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It is brilliant because you are effectively determining the height above the runway to commence the flare without having to actually think about or try to estimate what the aircraft's height actually is! Instead it utilises basic triginometry ie "the branch of mathematics dealing with the relations of the sides and angles of triangles and with the relevant functions of any angles." so that you are only using horizontal distances along the runway instead. As a result, because the approach angle is 3 degress or 20:1, if you can discern a distance along the runway of 20 feet (6.1 metres) (about half the length of a single centreline marking) you can discern a flare height to within 1 ft!!!
Get it???
misd-agin - so at 120-140 kts we´re supposed to pick out a 20´ difference? Mind you we have to have this point identified 1, 2, or perhaps 3 thousand feet away when you compute the slant angle...
Now admittedly there is around 100 ft discrepancy between this distance and the first one. But if you divide 100 ft by 20, that is the actual discrepancy in terms of height. It's just 5 feet. At 750 ft/min that equates to 0.4 of a second! Split the difference if you like.
misd-agin - Only 5'? Dón´t know about you but a landing height misjudged by 5', or even 2' feet, is not one I would be proud of. And you´re sink rate at touchdown is much lower so it´s not .4 of a second. Flare typically starts around 20', or slightly higher, but takes several seconds to complete.
This theory ignores sink rates and the actual arrival weight of the arriving aircraft. Landing an airliner at light weights typically needs a later and or slower flare vs. landing one at max gross landing weight. That´s for a normal approach. Have a 'sinker' like the FO had this afternoon and your theory would have left him humbled.