B737 contaminated take-offs

[elcol]

Hi Guys
Looking for some help here on B737 contaminated take-off's and V1/VMCG.
I would very much appreciate it if you could tell me what it says in your performance manual if you find that in the calculation V1 is less than VMCG.
The CAP 698 used in the training world (which uses 737 data) now states that if V1 is less than VMCG take-off is not permitted which I believe contradicts reality and the AMC.
Your help is much appreciated.

[john_tullamarine]

Consider that, if V1 is marginally less than the (real world on the day) Vmcg, then you might have a control problem sufficient to

(a) give you a good fright (if you're lucky), or

(b) roll the aircraft into a smoking ball in the grass off to the side of the runway (if you're unlucky and/or not highly skilled in the tap-dancing required with a failure in the near Vmcg region

If you are a few knots slower .. my money is on (b).

Which leads to one of the advantages which can be arranged by using derate.

Not too sure which reality your concern relates to .. but the above is the real world reality, I'm afraid.

Your call.

[B737900er]

I thought you can make the vmcg and v1 the same speed if the v1 is lower than vmcg?

[john_tullamarine]

Providing you have enough runway distance.

This is all complicated by slush drag considerations for contaminated calculations .. so significant contamination realistically can't be dealt with in the same idealistic way as a dry runway situation. Accel stop can get very messy ..

[despegue]

Let us not forget that an assumed temp. Is not allowed, and in this case, your tables should at no point give a V1 lower than V mcg. If still the case, this is a no go as the performance is inadequate for ake off.

One more thing, on contaminated runways, it sometimes is not so much the stopping distance that is the most limiting actor in v1 calculations, but rather the one engine acceleration disance between V1 and Vr due to the increased of drag. This is particulary true for slush covered runways.

When braking action is considered good and runway is not contaminated ( the 25% and 3mm. Definition), then Wet figures can be used in a lot of companies.

[john_tullamarine]

sometimes is not so much the stopping distance that is the most limiting actor in v1 calculations

Indeed.

For low levels of contaminant, ASD might be the worry due to reduced braking capabilities while TOD isn't affected all that much.

As the degree of contamination increases, the bow wave slush drag assists stopping but hinders going. Eventually a point is reached where the slipping problems for stopping are overcome by the drag benefits .. but the downside is that eventually the wheel drag overcomes the OEI thrust component and it all gets just a bit too difficult to contemplate.

My take ? If there's much in the way of stuff on the runway, time to consider coming back some time later and having a look again then ..

And the whole lot is further complicated by the variability of the specific slush ... talk about commercial interests competing with rational engineering and pilot command decision making ..

[Sciolistes]

Try a lower fixed derate from the N1 LIMIT page. This will reduce Vmcg.

[elcol]

Thanks.
What I am specifically looking for is what it says in the 737 (variant) performance manual.
In the old CAP698 it used to say "If adjusted V1 is less than V1(MCG), set V1 = V1(MCG).
In the new CAP it states "If adjusted V1 is less than VMCG take off is not permitted"
I think the latter statement is incorrect so any quotes from performance manuals will be much appreciated.

[BOAC]

Try a lower fixed derate from the N1 LIMIT page. This will reduce Vmcg. - and what does that do for V1?

My (old) perf manual conveniently ignores the problem in the 'text' (while the Boeing QRP says adjust V1 to Vcmg.) but allows for V1 to be raised to Vcmg when operating 'outside the tables' (?uncontam?).

We are getting back to the old days, I fear, of Vgo=Vcmg and Vstop=V1 and in between, well, you hope it doesn't happen:)

I personally think 698 is correct - ie 'don't go (routine) flying'. Maybe a question to your ops department, elcol? They need to take account of 698 in their manuals.

[elcol]

Okay the answers been found thanks to "Imbraceablecrunk"
On the Classic if V1 was found to be less than VMCG then V1 was increased to VMCG but on the NG take-off is prohibited.
The reference is Boeings Jet Transport Performance Methods:
http://www.theairlinepilots.com/forumarchive/quickref/jettransportperformancemethods.pdf

Thanks to all

[RMC]

Elcol was this a real situation? I have only ever had this question come up when ferrying an empty aircraft? Last time I used contaminated figures I believe the V1 reduction was around 15 knots but still a significant gap between V1 and Vmcg at that weight.

