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When I'm flying over the Rockies, does the plane stay at a constant 38,000 feet over the Earth's surface, or does it climb to 38,000 feet above the mountains?
When an aircraft flies at 38,000 feet (11,600 meters), this means it is flying 38,000 feet above sea level. This usually does not change when flying over mountains, since a specific flight path has been established that would not bring the plane over high peaks. Flying at this set altidue not only allows the aircraft to perform at its best, it also avoids airplane colision.