Despite that speed limit of 250, in the USA...
For a 747, we request "high speed" departure due to performance requirement.
Some guys simply say "Need 280 for performance".
ATCO confirms by clearing us for that speed.
Never had departure control denying us that speed.
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You see, heavy (377 tonnes) our V2 flaps 20 is 180 KIAS.
To retract flaps, we need V2 180 + 80 = 260 KIAS.
To maneuver clean, we need V2 180 + 100 = 280 KIAS...
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Happy contrails