Big jet vmc

Cyper42

Cyper42

Well-Known Member
Hi thought of a question I can't find an answer to. Do 4 engine birds have multiple vmc airspeeds? For example do they have an inner outer and total engine failure vmcs or just the worst/ highest one like on smaller twins?
 
Cyper42 said:
Hi thought of a question I can't find an answer to. Do 4 engine birds have multiple vmc airspeeds? For example do they have an inner outer and total engine failure vmcs or just the worst/ highest one like on smaller twins?
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V1 ensures you are above Vmcg for all of the parameters for your takeoff. Vr is always above Vmca and you climb out at V2(maybe plus something). At that point you don't really care what Vmc is because the other speeds have considered it.
 
To elabroate on the above response, V1 ensures you are above the vmca critical engine for a single engine out. VMCA-2, where you lose 2 on the same side, is significantly higher then V1 and often V2 at light weights.
Example A340
VMCL = 125 KTS; VMCL-2 = 157 KTS
(VMCL = VMCA in landing config)
 
Specifically 254 potentially different Vmc speeds. 2 to a power of 8, minus 2, for all the different possible combinations of engine failures except all operating and all failed.
 
kyl said:
To elabroate on the above response, V1 ensures you are above the vmca critical engine for a single engine out. VMCA-2, where you lose 2 on the same side, is significantly higher then V1 and often V2 at light weights.
Example A340
VMCL = 125 KTS; VMCL-2 = 157 KTS
(VMCL = VMCA in landing config)
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Indeed. Like @z987k said, V1 is (by definition) in excess of Vmcg, Vr is in excess of Vmca, and approach and landing speeds are in excess of Vmcl.

I've only ever seen Vmc-limited takeoff performance numbers when Aerodata goofed with a discontinuity on the Brasilia, incidentally. At least for us twin-jet guys and gals, field length of (especially) climb limits are the harder things to satisfy.
 
Where Vmc becomes a serious limitation is when there is slush/ice on the runway, then vmcg often drives V1 way up.
 
And per regs, published Vmca (the speed manufacturers put in the AFM) is considered with the most critical engine inoperative. So as Screaming_Emu said, there would be only one published Vmca for the worst case scenario, but the actual Vmca would vary depending on which engine failed.

See CFR 25.149
 
kyl said:
Where Vmc becomes a serious limitation is when there is slush/ice on the runway, then vmcg often drives V1 way up.
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It may drive V1 up, but the accelerate-stop equation will probably drive V1 right back down again.
 
BobDDuck said:
It may drive V1 up, but the accelerate-stop equation will probably drive V1 right back down again.
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It can't, since V1 cannot be lower then VMCG.
V1, by definition, is the decision speed where, with the one most critical engine failed, there is sufficent performance and control for the takeoff to continue at that power setting. V1 cannot be lower then VMCG as the aircraft will veer off the runway if the takeoff continues at speeds below VMCG.
In the situation where the runway is contaminated and ASD/TORA is limiting, the only way to get V1 down is to thrust limit/derate engine output, thereby reducing VMCG. Its one of the few times where it hurts to have too much thrust on a short runway.
 
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kyl said:
It can't, since V1 cannot be lower then VMCG.
V1, by definition, is the decision speed where, with the one most critical engine failed, there is sufficent performance and control for the takeoff to continue at that power setting. V1 cannot be lower then VMCG as the aircraft will veer off the runway if the takeoff continues at speeds below VMCG.
In the situation where the runway is contaminated and ASD/TORA is limiting, the only way to get V1 down is to thrust limit/derate engine output, thereby reducing VMCG. Its one of the few times where it hurts to have too much thrust on a short runway.
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That makes sense. Of course most manufactures don't allow for a derate/flex on a contaminated runway though. I wonder if there would be a scenario were you couldn't depart because of the math on that.

EDIT: Duh... of course. If you can take off on a runway when it isn't contaminated but can't when it is, that's because the reduction in V1 due to stopping ability is greater than VMCG. I'm not so speedy tonight.
 
gotWXdagain said:
Specifically 254 potentially different Vmc speeds. 2 to a power of 8, minus 2, for all the different possible combinations of engine failures except all operating and all failed.
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It'd be fewer than that, though right, because if you lose engines that are opposite one another there'd be no controllability hit.

If you lose #1 and #8, or #2 and #7 and so on then you shouldn't have any appreciable controllability problems all things being equal.
 
ppragman said:
It'd be fewer than that, though right, because if you lose engines that are opposite one another there'd be no controllability hit.

If you lose #1 and #8, or #2 and #7 and so on then you shouldn't have any appreciable controllability problems all things being equal.
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That is correct and something I didn't think about. So to answer the opening poster's question, the reason there's only one Vmc speed published is because if you took all possible conditions and all possible failures there's be more Vmc speeds than there would be actual indicated airspeeds the plane is capable of flying.
 
Derg said:
If you're anywhere near Vmc in a jet, man, you're going to be walking down that brightly lit passageway toward the light pretty soon.
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Not all of them :)

The D model Falcon 20 that I flew years ago had a 115K VMC. When you looked up Vref on the charts it would decrease with weight until it got down to 115K and never went lower. At our normal landing weight Vref was always 115K.
 
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