Just to make sure we're talking the same terms here:
"Contamination" or "clutter" is any measure of standing water, slush, dry or wet snow that impedes acceleration.
"Wet" or "slippery" is degraded stopping ability.
1" or 3" (or more) of HARD packed snow (or ice), will not slow acceleration, therefore no "clutter" or "contamination" penalty is made. The same holds true for "wet" or "slippery" runways with NO measure of clutter.
The "wet" 15 foot screen height model is only for a/c that come under the new Part 25 rules for about the last 3 years. And...that was changed to allow new models of two engined a/c to take off at heavier weights. (Symmetrical reverse doesn't help much on a 777.) All it does, in effect, is lower the V1 to get it stopped in case of a "slippery" abort. This lower V1 will require a greater distance between (the lower) V1 and (the same) Vr which in turn will put you lower over the screen. Any a/c certified under SR422A, SR422B, and many under Part 25 over the following 38 years do not consider the degraded braking for RTOs.
(There is nothing, however, from preventing your carrier from instituting more realistic performance data, and some do.)
mutt-
Perhaps the above is the case at your carrier. Or, perhaps it's unique to your model a/c, which I haven't flown.
You said: >>If you take a look at some charts (particularly the B747) you will see that your V1=VMCG weights will actually increase as the depth of contamination increases, <<
I have in my office, (and I looked at all of them today), the performance sections for 767, DC-9, and DC-8 from many different major airlines including SAS, UAL, NWA, JAL, SwissAir, etc. NONE of them had an adjustment for V1 in clutter. Granted, none of them were 747 manuals. I'm not saying that some don't, but it's certainly not universal. (And for what it's worth, I've never heard of that before.)
Also, you said: >>the contaminant builds up in front of the wheel during the stop, this ploughing action assists you with stopping, hence the ability to take a greater weight.<<
You're not suggesting that you can takeoff at heavier weights on slush covered runways than you can on a dry runway, are you?
Also, regarding your above statement: Once the throttles are at idle, spoilers are deployed, and you get on the brakes, the a/c really doesn't care if it's being asked to stop from 130 kts. after an abort, or from 130 kts. after a landing. If the "ploughing" effect will help it stop after an abort, it will also help it to stop after a landing - meaning you can stop it in a shorter distance (in either case) with contamination than you can with a dry runway.........I don't think so.
Is that what you meant? Perhaps I read it wrong.
[This message has been edited by quid (edited 05 January 2001).]