What a discussion. I think Intruder was right at the 5'th of october already.
Drag curve shifts right with an increase in weight. Hence, best L/D increases, and therefore best glide speed increases with weight.
With a set speed of 300 kts in the FMC, which is well over best glide speed, that speed is closer to L/D for a higher weight. Remember, the speed was fixed. Therefore that speed is comparatively more efficient, and you glide longer.
If the descent speed was set to ECON, the distance shouldn't shift with an increase in weight. Only the speed should differ. That's why we operate a low cost index and ECON descent, giving us a speed of about .77M/255kts during the descent. Then T/D usually is around 120-140 miles, non-dependent of weight.
Try selecting ECON descent speed and then changing the weight. TOD should stay where it is.
Last edited by KristianNorway; 14th Oct 2009 at 14:11.