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Vmcg and Xwind

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Vmcg and Xwind

Old 10th May 2006, 00:54
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SIC
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Vmcg and Xwind

Havent slept for three days due jetlag and cannot trust my own knowledge ( lack thereof rather ) SO here goes.

When a manufacturer states the Vmcg is for example 120 kts on a specific rwy,weight etc. it is a still wind Vmcg - right???

So if you have max crosswind lets say 30 kts - doesnt that actually invalidate the Vmcg you get from the book?? ( should you lose the critical upwind engine )

Are Vmcg testing during certification done in no wind conditions??
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Old 10th May 2006, 01:05
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Vmgc and Crosswind...

Yes, you're right. I've seen Boeing charts on the 737-200 for this. My memory is poor (as with the rest of me :> : :>), but I'm thinking the increase in Vmcg is significant. Loss of the downwind engine is a problem with high crosswinds in terms of Vmcg. An increase of as much as 10 to 15 kts is possible.

Regarding certification, etc., I can't offer any information. There are guys here in PPrune who are very knowledgeable in this area, though.

Good question!!!!!


Tom
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Old 10th May 2006, 01:12
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I think the early FARs were good for 7 knots XW, but not sure today if that holds on todays models-Just becase the FAA doesn't allow for it curently doesn't mean the airframe people don't build for it. I always wondered if they (manufacturers) may secretly have a Vmcg correcred to max XW's
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Old 10th May 2006, 01:32
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Try a search as this gets reviewed regularly.

(a) old UK BCARs had a 7kt Vmcg

(b) otherwise generally nil wind

(c) the certification number is a line in the sand for AFM purposes.

(d) real world Vmcg will increase for adverse crosswind (UPWIND engine failure is the problem .. think about it .. gives an into wind yawing moment due to the thrust asymmetry and also due to the weathercocking)

(e) increase varies with Type but typically 0.5 to something in excess of 1.0 kt/kt.

This increase doesn't invalidate the AFM data .. but it sure does need to be considered by crews if you are very light and have an aft CG .. ie short range ferry operation. It is quite easy, under such circumstances, to find yourself in a Vmcg departure situation with a failure near V1.

One of the other considerations is training .. I did some work for one operator which routinely used quite high V2 overspeed schedules but also had a short ferry operation which they could do at min speed. A few sim exercises opened more than a few eyes to the problems.

Unless there is a dollar in it for the OEM, I wouldn't necessarily take the view that the OEM will put non-required fat into the aircraft design ...
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Old 10th May 2006, 02:22
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Originally Posted by john_tullamarine
Unless there is a dollar in it for the OEM, I wouldn't necessarily take the view that the OEM will put non-required fat into the aircraft design ...
Sometimes, it's all we can do to keep the muscles attached to the bones, never mind worrying about the fat ....

As for the testing, the target is always 0kts today for test. Ideally not just 0kts xwind, but 0kts head/tail too, because we want stable conditions for repeatability (one argument againtst the old requirement is "what is a 7kt xwind?" - is a pure 7kt at 90 degs equivalent to a 14 kt wind 30 degs off the heading?, and so on). Plus the windier the conditions the more variable - do we take the average along the run, or the instantaneous value at engine cut? ... and so on. Many of these arguments can still be had about crosswind testing, too.
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Old 10th May 2006, 06:45
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Vmcg and X-wind

Forgive me, Gentlemen, I was incorrect. It IS the upwind engine that is critical in this matter, not the downwind engine. :> :> :> :>

Tom
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Old 10th May 2006, 09:37
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Unless there is a dollar in it for the OEM, I wouldn't necessarily take the view that the OEM will put non-required fat into the aircraft design
Following one of our previous discussions on this subject, i got a PM from someone in Boeing flight testing. Basically there is NO fat!

rhovsquared,

UK CAA/ BCAR had a 7kt requirement, FAA never had it.

Mutt
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Old 10th May 2006, 10:39
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It is "interesting" to watch Vmcg testing ... departure is VERY speed critical and it can go from nice to nasty in the space of several knots for the Vef speed ... one of these days I will dig out some long lens videos and post them somewhere with a link from PPRuNe ... very much a case of now you see it ... now you don't ...

Before playing with this I was a lip service to V1 min man .... took about two takeoffs watched through the video in real time to convince me that I had missed the point along the way somewhere ... instant conservative convert.

Those who play with takeoffs which are near Vmcg limited in a strong crosswind are far, far braver than I ..
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Old 10th May 2006, 11:07
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As in most things in aviation, it is all about 'acceptable risk'..If an upwind engine fails on a heavy 747, just above V1, with a strong crosswind then the aircraft will probably not stay on the concrete. However, somebody has calculated that the chances of this happening are below ten to the power of seven, which is how 'acceptable risk' is defined in aviation.
When we used to fail engines on base training we always had to calculate the actual Vmcg, as we knew that an engine was going to fail. We still carry out three-engine ferry flights, where the CAA impose a 7kt crosswind limit from the side of the U/S engine for this reason. There is no limit from the downwind side.
On a lightweight take-off on a 747, the actual Vmcg is much lower than the published figure due to the derated thrust. However, if full power is used due to an MEL item then this can be a problem.

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Old 10th May 2006, 11:22
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(a) depends on the Type .. but, in general, the problem isn't with a heavy aircraft as the V1 will be driven up well above V1min and the problems are more with optimising the climb rather than getting off the ground in one piece.

The Vmcg problem usually is for short range ferries and similar ... minimum/low weight aircraft where the option of V1min becomes feasible. IF you happen to have an aft CG and IF the runway is wet and IF there is a strong wind .. then you may be in the land of potential trouble ... unless your bet that the OTHER SIDE engine is going to fail proves to be true in the event of an OEI situation ..

(b) no problem with (a) if there is no option (short runway) and you can't delay until the wind dies down. One may choose to accept the risk ..

(c) however, and this is the reason that several of us like to see this sort of thread pop up every now and then ... if you are doing the takeoff from a very long runway and you had the option of adopting a higher speed schedule rather than (without thinking much about it) just going with the normal RTOW schedule .. then would it not be reasonable to think that one might feel a little silly during the gyrations associated with the sudden runway edge departure ?

(d) I'm sure that our departed colleague, Lou, would take issue with the reliability analysis ...

(e) .. where the CAA impose a 7kt crosswind limit from the side of the U/S engine .. .. now, that appears to me to be the height of illogic (unless you have certificated Vmcg2 data for the case ... now that would be interesting) ? Could you perhaps venture some reason as to why one might worry about mixing two unrelated situations ?

(f) .. if full power is used due to an MEL item then this can be a problem .. d'accord.

... however, as one old freight dog gnawing over a bone with another ... pass, brother ..
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Old 12th May 2006, 23:31
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About a year ago I found this Boeing doc on narrow runways, x-wind, Vmcg etc.
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Old 17th May 2006, 16:33
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Dusk2Dawn very inetresting I liked the very honest manner in which boeing Addressed that issue

Mutt and John Tullamarine oops regulatory mixup my apologies across The Pond
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