John,
If that's Milt, I'll take up the argument with him in Canberra next week ....
(a) gross is determined from flight test degraded to fleet average engine performance .. I would have to dig into the archives to get the delta used ... just can't recall it now.. a long time since I looked at that side of the table ..
(b) gross to net is to "cover" all the odd bits and pieces .. pilot variation, etc., which can be expected to compromise the gross assumption .. but beware turbulence ... although I have no idea whence he got it Roger G at Ansett used to quote a notional 1 in a quarter million probability of achieving net.
(c) gross to net delta applies throughout the takeoff from the end of the first segment .. with a corresponding reduction in acceleration capability for the third segment
(d) gross to net not specifically related to obstacle clearance but the net capability in the AFM is used in constructing the NFP profile which is then overlaid on the obstacle profile to juggle RTOW
(e) note that the certification 2.4/1.6 is a minimum WAT limit for maximum weight departures and may need to be increased to achieve whatever obstacle clearance is necessary for a given departure analysis.
(f) while the calculated clearance is of great interest in the early departure, by the time we get to the fourth segment, the gross to net delta has fed in a big margin of fat .. depending on what the aircraft is actually achieving.
(g) the delta for any given bank angle can be calculated simply but needs some knowledge of the aircraft drag characteristics to get a reasonable answer. 0.6, or thereabouts, appears in a number of AFMs. Just to convince myself that I don't have Alzheimers the DC9-33 manual quoted 0.57 ( .. or was that the price of a loaf of bread ... just can't quite remember ..)
(h) the obstacle clearance calcs start from the end of the TODA, while the gross to net delta starts from the end of the first segment. Although not usual to see, there is no reason why the first segment can't extend beyond TODA .. in which case, the first bit of the obstacle clearance calcs are based on the first segment capability. However, the first segment performance normally is so abysmal that it is often a toss-up whether one is better to pull the weight back to bring the first segment into the TODA.
... now waiting while Mutt and Old Smokey find the deliberate errors above ..
Last edited by john_tullamarine; 31st Oct 2004 at 22:25.