[BOAC]

elcol - I have not perused the entire pdf you linked, but I cannot see any distinction between the Classic and the NG deriving from it. I have obviously missed something, so could IC or you perhaps amplify what he thinks? The change from Vmcg to V1mcg has been incorporated in the Classic QRP as far as I can see, and still no mention of NOT adjusting V1 upwards for contam. It is more than possible that company OMs still refer to 'Vmcg' for Boeings, and of course this particular specific figure can no longer be found in the QRP.

[elcol]

Maybe I'm getting my knickers in a twist!
Being a "has been" in the training world (with limited access to manuals) I've been looking at CAP698 ,1999 edition, and CAP698 2006 edition, contaminated take-offs. In the former on a contaminated take-off if V1 was less than VMCG then it would be increased to V1=VMCG. The later CAP prohibits the take-off. I've been trying to establish the reason why hence my original post asking what was stated in current performance manuals. I think the description given in my link on minimum control speed considerations page 20-27 confirms why the later CAP is correct.
RHC- not a real situation thankfully.
BOAC. If I've got it wrong please say so. Happy to IC if you prefer.

[BOAC]

elcol - as per post #9 I agree with you. I would like to know what you/IC have found in the Boeing doc that supports 698. In your post #10 you said:

"Okay the answers been found thanks to "Imbraceablecrunk"
On the Classic if V1 was found to be less than VMCG then V1 was increased to VMCG but on the NG take-off is prohibited."

It is the bits I have bolded I am seeking info on.

[Denti]

Try a lower fixed derate from the N1 LIMIT page. This will reduce Vmcg.
- and what does that do for V1?


Lower thrust, therefore lower VMCg and a lower V1 becomes usable. Double figure V1s are achievable that way pretty easily.

[ImbracableCrunk]

I don't think the NG (or CL) prohibits takeoff with V1<Vmcg. All of my performance manuals say the same about increasing V1 to Vmcg.

My thought is that maybe the CAP changed with the new definitions/certification and they neglected to mention that, "You can't take off with V1<Vmcg but you can just increase it to Vmcg."

It's kinda like saying "You can't take off with snow and ice adhering to the wings" and forgetting the bit about "but you can de-ice."

[737aviator]

In my Ops manual, 737-800, we are permitted to increase the 'adjusted V1' to equal V1MCG in a situation where V1 is lower. However as has been said, I'd struggle to find a situation this is necessary in normal ops as VMCG is around 100kts and roughly the max correction to the RTOW V1 is 20kts.

[elcol]

BOAC: I think I was too hasty in making that statement given 737aviators post. I suspect that the early CAP was based on the first description of V1 for the earlier airplanes and the later CAP based upon the second V1 description for later airplanes. This is from chapter 17. What do you think? (its not copied over very well)
minimum V1 -the earlier airplanes



V1 has a minimum allowable value. That minimum depends on the airplane model, as the regulatory requirements changed in this regard some years ago.

For earlier airplanes, the V1speed was never allowed to be less than the ground minimum controlspeed. The intent here was to ensure that a continued takeoff would never be attempted from a
speed less than the speed at which adequate directional control is assured with an engine inoperative.

minimum V1-the later airplanes




For later airplanes, the rule is a bit different. For these airplanes, the engine failure speed is not
allowed to be less than the ground minimum control speed
. That’s different, isn’t it? Remember
that V1
is, by definition, the speed occurring one second after engine failure. If the engine failure
must not occur at less than the ground minimum control speed, then the V1
for these later airplanes may not be less than the ground minimum control speed plus the increment of speed gained during the one second interval between
VEF and V1. That new speed, one second of acceleration
faster than
VMCG has the name V1MCG and it is the minimum allowable value of V1 on
the later airplanes.

[BOAC]

Again I agree. The only QRP I have access to is dated 2004 and that permits adjustment of V1 toV1mcg for contam.

[BOAC]

Again I agree. The only QRP I have access to is dated 2004 and that permits adjustment of V1 to V1mcg for contam. As we know, the chances of being in this position are low.