Fair comments Mr Levitator, I feel that if this thread continues much longer that it will boil down to an argument of when does Damp become Wet.
For a runway that's only Damp, or has a few puddles, I'd suggest that only minor reduction in acceleration and stopping performance occurs, and you're better going with Dry Runway data, in that way you'll have Reverse Thrust as an additional and bonus retardation method, in other words, you have a bit in reserve. With Wet Runway data, you have expended ALL of your reserves, if you're at Accelerate-Stop limited MTOW, and the wind is 1 knot less favourable than as planned, then an overrun is on the cards. The only bonus in hand is - Can you initiate the RTO in a lesser time after Vef than the 1 or 2 seconds assumed?
Having said that, I cannot argue more strongly than to say that if the runway is TRULY wet, then you must use the Wet Runway data, as, even though all of your reserves of performance have been used, the data considers the significant degradation in acceleration to Vef, and the stopping performance thereafter.
There's nothing wrong with a 15' screen height if all goes as planned, but consider again that 1 or 2 knot less favourable wind than planned, if it knocks 10 to 15 feet off the standard 35 feet, then we all live another day, but knock 10 to 15 feet off 15 feet, and it's another story.
There is the bonus factor that only half of the Headwind and 1.5 times the Tailwind is considered to acommodate unforseen wind variations, true, but for a zero wind Takeoff, there's no factor whatsoever.
It seems as though Bananasbananas and I were simultaneously posting, and the factors that he mentions are very valid. I've done it for real many times with real failure at Vef during certification flying, and the clearance at the screen height is very very scary.
Regards,
Old Smokey