Contaminated Performance Bamboozler
 

Anyone out there who can clarify this for me?...

Have been looking over Contam Perf the last couple of days in preparation for the winter.

My query is this:

When calculating a contaminated V1, my Company Ops Manual states that if the calculated V1 is less than V1mcg, then you should set V1=V1mcg.

But surely then you are above the speed at which you are guaranteed to stop within the remaining distance if you decide to reject. For example, on the 737-700 at about 60 tons, using QRH poor braking action slippery perf, the V1 reduction is about 31knots, resulting in a typical V1(contam) of about 97 knots. V1mcg is typically 103 knots, so at 97 knots you are too slow to guarantee control if you decide to continue the take-off. This why we are asked to raise V1(contam) to 103 knots.

However, this extra 6 knots must surely put you at severe risk of an overrun.

Would it not be more prudent to reduce your aircraft weight until V1(contam) raises a few knots to equal V1(mcg), therefore guranteeing that at V1 the decision you have made will ensure a safe stop, AND a safe take-off???

Any clarification would be much appreciated from performance boffs...

Cheers,

DTD.

PS, for the critics: No, I would not consider taking off from a poor braking action surface, and yes, I would rather wait for it to clear. Just a query, that's all.

If you were to use a derate (not assumed temp), the Vmcg would reduce.

True, but my query is:

Is is safe / correct to raise a V1(contam) to equal V1mcg, therefore leaving you above the speed that would guarantee a stop before the end of the ASDA?

This is what my Company manual states to do. Anyone help?

DTD

Very intresting topic indeed. Did you try to consult with 1 of your top captains? I'm gonna review the perf part my self and also ask the captains together with the performance guy within my company. Will get back to you soon.
cheers and happyXmas.


Filip

If I remember correctly there is a „V1(MCG) Limit Weight“ table, where you calculate the max „slippery runway limited weight“ from field length, braking action and press.alt.

What aircraft type and certification authority?

Mutt

Aircraft B737-700, 20K powerplant

JAR Perf

Can't Find anything relating to that in Company OM's but will check Boeing's 737 FCTM.

Given that AIC 15/2006 states:

Operations from contaminated runways, by all classes of aeroplane, should be avoided whenever possible.

and

aeroplane performance relative to a particular contaminated runway cannot be scheduled with a high degree of accuracy

and

the use of a contaminated runway should be avoided if at all possible.

May I ask why you would want to take off from a contaminated runway? If you do take off, and an accident ensues, there will be an enormous amount of evidence that you have done the wrong thing, and your only response will probably be that it was commercially expedient.

So that would be wait for summer the the nothern Nordic regions! Biggest problem in my experiance is with crews flying in these conditions without both specific training and a great deal of experiance of extream wx ops.

hmm
Given that Boeing grandfathered a bunch of stuff on the later 737s, I wonder if their performance data is also grandfathered. Data on other than dry runways used to be optional, rather than required for certification. In which case there will be no specific rules about scheduling V1 ...
To answer the original question:
It isn't as "safe" as your normal operations on dry, in that indeed, you are not guaranteed to be able to stop in the case of an engine failure between your V1stop and the V1 you're actually using. It would indeed be prudent to reduce the weight a bit to buy back that margin.

But, there's no hard line in the sand for much of certified performance - no matter how much we kid ourselves by quoting distances to the foot and scheduled speeds to the tenth of a knot! (I've seen both in worked examples in flight manuals, and it's frankly farcical). So in fact it is "safe" because we don't have to dig a bunch of airliners out of overrun areas every spring. By which I mean that operational experience indicates that in practice the reduction in safety margins compared to the "dry" case is justified by the absence of an effect on accident rates.

Its exactly the same logic as for reduced wet screen heights, allowed use of all braking devices on wet runways, assumption of zero x-wind for Vmcg, and so on.

Thanks for that KG, but in future try reading ALL of the original post, particularly the last line...

Thanks MFS.

I'd agree with you, as I think the safety margins probably would be very conservative.

However I would be interested to see how I would stand legally if there WAS an overrun. Have I misinterpreted the rules - when the OM tells me to set V1=Vmcg, is that actually telling me to increase the t/o weight to raise the V1(contam) until it equals Vmcg, or just use Vmcg INSTEAD of V1(contam)?

Here's an excerp from our B737-800 manual. There are some days when you're simply not meant to fly.

