nomorecatering
22nd Jun 2012, 06:22
What is the most efficient method of a jet descending for minimum trip fuel. Discregarding ATC or manouvering requirements.
Maintain cruise alt then descend at Mmo and 3-4000ft/min or an earlier top of descent point and descending at a lower speed closer to a best L/D speed maybe 2000 ft/min. Assuming in both cases a constant descent profile till final approach.
Wizofoz
22nd Jun 2012, 06:37
Best L/D on descent. You use less fuel while descending, so you want to BE descending (at Flight Idle) for as long as possible.
Shredder6
22nd Jun 2012, 08:04
Yep, Best L/D ratio. Although THE most efficient method would be to shut down both engines and glide in at that descent speed/angle and land.
Since this is not really that practical the FMC will calculate this angle based on the best L/D ratio with a small variation for residual thrust at flight idle, entered winds, temp deviation and QNH. Also altitude and speed constraints although in an ideal world there would be none of these.
Where this angle intersects the cruise level is the top of descent point, any variation from this point (distance) will be less efficient, as descending before ToD requires thrust (therefore fuel) to get back to the most efficient descent profile, and going past ToD burns more fuel at cruise power, even if descent is at a higher speed.