Philadelphia ? Derrie-Air has been exposed.

Readers of The Philadelphia Inquirer and Philadelphia Daily News opened their papers Friday to see ads for a new airline called Derrie-Air, which purportedly charges passengers by the pound.

But the new carrier will never get off the ground. It’s a one-day advertising campaign about a fake airline by Philadelphia Media Holdings, the papers’ owner, and Gyro ad agency.

In light blue banners throughout the papers – as well as on their Web site, Philly.com – Derrie-Air cheerily trumpets its policy: The more you weigh, the more you pay. The ads direct readers to the Web site www.flyderrie-air.com.

Philadelphia Media Holdings spokesman Jay Devine said the goal is to “demonstrate the power of our brands in generating awareness and generating traffic for our advertisers, and put a smile on people’s faces.”

The company will track traffic to the Derrie-Air site. Devine said there’s already buzz about the campaign on online blogs.