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Fergie's headed for a runway ... we're just not sure which type!

The Black Eyed Peas babe was all set to board a flight out of LAX Thursday afternoon, but her outfit choice left onlookers scratching their heads.

With her hair pulled back in a tight bun, Fergie made her way through the airport in skintight brown leather pants and a wild-printed flowing top.

She looked like a model straight out of the '70s ... but how comfortable could that be to fly in?!