And it is one that is often ignored.
"I tend to think it's old school," said traveler Cory Balousek. "I think the technology has changed, but why take the chance?"
"I can't imagine my measly cellphone is going to cause anything bad to happen," said another traveler.
But according to a confidential airline industry report obtained by ABC News it can. The report documents 75 separate incidents that pilots and engineers believe are linked to cellphones and other electronic devices.
Incidents include the autopilot disengaging by itself, the GPS not reading correctly, and rapid changes in cabin altitude and altitude control.
"The equipment that feeds the data to the cockpit, the radios, and the communications and the navigations equipment, the equipment itself is located under the floor in the airplane or in the overhead or in the walls," said Dave Carson from Boeing.
According to Boeing, passenger devices radiate signals that can disrupt equipment, including those for an instrument landing system used in bad weather. To say safe, officials say it is best to follow the rules.
"I turn them off," said traveler Kim Hartmann. "I don't want to risk anything. Turn it off, put it away."
New planes with WIFI capability are prepared. They're specially built so that the signal doesn't interfere with the aircrafts.