Some nice 'home video' footage from a Pan Am LHR-LAX flight. 747-121.
www.youtube.com/watch?v=t6_iwIso4zY
Holy moly...I wasn't sure it was going to lift off.
Then I wasn't sure it was going to climb.
Interesting comments from the video source :
The altitude for most of the journey was 29,000 ft. As fuel was used up and the aircraft became lighter, the 747 climbed to 33,000ft over the Grand Canyon and into California.
I kinda thought it may have gained more height by the end of the long flight, but what would I know?