Seminole27p,
An easier way to look at it is that the FAA required that the demonstrated length be 60% of the published length, ie: the buffer or fudge factor was 40%.
The first time I worked with these numbers, they were in SFAR 422B, which preceded FAR 25.
For may years, the 15% (on the 100%, not the demonstrated length) was a recommendation for a wet runway, ie: Captain's discretion. I won't even try and go into the details of "wet runway" definitions, but CASA want to make 15% mandatory if it is raining --- ie: no recognition of the fact that all our major airport runways are grooved, or have a porous overlay --- ie: the equivalent of a dry runway, as long as there is no pooling of flooding.
In a wonderful demonstration of in-depth knowledge of these numbers, and involving a small Cessna jet, a "well knows civil aviation safety authority" demanded the Cessna published landing lengths (already factored) be factored again (in the "accepted operations manual") by a second 1.67, and the result be further factored by 15% for a wet runway.
This resulted in the farcical situation that the only runway that this little aeroplane could use, in the wet, in the state of NSW, Australia was RW 16R/34L at YSSY, and there were precious few it could use in the dry.
It took quite some time to get the message across that this approach was just plain wrong.
Last edited by LeadSled; 24th Jul 2013 at 02:37.