c. So why does my 777 require more time and distance to descend from cruise level when it is heavy than when it is light?........
Seems to me that our heavier 777 is a better glider than its lighter friend for the given descent airspeed.
Could it be that the heavier one is operating at a higher L/D than the lighter one? I believe so.
Why does the heavier one has higher L/D?
Because at the same airspeed heavier one needs to fly at higher angle of attack, the L/D ratio at this higher angle of attack is more than the one for the lighter 777.
How do you know the L/D ratio is higher at higher AOA?
For that we must take a look at the L/D vs angle of attack curve, and plot the operating points for these two cases.
Is the L/D ratio higher for all angle of attacks higher than the one at which the lighter 777 descends?
No, with increase of angle of attack the L/D ratio increases first, then levels off and then gradually decreases.