When the BCAR(UK) circa1980,first implimented the 15' screen they were the loudest in saying it was a COMPROMISE of the safest considerations at that time,for operations off a WET runway.
Safest in the fact that if one needed to stop,after the failure at Vef,then one had a Slower speed to stop from(reduced V1),but the longer acceleration,after the reduced V1,required more distance to GO.ergo the 15' screen height.
AS they say what constitues WET ,well the Airbus guys have their reference in Chapter 2.04.10(approved by the JAR),at the moment nodefinition exists in the FAA world,although Boeing sells performance 'B' to non FAA carriers.
The Certification bodies always felt that the 15'screen was at the end of the runway and it wasn't until 'Harmonization' meetings(FAA/JAR) that found out that according to the JAR rules it could be at the end of the clearway(ergo following the rules that the flight path to the screen must be over runway for 50%,one was left with crossing the end of the runway at 9' ?????).
I believe that the latest ruling by all bodies is that the 15'must be at the runways end..If this is the case then there can be little difference to crossing the end at 15'enroute to a DRY days 35' at the Clearway screen..
Facts from Airbus,On WET ops: Reverse thrust credit(if RevThrst inop ,offload payload ).....WET runway is 3mm of rain or shiny..
