Sembra che fosse Fl 600 non Fl400....
SR-71 Blackbird was crossing the control-zone of London Control. Evidently the controller didn't know the service ceiling of this aircraft is around 30,000 metres (yes metres, not feet!). There are so many variations on this item that the original event has evidently become part of aviation folklore or joke-lore.
Pilot: "Radar, Good Day, Airforce Blackbird, request FL 600(!)"
Controller (amused): "Sir, if you can reach, you are cleared FL 600"
Pilot: "US Air Force Blackbird, leaving FL 800, decending Level 600..."