A few years ago I was caught in the roughest descent I’d ever experienced in a commercial airplane. As the pilot’s voice came on, informing us that San Francisco was unapproachable and we were being redirected to Oakland, passengers began making cell-phone calls to their rides – hiding the phones from flight attendants, of course, since a federal law prohibits cellular calls on an airplane.

After some jarring and tense minutes, we landed: in San Jose. Once again, the cell phones came out – still in violation of the regulations – as people called to apologize to loved ones.

But soon these furtive maneuvers might not be necessary. The Federal Communications Commission (FCC) is currently reviewing a proposal to lift the ban on using cell phones in flight. Although there is no date set for its decision, both telecommunications companies and airlines are anxious to provide more services.

So why not allow cell phones anyway? Are they actually a danger? The FCC is evaluating the possibility that cell phones could either block satellite signals or disrupt ground-based towers. To most observers, though, other potential safety issues are more worrisome. The Federal Aviation Administration (FAA) is concerned that cell phones might produce significant radio frequency interference, possibly disrupting avionics, including a plane’s Global Positioning System (GPS) receiver.

These worries were inflamed recently by an article in the March 2006 issue of IEEE Spectrum, the monthly publication of the Institute of Electrical and Electronics Engineers. The article, “Unsafe at Any Airspeed?”, by Bill Strauss, M. Granger Morgan, and Daniel Stancil, researchers at Carnegie Mellon University (CMU), recapped Strauss’s 2003 PhD thesis, which revealed that people do sneak cell-phone calls during flight – and that it could, in some circumstances, lead to interference with avionics systems through a process known as intermodulation.

Intermodulation occurs when two radio signals of different frequencies interact, potentially causing spikes in new frequency ranges. As Strauss and his colleagues pointed out in their article, signals within the two main cell-phone frequency ranges used in the United States (the cellular band at 824 to 849 megahertz range, and PCS at 1850 to 1910 megahertz) do not interfere with those used by most aircraft navigation aviation systems. Yet Strauss, who carried radio-monitoring equipment on several commercial flights, reported seeing intermodulation effects from cell-phone signals “in the frequency bands used by an aircraft’s GPS and distance-measuring equipment.”

However, according to David Carson, a lead engineer in cabin systems engineering at Boeing, the CMU report does not justify the hysteria evident in some articles on the topic. Carson is also co-chair of a special committee on portable electronic devices for RTCA, a private, nonprofit aviation consulting organization in Washington, DC. The FAA has commissioned the RTCA committee to produce a report on the inflight use of cell phones and other portable devices. The report, which examines “intentionally transmitting” devices, including cell phones and computers with Wi-Fi cards, will be completed in December 2006.

Carson notes that Strauss was a founding member of the special committee in 2003, and that part of his work there became his PhD thesis. “The thesis was an inflight study of whether transmissions from cell phones occurred,” Carson said. “And the conclusion was, yeah, people do use cell phones on airplanes despite the ban.”

Carson says that “the potential to have interference with airplane systems is real.” But he adds that the few airplane systems that could be affected by radio frequency interference, such as the public address system and wireless tire pressure gauges, are not critical to flight safety. The RTCA released studies of earlier portable-electronics technologies in 1963, 1988, 1996, and 2004, Carson says, and each time the airline industry responded by placing better shielding around airplane electronics and adopting other mitigation strategies. As a result, no airline crash has ever been attributed to radio emissions from devices brought onboard by passengers.

Strauss acknowledges that his study is only suggestive. “It’s not an ‘Oh, my God’ situation,” he says. “But it’s not a light situation either.”

Because of restrictions placed on the study’s methods by the airlines, Strauss says, he was not able to monitor avionics and cellular frequencies simultaneously, just sequentially. This meant he could see spikes in the cellular frequencies when a phone was being used, then switch to watch corresponding spikes in avionics frequencies. The technique provided strong evidence of correlations between cell-phone use and increased interference at avionics frequencies, he says, but it did not prove causation.

Strauss therefore stresses that their study should not be read as an argument that cell-phone use in airplanes is unsafe, but rather as a caution. “We can’t tell conclusively if it’s safe or not,” he says. Safety determinations, Strauss adds, would depend on what new safeguards airlines put in place.

One such safeguard could be cellular technology itself. The leading candidate for an inflight cell-phone system, the “pico cell” approach, reduces each phone’s ability to generate radio frequency interference.

Pico-cell systems use small transmitters that collate all the signals from in-cabin cellular calls and transmit them directly to specialized ground- or satellite-based networks. This means phones can operate at their lowest power setting, reducing the likelihood of radio frequency interference. (Most proposed pico-cell systems include an off switch accessible to flight crews, in case a passenger’s phone becomes a disturbance.)

Qualcomm, Airinc, and a few other companies are developing pico-cell transmitters. But the technology isn’t mature. In one test in 2004, Qualcomm and American Airlines found that an installed pico-cell system could handle only 100 cellular calls and other wireless connections at once. If a cell phone is denied pico-cell access, it will automatically try to find another tower on the ground – as it would without the system in place.

Alternately, airline passengers with Wi-Fi-equipped laptops could use voice-over-Internet calling systems, such as Skype, if the FAA approved its use in U.S. airspace and access were available. According to a July 2005 memo documenting a hearing before the U.S. House of Representatives Subcommittee on Aviation, the FAA has approved a Wi-Fi system developed by Verizon and United Airlines for Boeing B-757-200 aircraft. Also, Boeings’ Connexion Wi-Fi service is already offered on international flights by Lufthansa and other airlines.

But such systems won’t be as attractive to users as their cell phones: airlines are sure to charge for Wi-Fi access, not everyone subscribes to a voice-over-Internet service, and Wi-Fi systems are far more expensive to install than pico cells – approximately $500,000 per aircraft, as opposed to $100,000 for a pico-cell system.

And there’s the social aspect. It remains to be seen whether most travelers want airplanes full of people talking on cell phones. Out of more than 8,000 comments submitted to the FCC since it asked for feedback in December 2004 on lifting the cell-phone ban, an overwhelming number were opposed, according to the FCC. And a USA Today/CNN/Gallup poll recently found that 68 percent of occasional air travelers are against lifting the ban.

The FAA even has a term for the problem: “the Annoying Seatmate issue.” But spokesperson Les Dorr says the FAA has no mandate or desire to regulate the social aspects of air travel, and can get involved only if it receives reports of cell-phone use interfering with the cabin crew.

Or, as the RTCA’s Carson put it, it seems that many airline passengers would like to be able to use cell phones – they just don’t want anyone else to.