B737 V1, VMCG & Snow
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B737 V1, VMCG & Snow
My flight today was very light in a B737-7 and we were taking off from a runway that had less than 1" dry snow. Our performance is automated and ACARS spits out the results and the FMC loads itself.
It came up with V1=108 and Vr and V2 much higher. The Vmcg for our airport/temp was 108 when we looked in the books. Also, according the books, the V1 should have been 102, had it not been limited by the Vmcg.
So V1 is raised to Vmcg, but what happens to the 6 knots where V1 was 102 and then raised to 108? How is this accounted for in the accelerate stop distance?
It came up with V1=108 and Vr and V2 much higher. The Vmcg for our airport/temp was 108 when we looked in the books. Also, according the books, the V1 should have been 102, had it not been limited by the Vmcg.
So V1 is raised to Vmcg, but what happens to the 6 knots where V1 was 102 and then raised to 108? How is this accounted for in the accelerate stop distance?
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In most cases there is a range of acceptable V1 values. Perhaps this was such a case. Then the FMC is likely to calculate a V1 based on stopping and acceleration distances only, then adjusting the calculated value to ensure >Vmcg and <Vr criteria.
The exception, where there is only one acceptable value for V1, is the balanced field scenario. It's often not applicable to real airfields, where:
- the TORA is only just big enough for the "stop" case; therefore V1 cannot increase without increasing stopping distance unacceptably, and
- the TODA is only just big enough for the "go" case; therefore V1 cannot reduce without increasing post-engine failure acceleration distance unacceptably.
There are others on PPRuNe with much more knowledge than I. But perhaps this will help until you get more thorough advice.
The exception, where there is only one acceptable value for V1, is the balanced field scenario. It's often not applicable to real airfields, where:
- the TORA is only just big enough for the "stop" case; therefore V1 cannot increase without increasing stopping distance unacceptably, and
- the TODA is only just big enough for the "go" case; therefore V1 cannot reduce without increasing post-engine failure acceleration distance unacceptably.
There are others on PPRuNe with much more knowledge than I. But perhaps this will help until you get more thorough advice.
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As Oktas8 said, there is a V1min and a V1max with the a actual V1 being a sensible point between.
If you want to increase performance of a slippery runway, a fixed derate is a potential option to lower VMCG.
If you want to increase performance of a slippery runway, a fixed derate is a potential option to lower VMCG.
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Yup, in a 700 you can lower your VMCg to 98kts or so by using a lower derate, which can actually allow a higher take off weight in some cases.
Nevertheless, at least in EU-OPS land no certified take off data exists for contaminated or reduced braking action calculation, it is just advisory data, take it with a grain of salt.
Nevertheless, at least in EU-OPS land no certified take off data exists for contaminated or reduced braking action calculation, it is just advisory data, take it with a grain of salt.
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The book value for V1 is usually a balanced field figure, if you were doing the takeoff calculation manually, you would then find a weight decrement and a V1 decrement, with a proviso that if the adjusted V1 was below VMCG, then you had to round it up to V1.
There are three issues here, the first is your ability to control the aircraft laterally following an engine failure, this is the reason the V1 is rounded up to VMCG.
The second issue is the accelerate/go, as you have increased the V1, you have accounted for this.
The third issue is accelerate/stop, this is where it gets fun, if you are on a true balanced runway that gives a V1 of 102, and you have rounded this up by 6 knots, what guarantee do you have that you will stop on the runway?
Is it right to arbitrarily round V1 up by 6 knots? If this is acceptable on a contaminated runway, why can't we do it on a dry runway, why are we so insistent that V1 is a speed which you MUST HAVE initiated an RTO.
Mutt.
There are three issues here, the first is your ability to control the aircraft laterally following an engine failure, this is the reason the V1 is rounded up to VMCG.
The second issue is the accelerate/go, as you have increased the V1, you have accounted for this.
The third issue is accelerate/stop, this is where it gets fun, if you are on a true balanced runway that gives a V1 of 102, and you have rounded this up by 6 knots, what guarantee do you have that you will stop on the runway?
Is it right to arbitrarily round V1 up by 6 knots? If this is acceptable on a contaminated runway, why can't we do it on a dry runway, why are we so insistent that V1 is a speed which you MUST HAVE initiated an RTO.
Mutt.
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As many have said, this is not an exact science. Thus there needs to be some awareness of the situation and a philosophy, perhaps, of which is better; a go or stop takeoff. As was seen last winter, trying to stop well down a slippery rwy might not always be the best idea. This is where it is necessary to understand that the figures are for guidance and there is no guarantee.
Having just read the latest publication from UK CAA about operation from contaminated ryws it is apparent that braking action info is also very subjective and for guidance only. It is emphasised that stopping distance is not guaranteed, neither is the directional control in crosswinds v braking action. Judgement is required. I wonder how much buffer airlines build in to their figure, and how much training is given to appreciate these parameters.
Having just read the latest publication from UK CAA about operation from contaminated ryws it is apparent that braking action info is also very subjective and for guidance only. It is emphasised that stopping distance is not guaranteed, neither is the directional control in crosswinds v braking action. Judgement is required. I wonder how much buffer airlines build in to their figure, and how much training is given to appreciate these parameters.
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Interesting question mutt. Yes, we can and do change V1 even on dry runways by a lot. On the 737 there is a range of around 30 to 35kts in which the V1 for a given runway can shift depending on used performance criteria. Sounds weird, but will lead to different used speeds and power reductions for each individual case. Of course improved speed schedules will lead to higher speeds including a higher V1 and at the same time to a very low or not existing stop margin, whereas a full thrust take off will lead to very low speeds, including a low V1 and very big stop margins.
Only at a runway where we have absolutely no stop margin and we are truly runway limited will there be only one solution. That might be a common occurrence in longrange aircraft, in shorthaul aircraft like the 737 it is pretty rare.
Only at a runway where we have absolutely no stop margin and we are truly runway limited will there be only one solution. That might be a common occurrence in longrange aircraft, in shorthaul aircraft like the 737 it is pretty rare.
