SOON AFTER FRONTIER AIRLINES FLIGHT 469 departed Baltimore for Denver on the night of December 2, 2003, passengers in the Airbus A319 twin-jet doled out $5 each to rent headsets for the carrier’s television service. The woman in seat 14E, however, had a cheaper alternative: She powered up a hand-held TV set.

Bad move. A flight attendant swooped in and told her to turn it off. Portable TVs are taboo on Frontier airplanes, one of the details illustrated in the laminated safety brochure that no one seems to read. After the attendant moved away, 14E turned her TV back on, only this time she hid the portable’s blue glow under an airline pillow.

Turning on a portable TV inflight seems innocent enough. The airlines offer their own television and phone service, so why should using a portable version of either be a problem? The truth is that portable electronic devices, such as mobile phones, compact disc players, and remote-controlled toys, can emit powerful electromagnetic radiation that can muck up an aircraft’s navigation and communication systems and actually endanger a flight. Airline telephones, on the other hand, transmit radio signals to and from antennas mounted externally on the aircraft, and such phones meet Federal Aviation Administration specifications that prevent them from interfering with the aircraft’s radio and navigation systems. Portable electronic devices do not currently meet such FAA requirements.

Although 14E’s disobedience, duly noted by a passenger across the aisle—me—did not appear to affect how the aircraft handled, such apparently innocent diversions have caused problems on other flights. The captain of a Boeing 737 airliner on an instrument approach to Baltimore-Washington International Airport one night in March 2003 reported that his course indicator, called a localizer, had been centered during the approach, then suddenly showed a full deflection. Just then the aircraft, flying on autopilot, broke out of the clouds—at an altitude of 2,500 feet and a full mile off course. The incident is described in NASA’s Aviation Safety Reporting System (asrs.arc.nasa.gov), a service that allows people to anonymously report aviation problems.

The 737 pilot theorized that after announcing that the United States had started attacking Iraq (information he had received from air traffic control), one or more passengers had placed calls on their mobile phones. His suggestion for prevention: Never make an announcement to passengers that might encourage mobile phone use during a flight.

Bruce Donham, who has spent a decade studying such interference for Boeing, recalls several incidents when the manufacturer was informed of anomalies—like an autopilot turning itself off during cruise or an airplane banking on its own—and advised the airlines to purchase the suspect portable electronic devices for tests. To the frustration of Boeing engineers, follow-up testing never duplicated the problems, either on subsequent flights or in the lab. “We think it’s a very low risk,” Donham says of the threat from electronic devices, “but we have to gather data to prove it out.”

The government first began investigating disruptions from carry-on devices in the early 1960s, when an FM radio was blamed for an incorrect off-course indication. The U.S. Radio Technical Commission for Aeronautics (RTCA), an FAA advisory group, called together government, industry, and academic experts to investigate the problem. Decades later, RTCA continues to study the threat: Its 1996 findings and associated advisory circulars published by the FAA form the basis for airlines’ ground rules on portable electronic devices in the air.

The advice calls for some electronic devices to be turned off whenever an aircraft is below 10,000 feet to “lessen the possibility of interference” during takeoff and landing, and encourages carriers to explain to the public the reasons for the prohibition. For mobile phones, the FAA defers to Federal Communications Commission rules, which prohibit their use when airborne. Though interference with aircraft is a potential problem, especially with mobile phones that boost their power output when searching for service, the FCC’s concern is that a mobile phone roaming at 35,000 feet will contact multiple towers at the same time, causing disruptions for ground-based users. Aside from mobile phones, the FAA leaves the ultimate decision on what can and can’t be used to the carrier and the pilot.

Frontier Airlines’ safety cards give passengers pictures of the dos and don’ts, but not explanations. Passengers are forbidden to use radios, radio-controlled toys, and TVs—ever. (Because they’re tunable over a range of frequencies, TVs and radios could be especially troublesome.) Laptop computers, mobile phones, video games, and CD players may be used when the flight crew says so.

