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Calling V1 Early vs Vmcg

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Calling V1 Early vs Vmcg

Old 16th Dec 2011, 12:05
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Calling V1 Early vs Vmcg

I recently had a snowy takeoff where V1=Vmcg.

http://www.pprune.org/tech-log/46953...vmcg-snow.html

My airline has a policy of calling V1 five knots early. So, if V1=Vmcg and we call V1 five knots early, that sounds like a recipe for runway excursion. Am I nit-picking?

If your airline calls V1 early, does it take Vmcg into consideration?
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Old 16th Dec 2011, 12:11
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I don't understand! Are you saying your airline actually LOWERS V1 by 5kts?? Is this company/regulatory approved? A very dubious practice. Surely IF V1 is called 5 kts early it is advisory and not actionable? IE In the situation you describe, no continued take-off would surely happen until V1? Re-assure me! I can see some perf guys getting quite edgy here.
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Old 16th Dec 2011, 12:22
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on such a rare accasion you may with this technique indeed run into problems. the v1 of course cannot in none case be lower than actual vmcg. on lightweight like in the other thread said a derate may help.

the idea of calling v1 early is that when you at the exact v1 speak out "vee one" you are actually slightly above this speed finished the call. thats why many carriers replaced this call out with the callout "go" speaking it out at the actual v1. and this means- starting speaking go means we go.

but thats hairsplitting , there are some margins in it .generally when v1 is at or very close to vmcg it might be a good idea to abort until positivly above vmcg. 99,99% ever tried a go with an engine failure at exactly vmcg=V1 whenever on a simulator, so barely anybody can say how a real 737 on a snowy runway will handle at such a situation- and it might be not pretty.
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Old 16th Dec 2011, 13:00
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I don't understand! Are you saying your airline actually LOWERS V1 by 5kts??
Yep. It bothers me, too.

I talked with a safety guy about this. . . he definitely danced about that very point. It went something like this: "Weellllll, we don't really lower V1, we just call it early." I'm not sure how they rationalize this. But it drives me batty.

I also worry about how this affects Accelerate GO distance if you've now got 5 more knots to cover on 1 engine.
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Old 16th Dec 2011, 13:45
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I suspect confusion is the simple answer here - what does your OM say about aborts/speeds? Surely the -5 call is merely a 'heads up'?
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Old 16th Dec 2011, 13:46
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One of the airlines I work with calls V1 5 kts early. After discussion their reasoning was thus:

The Captain is responsible for deciding.
He is prompted to decide by the call of "V1" to which he responds with "Taking off".
The call "Taking off" is a comitment to go.
If an engine fails at the same time as the co-pilot is saying "V1" the captain could still stop and he will have initiated the first action below the actual V1 speed.
If the engine failure occurs while he is saying "Taking off" the actual speed will already be at or very close to V1 and he will continue the take-off.

Note that the language this company uses is not English and therfore the call "taking off" is much shorter that the english translation.

So the concept is to limit the action to V1 by having the captain decalre"taking off" at V1.

I have no opinion either way
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Old 16th Dec 2011, 13:52
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Seems to me that all you are doing is ensuring that the V1 call is completed AND understood by the PF by the time the speed reaches V1 which is entirely as it should be.

I believe the 777 makes an auto call (or at least has an option to do so) for V1 for the same reason, can any 777 drivers comment on when the call is started? I suspect it is about V1 -5kts depending on the rate of acceleration .
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Old 16th Dec 2011, 14:09
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Company manual.

After “V1” call, the Captain’s hand should be
removed from the thrust levers.
At 5 kts below V1, call, “V1.”
Most of the reject takeoff discussion is from the FCTM, but this paragraph is slipped in:

Since V1 is the maximum speed for initiating an RTO, the decision to stop
must be made prior to V1. To account for reaction time, V1 is called 5 kts
prior to the actual V1.
Most, if not all CAs remove their hand from the TLs at the V1 call.

Last edited by ImbracableCrunk; 16th Dec 2011 at 16:38.
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Old 16th Dec 2011, 15:33
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IBC,

Not sure which FCTM your reading but thats absolute garbage with regard V1 auto call out.

The B777 calls V1 at precisely V1 as entered in the FMC.

From the manual....

"Takeoff V1 Airspeed"

"The voice unnunciation VEE ONE sounds when airspeed reaches V1 during takeoff"
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Old 16th Dec 2011, 15:44
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A good starting point for this discussion might be to first read the approach taken by the certification authorities when defining these speeds.

