BOAC, JT, et al. Old, but not a really an old dinosaur …?
Certified landing performance does not include any margin.
Operational landing performance does have a margin, but its ‘use’ unfortunately has many varied views.
When and where the margin came from is far from clear. The matter was researched recently by the ICAO working group on runway friction without a definitive conclusion; strong contenders are RAE runway surface research (1960s -), wet runway braking and early FAA tests.
There is an interesting view associated with proportionality from innate human judgement; i.e. a one-to-one fight with an adversary should be not considered, whereas two-to-one looks much better. As this sequence is developed, the 1.67 margin looks about right!
However I digress; according to the UK CAA (defunct AIC ?), factored performance provides a safety margin for the normal variations in daily operations (normal not defined), but not predetermined excesses. Furthermore, while the margin might accommodate a speed up to Vref+15, it will not compensate for high speed / TCH or long touchdown simultaneously (this can be evaluated with modern rules of thumb, e.g. AC 91-79).
Overall the message is Vref+5 at TCH, on speed, and good TD position.
The margin is for human variability, not pre-planned deviation. We should not kid ourselves that we can control our variability.
Info:
http://www.nlr-atsi.nl/downloads/ana...ssues-rega.pdf
http://www.nlr-atsi.nl/downloads/saf...s-in-cross.pdf
http://www.nlr-atsi.nl/downloads/cro...affect-you.pdf