Just to clear the air on a few well considered responses........
BizJetJock, Yes, I'm well aware that the JAR does require that the normal 'Dry' Takeoff Weight is the upper limit of Wet Runway considerations. You are right, it IS a geographical difference, not all countrys make that distinction in their legislation. In my Performance number crunching, I use a composite of JAR 25, FAR 25, and the Australian CAO 20.7.1B. There are various reasons for this (mainly to satisfy all of the regulatory authorities), but also as a safety issue.
In my earlier computer runs to calculate RTOWs, I encountered about 10% of cases where the Wet RTOW exceeded the Normal Dry RTOW. After much thought, I realised that 2 major safety margins are eliminated for Wet operations, thus leading to better weights with Wet figures, as
Denti and
PBL might say - "Daran habe ich gar nicht gedacht!". From that time onwards, I've used the 'composite legislation' approach.
lion-g asked "
Can somebody explain why is there a reduced SAFETY margin for wet runway ?". Normal rules apply to the dry runway condition where (i) one means of stopping the aircraft must be held in reserve. Typically, Reverse Thrust is the reserve means not considered, and it's contribution are a bonus, and (ii) The screen height is reduced from 35 ft to 15 ft.
THE LEGISLATION OBLIGES US TO FOLLOW THE NORMAL DRY RUNWAY RULES, WITH CONCESSION AGAINST THE 2 SAFETY REQUIREMENTS MENTIONED ABOVE ONLY IN THE CASE OF WET RUNWAY OPERATIONS. To do otherwise is patently
ILLEGAL.
Let's take a case where the runway is Dry, but you check Wet runway data and find that the 2 RTOW limits are exactly the same. Some cowboys will use the Wet data for an "improved" V1, but have effectively eliminated the afore-mentioned 2 safety considerations REQUIRED -
ILLEGAL! If you suffer an engine failure before V1, you have obviously improved your RTO performance even without Reverse Thrust (because of the lower V1). If the failure is recognised a micro-second after V1, your screen height will be 15 ft, which is much less than the MANDATORY 35 ft required for Dry runways.
It is illegal to use a concession outside of the parameters which allow the use of the concession. Do so at your own risk, but please not when I, or my family, are sitting down the back!!!!!!!
Regards (but not to the cowboys),
Old Smokey