V1 - Vr split on long runways
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V1 - Vr split on long runways
Hello Chaps,
here is a question which I know may have been asked many times before, but after searching I couldn't find a specific answer.
I fly Boeings and my company uses a Computer Take-Off Performance Program (CTOP) to calculate V-speeds.
Q. When operating from a VERY long wet runway without obstacle hazards (eg MLA), why would CTOP return speeds with a split between V1 and Vr ?
i.e. there is absolutely no way the aircraft would run off the end of an 11000' runway if a reject was made between V1 and Vr.
Thanks.
here is a question which I know may have been asked many times before, but after searching I couldn't find a specific answer.
I fly Boeings and my company uses a Computer Take-Off Performance Program (CTOP) to calculate V-speeds.
Q. When operating from a VERY long wet runway without obstacle hazards (eg MLA), why would CTOP return speeds with a split between V1 and Vr ?
i.e. there is absolutely no way the aircraft would run off the end of an 11000' runway if a reject was made between V1 and Vr.
Thanks.
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Hi Robbie-Rocket-Pants,
For every take off there will be a spread of V1s.
V1 GO will allow you to suffer an engine failure and still accelerate to VR.
V1 STOP will be the last point you can safely stop.
Practically V1 GO can't be lower than VMCG, and V1 STOP can't be bigger than VR.
Your CTOP has probably selected a V1 midway between V1 Go and V1 Stop.
For every take off there will be a spread of V1s.
V1 GO will allow you to suffer an engine failure and still accelerate to VR.
V1 STOP will be the last point you can safely stop.
Practically V1 GO can't be lower than VMCG, and V1 STOP can't be bigger than VR.
Your CTOP has probably selected a V1 midway between V1 Go and V1 Stop.
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Thanks rudderrudderrat.
I still don't understand why CTOP doesn't set V1=Vr in such conditions. For example I would much prefer to reject if I had a fire rather than be "forced" into taking it into the air.
I still don't understand why CTOP doesn't set V1=Vr in such conditions. For example I would much prefer to reject if I had a fire rather than be "forced" into taking it into the air.
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changed "VR >= VMCG" into "VR >= VMCA", typo
I'm a newbie, and I might well be wrong, but here's how I would answer your question, Robbie-Rocket-Pants:
V1 for the wet runway will be slower than V1 for a dry runway. This is because
a) the screen height can be reduced to 15 ft which reduces TODR and
b) reverse thrust may be taken into account for the stop case which in many cases will decrease ASDR.
On the other side, VR is still limited by the requirement VR >= VMCA. In other words, V1 decreases while VR cannot decrease.
V1 for the wet runway will be slower than V1 for a dry runway. This is because
a) the screen height can be reduced to 15 ft which reduces TODR and
b) reverse thrust may be taken into account for the stop case which in many cases will decrease ASDR.
On the other side, VR is still limited by the requirement VR >= VMCA. In other words, V1 decreases while VR cannot decrease.
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I still don't understand why CTOP doesn't set V1=Vr in such conditions.
On heavy 4 engine jets a split of 15 or so knots is the norm for long haul ops. In fact my mob tend to get mixed up on the very rare occasions when V1 = Vr, often we call "V1" and miss "Vr".
At max TOW, say 412 tons, dry, sea level, no obstacles the figures are around V1 154 Kt, Vr 170 Kt, V2 182Kt.
It figures that the V1 is closer to Vr on twins because they lose 50% thrust where quads lose 25% - one hell of a difference when you are trying to accelerate 200+ tons.
As an interesting aside, we were able to lift more out of NZWN, Wellington, by using RTG 1, a fixed derate, which lowered Vmcg which was the driver on this very short runway.
At max TOW, say 412 tons, dry, sea level, no obstacles the figures are around V1 154 Kt, Vr 170 Kt, V2 182Kt.
It figures that the V1 is closer to Vr on twins because they lose 50% thrust where quads lose 25% - one hell of a difference when you are trying to accelerate 200+ tons.
As an interesting aside, we were able to lift more out of NZWN, Wellington, by using RTG 1, a fixed derate, which lowered Vmcg which was the driver on this very short runway.
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RRR
that seems reasonable. V1 GO could be dangerously close to VMCG and V1 STOP is too much, by the time you realise there is a problem you are too fast. A speed in the middle is a good compromise.
Besides, I prefer to go airborne with an engine in flames than rejecting in marginal conditions.
Maybe the could refine the software and give V1 STOP when you have a large excess of runway where to reject the take off nicely
that seems reasonable. V1 GO could be dangerously close to VMCG and V1 STOP is too much, by the time you realise there is a problem you are too fast. A speed in the middle is a good compromise.
Besides, I prefer to go airborne with an engine in flames than rejecting in marginal conditions.
Maybe the could refine the software and give V1 STOP when you have a large excess of runway where to reject the take off nicely
If you have a range of V1s (assuming you are provided with this information by the Company via performance information etc) then it makes sense to pick the V1 which is most appropriate.
For example if you are departing a runway which has a vertical drop at the end of the runway (eg Luton, Funchal) select the minimum V1 since the last thing you want to do is go off the end in the event of a reject close to (maximum) V1. Also in the event of a continued engine failure once you have passed the end of the runway you'll have more than the screen height.
