Downwindabeam, the performance requirements you mention are for one-engine-inoperative flight.
The SIDs assume normal ops, that is, all engines operating. So your 747 departing on its 13hr journey will easily make a better gradient than 2.4%, easily doing better than 3.3%, probably happily clearing 6% if required. The
B717 (MD-95
) that is my ride often makes 8% gradients at moderate weights, both engines operating.
One inoperative is a different matter though. You will then fly your company's prescribed Engine Out SID (EO SID), which may have you flying a different path over the ground to avoid obstacles, obstacle clearance based on your first- to fourth-segment limitations, one inoperative.
How to know what gradient you are making? Use this rule of thumb:
Groundspeed (kt) x Gradient (in %) = Rate of Climb/Descent (feet/min)
Example: YPPH Perth Intl, Australia. All SIDs require 4% to 2800' thence standard 3.3%. B717 initial climb V2+10 speed target at high-ish weights, 158KIAS. Call it 160kt.
160 x 4.0 = 640 feet/min
From that, 640 fpm is the minimum rate of climb to make the gradient at that speed. B717 usually achieves +1800 feet/min both engines operating.
However, suffer a total power loss on one motor after V1 and precise handling might have you going up at 400 fpm at high weights. 400 divided by 150 = 2.6%. Therefore, fly the EO SID, which has a longer upwind leg than the SID followed by a turn toward the ocean and away from the higher terrain. Alert ATC as to your performance limited situation, and advise brief tracking details. ATC then moves everyone out of your way (one hopes).
The groundspeed x gradient rule-of-thumb works for all aeroplanes. Say your light twin normally climbs out at 105KIAS and you need to make a 3.3% gradient....
105 x 3.3 = 346
So if you climb out at 350'/min your light twin will make the SID gradient. Dont forget we are referring to groundspeed, so factor that in - esp if the SID turns you downwind.