CONCORDE FLEW AT FL60 BECAUSE air is thinner so less air resistance and it had after burner engines, more efficient at higher. It had to fly at lower levels till burned off fuel, weight, then upwards to FL60. Also long trips as they all were WX is better there, fly over most all WX or easy to go around, but mostly due to EFFICIENTLY, SPEED, AIR TRAFFIC, AND FUEL BURN. I am B-747 Capt A-380 Capt, but much time in F15 and 18s same theory there also. Hope this helps you some. You can also google your questions there.