The fact that more than four decades of study has not cleared up the uncertainty that remains is testament to the complexity of the issue. The basic science, however, is irrefutable: An aircraft’s flight navigation and communication systems receive radio signals from ground stations and orbiting satellites through antennas mounted on the aircraft’s exterior. Electronic boxes at various locations inside the aircraft process the signals gathered by the antennas to provide information for cockpit displays, and cables running throughout the aircraft route communications between the antennas, boxes, and cockpit displays. Portable electronic devices can corrupt these avionics networks in two ways. If a passenger talks on a mobile phone or watches a TV inside the aircraft, radio waves emitted from these devices can flow through aircraft windows and leak past door seals. Engineers refer to this as “front door” interference because once outside the aircraft, the radio emissions can be picked up by the craft’s antennas, which then are less able to pick up and send transmissions to and from, say, an air traffic control tower on the ground or an orbiting navigation satellite. And, though less likely, the same emissions from an onboard mobile phone or TV set can mingle with signals passing through cables running under the floor and in the airframe shell; they can even radiate directly into the aircraft’s electronic boxes. Both are examples of “back door” interference.

For front-door interference to occur, however, there must be overlaps between the frequency and power of two radio transmissions. A popular mobile phone, for example, broadcasts its intended signal at a frequency of 1,850 to 1,910 megahertz and a power level of 30 milliwatts. At the same time the phone is emitting its intended broadcast loud and clear, it is also putting out an unintended, or spurious, low-power background buzz of radio signals ranging in frequency from 100 to 2,000 megahertz. It just so happens that the very high frequency radio that air traffic control uses to communicate with cockpit crews broadcasts at frequencies of 118 to 137 megahertz, which falls within the frequency range of the mobile phone’s background buzz. Interference is not likely to occur, however, as long as the VHF transmission is sufficiently stronger than the phone’s background buzz. But the farther the airplane flies from an air traffic control tower, the weaker the tower’s signal is when it reaches the airliner. And if the phone transmits a signal that has the same frequency as the tower’s and is nearly as powerful, the two signals will compete with each other. Result: interference.

That’s one way to explain what happened to Richard Innes, a pilot who flies McDonnell Douglas MD-88s for a major airline. A year and a half ago, Innes was in cruise flight near Indianapolis, Indiana, when static over his headphones made it difficult to speak with his copilot. “It wasn’t hair-raising—more like annoying,” he says. Innes then made what’s known as a “PED announcement” to the cabin, asking passengers to turn off all portable electronic devices. The problem cleared up and Innes was inclined to leave it at that. “I’m not sure what people were operating in the cabin,” he says. “When you’re up in the cockpit, your main focus is flying. You don’t have time to play flight test engineer.”

It’s likely, though, that flight crews will continue to experience PED intrusions as airline passengers seek the convenience of electronics while in transit. However, some companies are developing technology to allow passengers to safely use cell phones during flight. Arinc/Telenor has designed a system to enable cell phone base stations on the ground to communicate with airborne cell phones via a device installed on each airliner. During critical phases of flight (takeoff, approach, and landing), the cell phones would be remotely disabled, and at 30,000 feet and above the phones would be turned on, transmitting at power levels that wouldn’t interfere with airline avionics. Arinc/Telenor expects to have a proof-of-concept demonstrator available later this year, after which testing and certification could begin. It could be a long time, though, before the Arinc/Telenor technology finds its way onto FAA-certified aircraft.

When it comes to electromagnetic phenomena, proving cause and effect is difficult because an unknown number of factors have to be considered, not the least of which is how many people and seats the electronic device’s signal had to pass through before it went through the “front” or “back” door (what engineers refer to as path loss) and whether external sources, such as lightning, ground-based military radars, and television and radio stations have contributed.

The RTCA committee is looking into onboard solutions for protecting avionics: installing window and door shielding that would prevent portable electronic device signals from reaching external antennas, getting manufacturers to build mobile phones and other carry-on devices that won’t interfere with aircraft electronics, and evaluating interference detection systems for the crew. Jay Ely, an RTCA committee member and electromagnetic interference researcher at NASA’s Langley Research Center in Virginia, says the current detector systems have problems involving the handling of false alarms.

There’s much discussion within the industry as to whether the guidelines in place are too strict or too liberal. Ely and his counterparts are of the opinion that more restrictions may be needed. During several weeks of tests on a Boeing 747 and a 737 in 2002, NASA found that an ultra-wide-band transmitter operating within FCC limits in a passenger cabin could wipe out depictions of nearby aircraft on a pilot’s collision avoidance system screen, as well as cause “erratic motion and failure” of the instrument landing system’s horizontal and vertical course guidance indicators, among other unwanted effects.

As for Frontier Flight 469, the pilot’s pre-landing pep talk—“Ah, folks? We’re about to begin our approach. Seat backs up, tray tables stowed, and all portable electronic devices off”—made at least one passenger—me—feel safer, since it got 14E to switch the TV off and put it away. Though the “why” may have been unclear, the authoritative baritone from the front office was not.

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