American Airlines announced Tuesday that it will expand in-flight Wi-Fi Internet service to its entire fleet. The airline, along with Delta and Virgin America, started offering Wi-Fi on select planes in late 2008. In-flight calls, however, are still prohibited. If I can surf the Web, why can't I use my cell?
It operates on a totally different frequency. Cell phones transmit signals at roughly the same frequencies as aircraft communications—pilot radios and radar range from below 100 to 2,000 MHz, and many phones operate at 850 MHz or 1,900 MHz. Your cell could therefore—at least theoretically—interfere with navigation. Wi-Fi, on the other hand, signals at a higher frequency—anywhere from 2,500 to 5,000 MHz—and thus won't get mixed up with the plane's transmissions.
In-flight Wi-Fi works like a moving Starbucks hot spot. The plane is rigged with three antennae—two on its belly and one on top—that receive signals from towers across the country. The frequency of those transmissions, 849 MHz, is within the range of airline communications. But they don't interfere with the plane's navigation, since 849 MHz is a dedicated frequency that was auctioned off and bought in 2006 by Aircell, which services American, Delta, and Virgin. (It's the same frequency once used by Airfone.)
But are cell phones on planes really that dangerous, anyway? Studies analyzing the dangers of in-flight cell-phone use suggest the risks are small but real. In 2003, a study by IEEE Spectrum concluded that "continued use of portable RF-emitting devices such as cell phones will, in all likelihood, someday cause an accident by interfering with critical cockpit instruments such as GPS receivers." A study produced by the Radio Technical Commission for Aeronautics in 2006 found that portable electronic devices can interfere with airplane communications and laid out testing guidelines for airlines to figure out which devices should be permitted.
The rationale for switching off other portable electronic devices is slightly different. Even if a device doesn't transmit a signal—think iPods, Game Boys, "anything with an on-off switch"—it still emits energy at a frequency that could, possibly, interfere with the plane's electronics. The Federal Aviation Administration requires all such devices to be off during takeoff and landings, but you're allowed to turn them on once you reach a cruising altitude—presumably because any interference would be minimal and temporary. There are exceptions, though, for necessary devices like hearing aids and pacemakers.
Some international airlines do allow cell-phone use. Emirates Airline permits in-flight calls as long as you use an onboard picocell network, which isolates the cellular communications from the pilot's. In the United States, the resistance to in-flight calls is strong, but often for social rather than safety reasons. Members of Congress have even introduced legislation to keep cell phones off planes, titled the Halting Airplane Noise To Give Us Peace Act, or HANG UP Act.