Mir From United States of America, joined Jan 2004, 22788 posts, RR: 53
Reply 3, posted (3 years 10 months 1 week 4 days 4 hours ago) and read 4299 times:
It is, in general, better to be higher. However, if you're too heavy for a certain altitude, then you'll burn more fuel than you would if you stayed lower down until you became light enough to climb up. Aircraft will have an optimum altitude for a particular weight, and and significant deviation from that will result in higher fuel burn. The optimum altitude will increase over the course of the flight as the aircraft burns off fuel and becomes lighter.
7 billion, one nation, imagination...it's a beautiful day