Why do we have to turn off iPods during takeoff?

Every passenger wants to escape the tedium of commercial air flight. Seems like a good time to catch up on e-mails or call home to arrange an airport pickup. Nervous fliers want to ease their fears with movies, music, or video games. But then the flight attendant makes the demand: "Turn off all electronic devices."

Todd W. Chavanne of Colorado Springs, Colorado, thinks he smells a rat. "I can understand cell phones, pagers, or anything that transmits or even receives signals," he writes. "But why exactly do I have to turn off my iPod, CD player, or anything with an on/off switch when we take off and land? Does it really interfere with the aircraft…and if so, how?"

The FAA regulation that covers this topic is Advisory Circular 91.21-1B. Agency spokesman Les Dorr says that "while an Advisory Circular is not mandatory, in practice all airlines and other operators adhere to its provisions. Airlines generally consider the ‘prohibited' period to begin when the cabin door closes for pushback." One big reason is lawyers. The FAA's regulation makes the airlines responsible for governing the use of portable electronic devices on their airplanes, ensuring that carriers err on the side of non-litigious caution.

The ban on cellular phones is fairly easy to understand. Not only do they transmit signals, they might interfere with cellular signals on the ground. For that reason, it isn't just the FAA that limits their use on airplanes. The Federal Communications Commission has recently supported the FAA's decision to keep cell phones turned off during the entire flight, not just during takeoff and landing.

But what about laptops, iPods, and other devices that don't have to receive signals to work? The reason these devices are banned is that they emit radio waves. All wireless devices do, and the navigation and flight control computers on airplanes are designed to sense even very weak signals coming from far away. Radio waves with just the right power and frequency can in theory introduce errors in computing equipment.

Whether they actually do remains a question. "Though many cases of electromagnetic interference have been reported over the years, with personal electronic devices suspected as the cause, it has proven almost impossible to duplicate these events," Boeing engineer Bruce Donham wrote in the company's internal publication, Aero, in 2000. Case in point: In 1995, a passenger's laptop computer was reported to cause the autopilot on a 737 to disconnect. Boeing bought the computer from the passenger and sent it to the lab for testing. Scans showed the laptop emitting frequency-range levels exceeding the company's pre-set limits for equipment on airplanes. However, "after lengthy attempts," the disconnect could not be duplicated. "As a result of these and other investigations, Boeing has not been able to find a definite correlation between personal electronic devices and the associated reported airplane anomalies," Donham wrote.

So the mystery lingers. "Sometimes [interference] appears, usually it does not," says Victoria Day, a spokesperson for the Air Transport Association, a trade group for U.S. airlines. "It may present pilots with an annoyance, or it may threaten safe operation of the airplane. During takeoff and landing when the airplane is closer to the ground, there is no room for error, and airlines take every precaution to ensure that no interference occurs."

In his query, Chavanne adds: "I always thought the real reason [for the personal electronics ban] was to be sure everyone in the aircraft can hear instructions in case of an emergency." He may be on to something. Airlines would likely use any means to get passengers to pay attention to those pre-takeoff lectures on emergency procedures. "There is probably something to the idea that they want you to listen to the briefing as well," the FAA's Dorr says. "Although I don't find any requirement in the Federal Aviation Regulations that mandates a passenger has to listen."