It's fuel burn, pure and simple. If an aircraft flies at low altitude it's much less fuel efficient. The air is thicker, causing more drag, more drag needs more power, more power needs more fuel. With a full load it probably won't reach it's destination. The aircraft would also fly much slower You'd be looking at stopping to refuel. DH described it as me driving my very thirsty diesel at 30 mph in third gear everywhere.Originally Posted by Keith, post: 202497
DH also said something about airspeed at different altitudes but I got confused.