The biggest safety threat facing airlines today may not be a man with a gun, but the man with the portable computer in business class. In the last 15 years, pilots have reposed well over 100 incidents that could have been caused by electromagnetic interference(电磁波干扰). The source of this interference remains a mystery, but increasingly, experts are pointing the blame at potable electronic devices such as portable computers, radio and mobile telephones. RTCA, an organization which advises the aviation(航空)industry, has recommended that all airlines ban such devices from being used during "critical" stages of flight, particularly take off and landing. Some experts have gone further, calling for a total ban during all flights. Although some airlines prohibit passengers from using such equipment during take off and landing, most are reluctant to enforce a total ban, given that many passengers want to work during flights. The difficulty predicting how electromagnetic fields might affect an aircraft" s computers. Experts know that portable devices give out radiation that affects navigation and communication of aircraft. 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 erous or not. What is said about the over 100 aircraft incidents in the past 15 years