Marco B
Subscriber
It may, for instance, be confined to a layer, with layers below it completely free of it. And if you can't fly at 10K, but can at 5K, you just fly at 5K.
That may not be an option if your intercontinental flight is to make it over the ocean, and not end up down in the sea instead because it ran out of fuel...
As I always understood it, jet engine fuel consumption is highly depended on the altitude the plane flies, probably mainly due to air drag (less at high altitude of course in the thin air), and another factor the efficiency of the engine.
http://forums.x-plane.org/index.php?showtopic=15483
Marco