maximum V1 associated with that Mass, you have unbalanced the V1, if you are willing to use an unbalanced V1, then there is nothing stopping you from unbalancing the V1 based on the original Mass, as i said we do this all the time with contaminated runway calculations.

It sounds like you have just become an advocate of optimized V1 :):)

BTW, that chart uses the term NORMAL V1, and shows it associated with a field length limit, this is somewhat misleading as it indicates that NORMAL V1 will always be balanced and field length limited, which is incorrect. It also shows that the Field Length Limit is longer with the lower weight, and this is once again incorrect, with a lower weight you will have a lower Field Length requirement.

If you plot the balanced V1's associated with a particular weight and field length, you will discover that they are linear and show that a higher weight gives a higher balanced V1, and inversely a lower weight gives a lower balanced V1.


Mutt

[truckflyer]

I am still trying to get my head around the graphs, to be honest. But I have been reading about something like this before too, where they stated that if you you was not FLL, than V1 would increase with mass, as we both agree on.

Same place I read that if FLL, than by reducing mass you would also be able to increase your V1 - I can't find this "book" or article where I read about this now, where I can find all the details about this.

But would this than be an unbalanced V1?

My logic behind the reducing the mass theory, was that you would be able to increase V1 - (with sufficient mass reduced), so that you would reach this higher V1 earlier. (as F = m x a which would be: a = F/m)

In my mind, I might be wrong, but this was my own theory behind it, you accelerate faster to a higher V1, because you have less mass, this means even though you have increased your V1, you will reach it at an earlier stage on the runway, because you are faster now.

You will also need a longer ASDR, but because you are lighter, there will be less inertia, to make you stop faster, although the balance between higher speed and less mass, will have be done trough performance operations/calculations.

Are these assumptions and logic above completely wrong? Or is there some major issue that I am failing to see with this?

The main reason I see that V1 increases with mass, when you are not field limited, is that you must have sufficient speed to be able to continue your take off, VR will increase with mass.
That means if you are very heavy, the gap between V1 and VR will most likely increase. So increasing V1, is to ensure that you can reach VR with one engine inop before end of runway. Which if the gap is to big, you might not reach, so you can than reduce VR by reducing mass!

If V1 is less than Vmcg, that means you reach V1, you will not be able to take off, but the only way I see from logic, is to make V1 equal to Vmcg. But obviously those calculations will be have been based on your current performance calculations.

Please I apologise if for what I have said, if I have completely missed the target here on this, but for me it did seemed logical way to explain, how that reducing mass would be able to increase your V1! And I would have thought there would be a mass that would still keep you within the balanced field requirement.

Your V1 must be high enough to stop and go, if you want to increase your V1 to a higher speed, you must find a way to reach that speed within a shorter distance than your older "to low V1", because as your V1 will be higher, you will need more distance to stop too.
So for me the logical conclusion that you can reach a faster V1 at a shorter distance, is to increase your acceleration, and to do this you must reduce mass, as Thrust / Mass = Acceleration

[Andy1973]

Firstly, I agree with the other posters that the original question is poorly worded and ambiguous.

The debate that has led on from it has shown some confusion, both about take-off optimisation and about the effect of V1 on take-off performance.

Effect of V1

The term "field length limited" covers 5 basic performance scenarios:

1) You are limited by take-off run, all engines operating.
2) You are limited by take-off run, one engine inoperative.
3) You are limited by take-off distance, all engines operating.
4) You are limited by take-off distance, one engine inoperative.
5) You are limited by accelerate-stop distance.

So, if we change V1 but keep all other factors the same (this includes weight), we can have a variety of effects:

a) By definition, V1 has no effect on the all engines operating cases (1&3).

b) If V1 is increased, the accelerate-stop distance required will increase. This is intuitive, as we must assume that the aircraft will stop from a higher speed than before.

c) The take-off run and distance required, with one engine inoperative will decrease. This is not as clear as point b, but remember that we said that all other factors will stay the same. So, we will still have to reach the same Vr for rotation. The higher V1 means we can assume that the point of engine failure (Vef) happens later and the aircraft spends more off the take-off run with both engines operating, thereby accelerating faster and using less distance.

So, the effect of a change in V1 on a field length limited take-off depends upon which of the factors is actually limiting!