Contaminated Performance Bamboozler

[Drop The Dunlops]

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.

[Zeke]

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

[Drop The Dunlops]

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

[Filip Bigün]

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

[dolly737]

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.

[mutt]

What aircraft type and certification authority?

Mutt

[Drop The Dunlops]

What aircraft type and certification authority?
Mutt

Aircraft B737-700, 20K powerplant

JAR Perf

[Drop The Dunlops]

V1 on contaminated runway...is it a 'stop' or 'go' speed? Flight manuals normally contain information that defines the V1 contaminated differently to a dry V1.

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

[Kit d'Rection KG]

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.

[swedish]

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.

[Mad (Flt) Scientist]

Aircraft B737-700, 20K powerplant
JAR Perf
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:
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?
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.

[Drop The Dunlops]

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.

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

[Drop The Dunlops]

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 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)?

[Blip]

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.

[Kit d'Rection KG]

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

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...

[Drop The Dunlops]

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.

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.

[RYR-738-JOCKEY]

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.

[Kit d'Rection KG]

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.

[FlapsOne]

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............

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.

[Brain Potter]

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

[tribo]

Useful info on these links:
http://uk.geocities.com/[email protected]/Boeing_Slippery_Runways.pdf
http://brahimtahiri.googlepages.com/Contaminated_and_Slippery_Runways.pdf
http://www.pilots.or.kr/upfile/aip/9AAAA_StoppingonSlipperyRwys2.ppt#11
http://www.dcs.gla.ac.uk/~johnson/infrastructure/slides/Tony.ppt#30
http://transport.swooplinux.org/B-narrow_rwy.pdf

[Kit d'Rection KG]

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............

For heaven's sake... Drop the Dunlops already had a go at this, and I reassured him (on page one of the thread) that I had read that line. Please, do pay attention at the back!

[rubik101]

As has been pointed out previously, all contaminated performance is 'ADVISORY' and has not been demonstrated by flight test. These numbers are arrived by mathematical models used by the manufacturers to arrive at very conservative numbers to GUIDE you in these circumastances.
As mentioned, POOR for a braking action is a non starter.
Also, many of the Nordic/Arctic have braking actions of medium and even better when the compacted snow is covered with grit.
If you take in to account all the available data and info from pilots who have taken off or landed prior to you then you should be able to arrive at a sensible set of numbers for most situations that you will meet in your career. Just don't be first in the queue for the dawn departures!

[RMC]

Kit - You need to pay attention. The reason DTD wrote his last line was to make it clear he did not intend to take off from a contaminated runway but was seeking to expand his knowledge.
The reason people are having a go is that the tone of your response was entirely inappropriate (which actually suggests you did not read the last line and are trying to save face... or you are non too bright).
There are too many people on PPRUNE who don't take the trouble to read,understand and then respond in a condescending/objectionable manner.

[Kit d'Rection KG]

The reason people are having a go is that the tone of your response was entirely inappropriate (which actually suggests you did not read the last line and are trying to save face... or you are non too bright).

My responses may have been somewhat robust, but were certainly not 'entirely inapropriate'. As to your second remark quoted above, would you say that to my face?

This is not a kindergarten - even though some here do seem to behave like children.

[RMC]

Evidently most people here have a different view ... you don't seem to realise how you come across to others (we have one or two trainers with your approach to good questions...they don't teach people very much).
My response to yours was robust and certainly not inappropriate...and if you can't take it don't dish it out.

[Kit d'Rection KG]

I'm extremely aware how I come across to others, thank you.

And as a trainer, I can assure you that I'm not of the old school. I am, however, perplexed that (a) some people ask such elementary questions here (Yours about cloud ceiling was a case in point - are you a professional pilot? Your profile says you're not), and (b) that people ask the questions here, rather than of their trainers.

And if you think that your response was not inappropriate, then one day somone is going to belt you for your impertinence.

Yes, I can take it, and yes, I do dish it out. Routinely in much better company than this.

[cwatters]

I'm somewhat surprised how long it took this thread to conclude that the runway would be too short. It was obvious to me just reading what the OP wrote and I'm only a ex glider pilot who never had to worry about this stuff.

Forgive me if I'm wrong but I get the impression that some of you might be "working the equations" without considering what the answers really mean. This suggests that a minor error in doing the sums could produce an answer thats just accepeted without question and leads to an accident.

The statement "in which case the runway is too short" should be in the SOP but it shouldn't need to be in the SOP if you get what I mean.

[Kit d'Rection KG]

cwatters, you're absolutely on the money, though not because any given runway is 'too short', rather, the dynamics involved are so complex that runway length soon becomes irrelevant, as the contaminant does its fiendish work in front of the tyres...

Not to mention the possible effects of wet snow flung up by the NLG trying to put the fires out, or the other odd stuff that goes on in wet snow, for example.

(Strictly, SOPs are not the same as performance calculations, but you're right that this is elementary stuff).