A little bit pregnant...
I always get a bit tickled with the differences between JAA and FAA. One which comes to mind is that of protected glide slope signals. In the U.S., either you're screwing up the signal, or you're not. Under JAA, you have protected areas for CAT I, CAT II, CAT III.
Same with the "calculations" for takeoff performance. Under FAA, the runway is either dry, or it's not. There is no 'damp' in the eyes of FAA. (Again, a little bit pregnant.)
In terms of actual airplane takeoff perfomance, there is no way the plane will perform better on a wet runway vs. a dry runway. That's the reality. But, as we say, figures lie, and liars figure. It's all in the calculations.
Depending on what system you use for doing your RTOW charts, it IS possible to have 'better performance' on a 'wet' chart vs. a 'dry' chart. And, there's the provision of using reverse thrust on a wet runway for stopping...you don't have this on a dry runway. And, the screen height is 35 feet for dry...15 feet for wet....and the list goes on.
In the event you have a performance calculation system that, sometimes, will show a higher allowable RTOW under wet conditions, you MUST (as per the authority of granting permission to utilize such a performance system) take the more conservative of the two. That is to say, if you can go to 100T dry, but 110T wet...and it's wet...you are limited to 100T.
Some systems (most of what I've seen used in the U.S.) already take into account for this, thus relieving the crews of having to compare the two figures (wet and dry) and taking the more conservative of the two.
My company uses a computerized system ('rented' from Sabre) that is outstanding. It does everything for you. Little chance for error. The FAA loves such a system.
But, FAA approves a lot of crazy stuff. For example, one carrier in the U.S. (It will remain nameless.), has the first officer with a clip board and a calculator glued to that clipboard...and he does it all by hand. Not much better than doing a takeoff performance calculation for your Cessna 172. (I've seen private pilots who fly single-engine airplanes have very sophisticated weight-n-balance and takeoff/landing performance programs in these little programable calculators....)
Of course, over 30 years ago, when I flew co-pilot on a 727...the captain, an ex-WW II guy who flew the Hump)...used to tell me, "You don't need all that fancy paperwork...you push the thrust levers up...and when she feels like she's ready to fly, you ease back on the yoke and fly the damn thing. It ain't rocket science!"
Ah, as crazy as it sounds, I used to love to fly with those guys....
PantLoad