SLUSH/STANDING WATER TAKEOFF - CFM56-7B26
GUIDANCE INFORMATION ONLY
(DO NOT TAKEOFF IF DEPTH EXCEEDS 13 mm)

To determine maximum takeoff weight and associated V1 (for a given runway) proceed as follows:-
(1) Find performance limit weight from specific takeoff table or generalised data and obtain
associated V1 for limit weight.
(2) Enter chart below with weight obtained in step 1, move vertically up to intercept the diagonal line
then move horizontally to the left to obtain contaminated runway weight limitation.
(3) Reduce V1 obtained in step 1, by V1 adjustment for contaminated depth from Table A below. Obtain
VR and V2 for actual takeoff weight from the same data source to obtain limit V1. Use adjusted V1 for takeoff unless adjusted V1 is greater than VR. If so set V1 equal to VR.

NOTE: If full V1 adjustment cannot be achieved (due min V constraints) then runway is too short.
Check Table B for a guide to minimum acceptable runway length for use with contaminated runways.

Thank you; I had done so. My point stands.

And for those who choose to operate from compacted ice and the like, I wish you luck, and hope that you never find yourself sitting off the end of the strip with the dawning realisation that a library full of books are heading your way...

Thanks Blip.

I think we are on to a winner.

My Company OM makes no such statement, but the note stated above suggests to me that it would be correct to NOT schedule a take-off if contaminated V1 is less than Vmcg.

Therefore as you pointed out you need a longer runway, or an increase in takeoff weight (iINCREASING take-off weight DECREASES the V1 reduction of course, contrary to my earlier incorrect statement).

And yes, as KG correctly stands by his point, we should not be operating from contaminated runways, and should do so at our own risk... However, my point still stands that if a confusing element arises from our Company OM's then it should be addressed, irrespective of whether you would personally opt to use it.


Thanks, any further useful info would be much appreciated.

DTD.

My personal view here is that the main problem is that many people are not very confident when it comes to cont. rwy calculations or adverse wx ops in general. I mean, how can you ask a question like " May I ask why you would want to take off from a contaminated runway?" How do you think aviation works north of 60 degrees latitude? It may take you an extra five minutes looking up the details in the manuals, and I have come across alot of captains who try to expedite the process by trying to "simplify" the situation based on previous experience. But the tables are there for a reason, to give us specific limitations and also to guide us. Of course there are cases where the only sane option is not to operate, like the V1 limited by Vmcg thing, but come on....3mm of slush/standing water on a 10000ft rwy is not necessarily a dangerous thing...if you open up the tables and get the correct figures.

Being a little more pragmatic...

If you absolutely insist, contrary to all the published advice, on operating on contaminated runways then...

...your company should have provided you with lots of training, and left you in no doubt whatsoever about any element of the proposed operation, and

...you really should make sure that everything is as much in your favour as possible.

KG you clearly hadn't read the last line of Dunlop's post otherwise you surely wouldn't have written what you did.

Here it is again............

Drop the Dunlops,
The key to contaminated performance is an understanding of the range of available V1s. Most V1s given by a decrement from a table are produced for simplicity, using a mid-range V1. Unless at limiting weight, there will be a minimum and a maximum possible V1; calculating and choosing the most appropiate is the nub of the issue. If the contaminent is such that there is an acceleration problem (slush) then V1=Vr is the safest conditon - ie you are not trying to accelerate through the contaminent with an engine out. Conversely, a stopping problem (ice) requires a (properly calculated) low V1, but this must not be lower than Vmcg/Vmca. In your case you don't have to actually increase the weight - just use a V1 equal to or less than the V1 for the max weight achievable in the conditions, provided it is above Vmcg.
A simple technique to solve your problem may be:
1. Calculate the maximum T/O weight for the contaminated runway and then calculate the associated V1.
2. Compare the speeds given by your (lighter) actual T/O weight to those for the max weight for the conditons.
3. If required you can enhance your V1 up to that for the max weight for the conditions in order to achive V1>Vmcg. If the max V1 is higher than your intended Vr then you can use a V1/Vr of unity.
My company uses this technique to produce a single Wet/Dry V1, rather than uneccesarily sacrificing screen height by having a lower V1 than necessary in the wet.
Do not use a V1 lower than Vmcg - this would be dangerous.
I would not use less than full power for a contaminated take-off.
Regards