Thanks for all the great answers.
Call 100: I've never tried the wet trouser test yet!
It seems that the ICAO 'Water patches' is equivalent to 'contaminated' in Jar-ops and presumably under EU -Ops also, which is >3mm water over 25% of RW (or by slush or looses snow equivalent to >3mm, or compacted snow or wet ice) (Paraphrased)
'Wet' could mean up to 3mm of water, which would cause quite a big risk of aquaplaning. This is probably why we need an extra 15% of LDA.
Under EU-Ops, the LDR may not be more than 60% of the LDA for a dry RW. If you turn this around. The LDA need to be at least 1.66 of LDR. But for 'wet' it is 1.92. This extra factor for 'wet' means that a fair number of a/c will not be able to land (or t/o) in 'wet' conditions at say LCY.
For EU-Ops, 'damp' performance figures are the same as 'dry'. Which is why we all love grooved runways which can be declared no worse than damp in all but heavy rain. (as long as the RW 'does not appear reflective'!)
Practically speaking, the stopping distance on a runway with up to 3mm of water compared with that on a grooved runway which is draining effectively is bound to be much greater. (with anti-skid in both cases).This is could be the difference between 'wet' and 'damp'.
With FOCOM 3/2009 operators are no longer allowed to assume a grooved RW to be 'damp' (and use 'dry' performance) if it has been declared 'wet'.
To conclude, it will help us all out (Operaters, ATC, & airport authorities) if runways are declared no worse than 'damp' unless they, as they say in the good book, 'appear reflective', otherwise declare them 'damp'.
