Cockpit Chronicles: A gallon saved... {Gadling}

Jul 2nd 2009 8:53AM Mike.

The one-word answer to your question is "physics." As the speed of a vehicle approaches the speed of sound, the drag starts to go way up. In aerospace, this is often referred to as the "transonic drag rise". Without the transonic drag rise, drag basically scales with velocity squared. Once you approach or hit the drag divergence Mach number, the overall drag rises much more rapidly than just an extrapolation of the velocity-squared value.

As to your question about "descending as late as possible", it is because turbine engines operate more efficiently at higher altitudes, close to their cruise design point. Also with the lower density at altitude, aircraft drag is lower as well (for a fixed speed). So the more distance you cover when operating efficiently, the lower the overall fuel consumption will be.