An interesting discussion.
On a more practical level with regards to everyday descent performance, I find new jet trainees have a little trouble explaining the effect weight on a typical jet descent. ie. Why does a heavier jet require more track miles for descent compared to the same jet at a lighter weight, assuming a flight idle descent at typical jet descent speeds (280kt/250kt below 10,000)?
The answer of course lies in how close an aircraft’s descent speed is to it’s current best L/D (driftdown) speed for the current weight.
Example;
B737 at 41,000’ – track miles required for an idle descent in nil wind;
70,000kg -142nm Best L/D speed: 247kts
50,000kg - 119nm Best L/D speed: 209kts
Descending at 280kts or 250kts below 10,000’, the heavier 737 will be descending at a speed closer to it’s current best L/D speed compared to the lighter example. At 250kts, the heavy 737 is descending at almost it’s best L/D speed and the lighter version is descending much faster than best L/D. At 280kts the lighter jet is even further from best L/D.
So the bottom line to remember? If you are held high on descent (and unable to increase your descent speed), a heavy jet is going to have more trouble getting down compared to a lighter one. At lighter weights and the same descent speed, you will have a much steeper descent path (higher rate of descent) compared to a descent at a heavier weight.
Last edited by Warped Wings; 19th August 2013 at 08:36.