听力原文： The biggest safety threat facing airlines today may not be a terrorist with a gun, but the man with the portable computer in business class. In the last 15 years, pilots have reported well over 100 incidents that could have been caused by electromagnetic interference. The source of this interference remains unconfirmed, but increasingly, experts are pointing the blame at portable electronic devices such as portable computers, radio and cassette players and mobile telephones. Radio Technical Commission for Aeronautics, or RTCA, an organization which advises the aviation industry, has recommended that all airlines ban such devices from being used during 'critical' stages of a flight, particularly takeoff and landing. Some experts have gone further, calling for a total ban during all flights. Currently, rules on using these devices are left up to individual airlines. And although some airlines prohibit passengers from using such equipment during takeoff and landing, most are reluctant to enforce a total ban, given that many passengers want to work during flights. The difficulty is predicting how electromagnetic fields might affect an aircraft's computers. Experts know that portable devices emit radiation which affects those wavelengths which aircraft use for navigation and communication. But, because they have not been able to reproduce these effects in a laboratory, they have no way of knowing whether the interference might be dangerous or not. (1)