The 60% and 70% rules are "lines in the sand", along with numerous other "lines in the sand" applying to operations. Why 35ft screen height, and not 30 or 40 feet, or even 10 Metres in our increasingly Metric world.
The 60% rule is a good one to account for the differences between standard procedures for normal (and non-normal) operations by ordinary pilots, versus the techniques used by test pilots in evaluating landing distance. Consider the following sample -
(1) The test pilot approaches the runway crossing the threshold at 50 feet and at Vref. We mortals stick to the 50 ft rule, but carry a minimum of 5Kt additive, or half the wind component if greater. Thus the landing speed will be greater.
(2) The test pilots chops the thrust as he/she crosses the threshold at 50 ft. My gawd!
, would we do that? (Typically we mere mortals slowly bring the thrust to idle during the landing flare).
(3) The test pilot aims for the 300M / 1000 ft touchdown point (so do we), but then "plants" the aircraft spot on the markers. We aim for it, and then flare at the appropriate moment, which by all of the rules of physics means that we MUST touch down beyond the 300M / 1000 ft touchdown point.
(4) The test pilot then applys
MAXIMUM Braking (the same as in a Rejected Takeoff) to bring the aircraft to a full stop on the runway. Do we want to do that on an every landing basis
I think not, the passengers, engineers, and the accountants will not be impressed.
SO - We land faster, keep approach thrust longer, land deeper, and routinely use much less braking than the test pilot, and we normally don't come to a full stop on the runway. We call it normal operations, and the regulatory authorities add a nice buffer to allow for it.
A Line in the Sand? -
YES, but a very good one at that.
Regards,
Old Smokey