NEW YORK -- American Airlines said yesterday that it would join the growing number of carriers that have banned the use of laptop computers, compact-disk players and other electronic devices during takeoffs and landings. The policy underlines an emerging safety issue in which questions outnumber answers.
Although there is no scientific evidence that such devices interfere with planes' navigation or communications systems, the restrictions being imposed by airlines stem from a growing number of reports by pilots that their navigation instruments were inexplicably disrupted in flight, and that they resumed normal operation when flight attendants asked passengers to turn off these electronic devices.
At least one carrier, Swissair, has reported that the disruptions then recurred when the CD players were turned on again.
Because American is the nation's largest carrier, its policy, which it expects to put into effect by July, is expected to draw new attention to a debate among airline-safety experts on whether the radio signals that all such devices emit can indeed throw off compasses and other instruments.
Though no accident is known to have been caused by a computer or other electronic device, even indirectly, the varying policies among companies present a sticky problem in an industry that makes every effort to speak with one voice on safety matters.
"We would much prefer there be one industrywide standard," said Tim Smith, an American Airlines spokesman.
A 1988 study commissioned by the Federal Aviation Administration found no conclusive proof of an effect on instruments, but it did recommend that the use of such devices be banned during periods before and after takeoff and landing, when most accidents occur and pilots have their greatest workload.
A second study is under way, with an interim report due in October, and a final report is expected next year.
In the meantime, FAA rules ban the use of devices such as cellular telephones that transmit or receive signals. But they leave rule-making on widely used laptop computers, compact-disk players and hand-held video games to the individual carriers and their pilots.
United Airlines, for example, allows the use of computers and the like during most of a flight, but Northwest Airlines early in March restricted the use of such devices at altitudes below 10,000 feet, as will American.
Most airlines ask passengers to stow their computers under their seats during takeoff and landing.
Delta, USAir and All Nippon Airways do not allow passengers to operate CD players at any time during a flight. These have been given this special attention because some pilots suspect that the circuitry of CD players emits signals that are more likely to affect a plane's instruments than, say, a cassette player.
Imposing even stiffer restrictions, the Jordanian national carrier prohibits the use of any electronics device by passengers.
Computer makers and other electronics manufacturers are subject to FCC rules prohibiting the emission of radio-frequency radiation from appliances beyond minuscule levels, which these manufacturers believe do not affect flight instruments.
"Our laptop computers don't emit enough radio frequency energy to disturb much of anything," said Mark Cohen, a mobile-systems product manager for the IBM Personal Computer Co.
Aircraft manufacturers also said that their equipment is shielded to an extent that it is unlikely that cockpit instruments could ever be unsettled by a computer or hand-held video game. "There's a very, very low likelihood," said David B. Walen, who studies electromagnetics at the Boeing Co.