The 400ft above the runway and the 1500ft above the runway are as was said earlier simply markers to set performance values against.
They are specified in the certification requirements of FAR -23 and FAR - 25 as well as CS-23 and CS-25.
They ensure that certain standards are met.
For example a CS-25 twin engine aircraft must have a certain minimum climb gradient in the second segment. 400ft is simply the minimum height that the second segment can end at.
That prevents some manufacturer from trying to certify an aircraft that they claim has a second segment climb gradient of the minimum required but only up to 200ft!!!!!
Just like the balked landing minimum climb gardient requirement these are simply certification requirements.
Since the 400ft above the runway is a certification requirement there is no relationship to obstacle clearance which is a totally separate issue.
Edit to say that what is also tied in with the 400ft lowest end of the second segment is the 5 minute take-off power limit (10 minutes on some aircraft).
Without this 400ft yardstick, some designer could claim that an aircraft which could only manage the minimum climb gradient to 100ft and which had to level at 50ft in order to accelerate to min clean within the time limit when at max weight should be put into production!!
Regards,
DFC
Last edited by DFC; 21st June 2009 at 18:26.