RTCA, an organization which advises the aviation（航空）industry, has recommended that all

　　airlines ban（禁止）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． