Hey TCASfail,
short post for clarification:
first of all, it is IMPOSSIBLE that under given conditions the required landing distance on a dry RWY is LONGER than for a wet RWY!
This is because of the additional safety margin of 15% on a wet RWY compared to a dry RWY!
The required landing distance is derived/calculated from several flown approaches from testpilots (50ft passing Threshold, Vref, TD at 1000' etc).
This idealistic (and unrealistic) ACTUAL Landing Distance is multiplied by 1,67 to get the REQUIRED Landing Distance DRY RWY!
If you mulitply this lengths by 15%, you get the REQUIRED Landing Distance WET RWY, which therefore equals to ACTUAL Landing Distance X 1,92!
Nice to know, that in general the landing performance on a wet RWY requires more than the 15% safety margin, therefore the ACTUAL safety margin reduces compard to DRY conditions!
Landing on a contaminated RWY:
The REQUIRED Landing Distance on a Contaminated RWY is the longer of the
-REQUIRED Landing Distance WET RWY or
-ACTUAL Landing Distance CONTAMINATED RWY x 1,15
If the second one is the limiting, your safety margin reduces to 15% compared to the ACTUAL Landing Distance. By the way, the data for actual landing distance on cont. RWY may be based on calculations - it has never been demonstrated on a real aircraft under these conditions!
Under normal conditions (no emergency) the Landing Distance Available MUST NOT be less than the REQUIRED Landing Distance! This is valid for planning as well as for the actual landing. It must be assured at all landings that legal requirements are met and Landing Distance Available is sufficient for your conditions!
Things are different in an emergency situation! For the 737 for example, the QRD gives you information for the ACTUAL Landing distance for different malfunctions (One Engine Out, Flapless Landings or Dual Hydraulic Failure etc.). Corrective values are given for different pressure altitude or wet RWYs. But you should keep in mind that these are again ACTUAL Distances. If this distance is equal to the Landing Distance Available, you have NO safety margin at all!
Cheers
Flexi