Net flight path and obstacle clearance
Friendly but heated discussion between self and old Air Force colleague needs resolution. Beers rest on this. Subject was the 0.8% factor applied to the one engine inop gross gradient of 2.4% at instant of gear retraction in a twin engine turbine type (737 for example).
Colleague states that the 0.8% degradation has nothing to do with obstacle clearance requirements, but is purely a figure that allows for less than optimum operation (worn aircraft and pilot etc)
I maintain that it has everything to do with obstacle clearance otherwise why have it in the first place?
My understanding is that when calculating obstacle clearance the net flight path (2.4% less 0.8%) which is 1.6%, is used to ensure all obstacles are cleared by at least 35 feet straight ahead or 50 ft in a curving take off.
In addition for the 737 the net gradient is further reduced by another 0.60% for more than 15 degrees angle of bank for the duration of the turn. Theoretically therefore the final net gradient could be just one percent during the duration of a curved take off (we are talking one engine inop of course).
Also if the screen height is reduced to 20 feet with a reduced V1 applied to wet runway, from where does the net flight path begin for obstacle clearance purposes - because normally the calculations start from the end of the first segment don't they (35 feet)