I you are taking off from a relatively long runway and are well below the field length limit but there is an engine out escape procedure (aka emergency turn) it might make sense to select the maximum V1 since stopping isn't a problem but do you want to be clambering round an emergency turn on one engine when obstacle limited?
Just selecting the "middle" V1 is not taking advantage - think about the situation and what makes more sense.
Not on every take off - on some there will only be one V1.
For example if you are departing a runway which has a vertical drop at the end of the runway (eg Luton, Funchal) select the minimum V1 since the last thing you want to do is go off the end in the event of a reject close to (maximum) V1. Also in the event of a continued engine failure once you have passed the end of the runway you'll have more than the screen height.
I you are taking off from a relatively long runway and are well below the field length limit but there is an engine out escape procedure (aka emergency turn) it might make sense to select the maximum V1 since stopping isn't a problem but do you want to be clambering round an emergency turn on one engine when obstacle limited?
Just selecting the "middle" V1 is not taking advantage - think about the situation and what makes more sense.
For every take off there will be a spread of V1s.
I still don't understand why CTOP doesn't set V1=Vr in such conditions. For example I would much prefer to reject if I had a fire rather than be "forced" into taking it into the air.
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Not on every take off - on some there will only be one V1.
By using Flex Thrust method, you can alter the performance, for the given runway length, terrain etc. so you end up with V1 go = V1 stop.
There will be a maximum temperature allowed for flex calculations, so if you reach max flex on a very long runway with no obstacles, you will generate a range of V1 go & V1 stop.
Our performance software allowed us to look at the V1 range with different flex figures.
I agree with fireflybob. Sometimes it's better to take the problem into the air and sort it out, sometimes it's better to stop - but you won't know which one was better until you've had tea and biscuits. V1 in the middle seems a very good compromise to me.
Last edited by rudderrudderrat; 5th Jan 2012 at 12:12. Reason: extra text
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Thanks for your replies chaps.
FireflyBob, I understand what your saying about limiting runways and that in such cases getting airborne would very often be the safest course of action.
My orginal question was only considering a very long runway with no obstacles. If it's totally possible to safely stop all the way up to Vr, I still don't get why I should be forced to take an aeroplane which may be filling with smoke into the air if I don't really need to.
FireflyBob, I understand what your saying about limiting runways and that in such cases getting airborne would very often be the safest course of action.
My orginal question was only considering a very long runway with no obstacles. If it's totally possible to safely stop all the way up to Vr, I still don't get why I should be forced to take an aeroplane which may be filling with smoke into the air if I don't really need to.
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depending on the type of aircraft, the actual take off mass as well the flap setting for t.o you may be brake energy limited regarding the split between v1 and vr .
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Hi Robbie-Rocket-Pants,
Having tried to explain the logic behind the maths and the CTOP presentation, if you are still worried about having a smoke filled flight deck which only became apparent after V1 but before VR - then I think you may be in the wrong profession.
I still don't get why I should be forced to take an aeroplane which may be filling with smoke into the air if I don't really need to.
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rudderrudderrat, I know I am being pedantic about it but my (very) hypothetical situation may be V1 145kts, Cargo Fire Warning 146 kts, VR 155kts, smoke in flight deck at 1000' and wishing I was on the ground. [Having thoroughly enjoyed being a pilot for 25 years I'm happy to continue in the profession !
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Q. When operating from a VERY long wet runway without obstacle hazards (eg MLA), why would CTOP return speeds with a split between V1 and Vr ?
A. It is likely that a fixed wet V1 reduction is applied regardless of runway length.
A. It is likely that a fixed wet V1 reduction is applied regardless of runway length.
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Robbie, is the data reflecting an increased V2 climb for 2nd segment clb performance and hence increased Vr ? Agreed Vmbe may well be in play and contribute to split.
Used to see this on 767 out of DXB on a hot day /inversion using southrerly runway. This can be the only reason I can think of.
Used to see this on 767 out of DXB on a hot day /inversion using southrerly runway. This can be the only reason I can think of.
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Vmbe is not a factor since I can increase the weight and get a higher V1. Also if the runway is dry V1=Vr.
I guess it must be the reason that KBPsen gave: "A. It is likely that a fixed wet V1 reduction is applied regardless of runway length."
I guess it must be the reason that KBPsen gave: "A. It is likely that a fixed wet V1 reduction is applied regardless of runway length."
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Hi Robbie ,
Ah. That's easy to fix.
Since the chance of a Cargo Fire Warning is time dependent and not engine power dependent, then simply delay any take off by about 5 secs (or the same time it would take you to accelerate from V1 to VR.) Thus ensuring that the hypothetical Cargo Fire Warning would now occur below your mid value V1.
but my (very) hypothetical situation may be V1 145kts, Cargo Fire Warning 146 kts, VR 155kts, smoke in flight deck at 1000' and wishing I was on the ground.
Since the chance of a Cargo Fire Warning is time dependent and not engine power dependent, then simply delay any take off by about 5 secs (or the same time it would take you to accelerate from V1 to VR.) Thus ensuring that the hypothetical Cargo Fire Warning would now occur below your mid value V1.
