http://ask.yahoo.com/20040720.html
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Dear Yahoo!:
Could a person open a plane's emergency exit during a flight?
Mild Lee Concerned
Dear Mild Lee Concerned:
We were relieved to discover that the answer is no. A Yahoo! Search on "airplane emergency exit door" resulted in a great Salon.com piece called "Ask the Pilot." Written by airline pilot Patrick Smith, the article covers a range of issues, including water landings and conversation in the cockpit.
As the article explains, emergency exits open inward, and pressurized air in the plane makes it impossible to open them in mid-flight. Safety hatches and exit doors can only be opened when depressurized.
Most problems with airplane exit doors happen on the ground. In 2000, a flight attendant was killed during an emergency evacuation, because the airplane hadn't been properly depressurized. He opened the door and was "forcibly ejected from the airplane."
Even if you're a seasoned traveler, it's always a good idea to listen to the safety instructions at the beginning of a flight. Here's a quick guide to aircraft emergency procedures if you feel like a refresher.