Google 'EASA CS25' and then read paragraph CS 25.107 for their interpretation on VEF and V1.
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Old 16th Dec 2011, 15:47
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A disguised call out for VEF or V event?what next?a call out for VLOF?
My airline uses this call out also,i dont agree with it but i go along.
Could be a problem if you reject at your company call of V1,ie your engine failed VEF at the real v1-10kts and you reject at the real v1-5.
You will therefore be below your VMCG...nose wheel stearing always a bonus in this scenario...
If you decide to continue at v1-5 (your airline v1) rather than stopping,this extra distance will increase your VR point therefore decrease your obstacle clearance...already low as 15ft by regulations..
Short runway was it?
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Old 16th Dec 2011, 16:43
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8che wrote:
IBC,

Not sure which FCTM your reading but thats absolute garbage with regard V1 auto call out.
Nothing I quoted was from FCTM.

I've flown 737 with the automated callouts. I think it will call V1 early, if required, so that it can complete that call and make the Vr call on time. Nothing to back that up on, but it's something I noticed.
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Old 16th Dec 2011, 18:29
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Your post indicated you were quoting from a FCTM.

Most of the reject takeoff discussion is from the FCTM, but this paragraph is slipped in:
Any how allowing for misunderstood posts I wasnt aware that the B737 had an optional auto VR call as well as a V1 call.

Boeing have not to my knowledge ever designed a system that calls a V1 other than your V1 calculation.
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Old 16th Dec 2011, 18:41
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I was responding to BOAC's question as to what was in the company manuals. Slipping something into the manuals from outside of the FCTM is what worries me.

As far as V1 and Rotate calls, you're right. I flew temporarily in Asia, and I'm back home, so I'm getting my variants and procedures a bit crossed. None has an automated Rotate call, but I still seem to remember that it was advanced so Rotate could be called on time. Not important, really.

I am worried about a V1 below Vmcg.
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Old 16th Dec 2011, 18:57
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Contaminated runway, no FLEX so TOGA, V1 called and you decide to reject, but still below Vmcg, no problem. Symmetrical thrust (depending on reverse selected). Should stay on the runway.

However, V1 called and you loose an engine and still decide to go. TOGA thrust on one side only. Still less tham Vmcg..............
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Old 16th Dec 2011, 19:05
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However, V1 called and you loose an engine and still decide to go. TOGA thrust on one side only. Still less tham Vmcg..............
And the airport was Ketchikan, Alaska. PAKT. Mountains on one side, slope down to the water on the other.

pakt - Google Maps
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Old 16th Dec 2011, 19:29
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The auto-callout V1 on the 737 starts at V1 minus 2 kts. Recently had the interesting case where it called V1 and the speed stayed at V1 -2kts for several seconds, so in fact after the autocallout was finished we were still not at or above V1.
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Old 16th Dec 2011, 19:34
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Google 'EASA CS25' and then read paragraph CS 25.107 for their interpretation on VEF and V1.
If you google you might as well look up how the certification is being done on vmcg.

From a practical standpoint;
1) The weight might be more favourable in comparison to the Vmcg testing
2) Most probably not at extreme (unfavourable) c.g.
3) Nosewheel steering still operative (and effective?)
4) Do a risk assesment on crosswind (B.A. medium to poor)
5) Take a moment in the simulator to asses how long the VVVVVVV oooonnnneeee call takes in relation to your acceleration and when you will decide to stop or go in this call
6) What is 30 feet from the centerline in the end

In my memory Boeing has an excellent article on t.o. margins (runway available) in relation to V speeds. Google helps here as well.

One of those magic traps introduced by a combination of certification criteria with performance optimising tools.

Last edited by have another coffee; 16th Dec 2011 at 20:06.
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Old 16th Dec 2011, 20:06
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Said article.

IMHO the 5 knot "early" call is nothing to worry about. Just a precaution against rejecting late (which can be disastrous) vs. deciding to go with a failure prior to V1 (which, even at 10 knots prior, still gets you off the runway with a good bit of clearance).
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Old 16th Dec 2011, 20:45
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aviatorhi, take care with your assumptions. The margins discussed in the Boeing document generally relate to time, which translates to distance.
In the diagrams showing V1, none of the options / margins changes the value of V1, which in some applications of ‘calling V1 early’ might happen.

CS 25 AMC section refers to the accuracy of published data; for V1 this is 1.5 kts, or less if it results in a change of 100ft or more in the performance distances.

From limited experience, small speed changes around the value of Vmcg can result in significant changes in the ability to control the lateral deviation, e.g 1 kt below a demonstrated Vmcg at limiting cg, etc, was the difference between a controllable 30ft deviation, and the side of the runway at the lower speed.

In aviation we often learn by knowing what the unknowns are; if we don’t know, then don’t do ‘it’.
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