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Answer
Hi ,
This is a classic climb gradient example. APG does not calculate a climb gradient solution. They could careless what the required climb gradient is. Notice it is leveling you off at 1620 ft (thus you have yet to clear the 2500 requirement). You say there is no special procedure. Are you sure it isn't requiring a hold? They almost always do since their analysis only goes up to 1500ft (the top of the Net takeoff flight path). If no hold is required, its because the climb requirement is due to noise abatement.
EFB-Pro keeps you above the Net Gradient all the way to the top (into the enroute structure, not a holding pattern). Our path truly is the same procedure as the listed DP.
Now, for how the 92429lbs is calculated. Go to Figure 6-4-8. This gives us the ref gradient that feeds the other charts. The top of the procedure is 2500ft. But how far downrange is that? Take the 2500ft and divide by the ft/nm (3.3% is 200ft/nm) . So the DP top is 12.5 NM or 76000ft downrange.
However we are not interested in clearing the gross gradient (2500ft) but the net gradient (3.3 - .8%) which is 48ft/nm less or in this case 600 ft. 2500ft-600 is 1900 ft.
So go into the chart at 76000ft up to 1935 ft. (We want to clear the "obstacle" by 35 ft, I am doing some rounding here which the calculator doesn't do). You will see that is around 2.5% REF gradient.
Now go to Figure 6-04-04 (sheet 2) and work your way backwards. (Our gross level-off is 2500 roughly) so the ref gradient goes up to 3%. Now go to sheet 1 and you will see the 2nd segment limiting weight is between 92000 and 93000 for the temp/pressure altitude. As you can see, EFB-Pro produces 92429 lbs.
APG (actually none of the runway analysis folks) do this kind of calculation. They simply take you to 1500 ft in 2nd segment, which is very close to the max wgt climb (WAT) value, in this case MGTOW (or inlimited as you say) and place you in a hold. I consider that more of a "VFR conditions" procedure.
The confusion to fly a different procedure, the hold that occurs very soon after liftoff , understanding when the different procedure can no longer be executed because its behind you, how to set up your FMSs, can be considerable and very prone to error. It is much simplier to "fly the DP whether you lose an engine or not."
If conditions are CAVU and the obstacles are not apparent (as in this case), then be more concerned with the Max Wgt Climb unlimited value. If its IMC, follow the published DP and use the published Net Takeoff Profile procedure as described above to place you into the enroute structure.
Hope this helps.
Jim
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