Originally Posted by CliveL
To simplify things without introducing too much conservatism they decided to approximate the curved path by two straight lines and guess what? the convenient break point was 400 ft.
To do their statistical analysis, they assumed an average takeoff technique.
The Average Take-off Technique assumed is not different in principle from the technique assumed in the previous ICAO code. (...) The climb is assumed to be made in straight flight up to a height of 400 ft at the Take-off Safety Speed; then turns, accelerations and changes of configuration are allowed; however, the possibility of turns being made before the height of 400 feet is reached is envisaged ...
EDIT:
In the SCOP Proposed Standards (Part IV of the report) the 'nettage' deduction from the gross path is 0.65% below 400 ft and 0.95% above 400 ft (for twins at 2*10^-6 IP). The 'administrative feasibility' is that the height chosen for the nettage change corresponds to the minimum acceleration height in the assumed Average Take-off Technique. (If one overlooks that 400 ft net has become 400 ft gross in Part IV).
The concept of variable nettage was not adopted in current regulation. In 1957 the US CAA/CAB published Special Civil Air Regulation No. SR-422 which specified that the gross FP must clear obstacles with a vertical margin of 35 ft plus 1% of the distance travelled from end of TODR.