I'm enjoying the discussion; almost something for Tech Log.
The mainstream of discussion is going down the path of considering performance on a very wet or flooded runway, and the side issue of grooving, which is the most logical path for the discussion to take with the information available so far.
At the same time, let's keep minimal texture and shallow surface water in the back of our minds as a possibility until we know the macrotexture and microtexture of the runway.
Most runways have reasonable macrotexture and microtexture and performance calculations assume those conditions being present. But it is possible to have a runway where the macrotexture or microtexture have been effectively destroyed by rubber build-up or by inappropriate maintenance or rehabilitation (trust me – I've seen this with my own eyes twice this year
). Under those circumstances, even a small amount of surface water can significantly affect the braking performance, and we can experience the braking problems that are normally associated with very wet or flooded conditions. One of safetypee's earlier quotes touches on this:
not provide an adequate safety margin for landings on wet runways, particularly those with low texture or rubber contamination.
Because 9 times out of 10, overruns in the wet are associated with lots of rain and with poor macrotexture or lack of grooving, I've been guilty of sloppily thinking in the past that overruns always go with these conditions. Clearly I am a dynamic/viscous hydroplaning or aquaplaning sort of person – i.e. occurring when an aircraft lands fast enough on a wet runway.
I was therefore surprised recently looking at an overrun accident which occurred in only light rain. It wasn't until I measured the minimal macrotexture and virtually nil microtexture of the runway that I remembered reverted rubber skidding - akin to viscous skidding in that it occurs with a thin film of water and a smooth runway surface.
Not saying for a moment that this happened here at Moncton – we simply haven't got the data to judge yet. But while I'm enjoying the discussions here about water great than 3mm and the rest of it, I'll still keep in mind the danger represented by light rain and smooth surface.