IMAGE: PETER DAWSON; PHOTO MANIPULATION: LAURA HOFFMAN

Is it safe to use cellphones on airplanes? The U.S. Federal Communications Commission (FCC) thinks it may be. In December 2004, the agency began soliciting comments on proposed regulations that would allow airline passengers to use cellphones and other electronic devices. To be sure, it acknowledges that a sister agency, the Federal Aviation Administration (FAA), has ultimate authority regarding regulations that govern airline safety. Yet a July 2005 report by a U.S. House of Representatives subcommittee, which held hearings on the matter, noted: "The FCC hopes to issue a final ruling in 2006, stating that its ultimate objective is to allow consumers to use their own wireless devices during flight."

In the meantime, more and more passengers are bringing cellphones, PDAs, laptops, DVD players, and game machines on board. All of these items emit radiation and have the potential to interfere with aircraft instrumentation. More and more passengers, however, do not believe that using portable electronic devices presents a risk to their safety. We, on the other hand, have had our doubts that such use was safe.

Over the course of three months in late 2003, we investigated the possibility that portable electronic devices interfere with a plane's safety instruments by measuring the RF spectrum inside commercial aircraft cabins. What we found was disturbing. Passengers are using cellphones, on the average, at least once per flight, contrary to FCC and FAA regulations, and sometimes during the especially critical flight phases of takeoff and landing. Although that number seems low, keep in mind that it represents the furtive activity of a small number of rule breakers. Should the FCC and the airlines allow cellphone use, the number of calls could rise dramatically. In addition, regulations already permit a wide variety of other portable electronic devices—from game machines to laptops with Wi-Fi cards—to be used in the air today. Yet our research has found that these items can interrupt the normal operation of key cockpit instruments, especially Global Positioning System (GPS) receivers, which are increasingly vital to safe landings. Two different studies by NASA further support the idea that passengers' electronic devices dangerously produce interference in a way that reduces the safety margins for critical avionics systems.

There is no smoking gun to this story: there is no definitive instance of an air accident known to have been caused by a passenger's use of an electronic device. Nonetheless, although it is impossible to say that such use has contributed to air accidents in the past, the data also make it impossible to rule it out completely. More important, the data support a conclusion that continued use of portable RF-emitting devices such as cellphones will, in all likelihood, someday cause an accident by interfering with critical cockpit instruments such as GPS receivers. This much is certain: there exists a greater potential for problems than was previously believed.

Although our data are more than two years old, they still represent the best available in this critical area of air safety. Ours is the first documented study of in-flight RF emissions by portable electronic devices and, we believe, the first such scientific measuring other than what has been done by individual airlines. And as far as we know, it is the first in-the-field examination ever into the critical question of emissions interference with the spectrum bands used for navigation. Yet despite the paucity of available data, regulators and the airlines seem poised to yield to public demands to allow the use of cellphones in flight and the use of other devices, such as PDAs, during critical phases of flight. We believe additional studies are needed to characterize potential risks, followed by regulations that ensure the safe use of radiating devices, and we conclude with a suggested five-point program for such studies. And we argue that in the meantime, the public needs to be more clearly informed about the risks of its current behavior.

Some folks doubt that there is a risk, arguing that the evidence of cellphone use on planes is merely anecdotal. However, take, for example, one flight on a Boeing 737 in the busy eastern U.S. air corridor. One of us watched a passenger pull out a cellphone and make a call shortly after the wheels left the ground. Normally, that would have been dismissed as just another undocumented story about possible cellphone use on a commercial airliner, but not this time: on this occasion, it was thoroughly documented. Unbeknownst to everyone on board (except one of us and the flight crew), an innocuous-looking carry-on bag was stuffed in the overhead luggage rack [see photo, "Overhead Instrumentation"]. It contained a broadband antenna connected to a compact portable spectrum analyzer. A laptop computer controlled the system and logged the data. The whole package had been carefully tested for safe in-flight operation and was allowed on board by the airline and the two relevant U.S. safety agencies, the FAA and the Transportation Security Administration. When the flight was over, we downloaded the data, and there it was—the clear spectral signature of that phone call.