Katharine McPhee -- Mile High Lawbreaker

11/24/2010 11:54 AM PST by TMZ Staff   "American Idol" alum Katharine McPhee just broke a cardinal rule in the sky --  turning on her cell phone at 36,000 feet ... and even worse, she bragged about it afterward!

McPhee caught a flight yesterday ... and after the plane took off, she tweeted, "I love illegally turning on my phone when I'm still in the air ... makes me feel powerful! A tweet from the sky. Hello Atlanta!"

It's a big FCC no-no to turn on your phone after take off -- the penalties include a hefty fine and suspension of her cellular service.

UPDATE: McPhee's rep tells us, "She posted when we landed in Atlanta. She was trying to be funny but obviously some people missed the humor."

Joking about airplane safety -- always a good laugh ...

Tags: Katharine McPhee, American Idol, Celebrity Justice

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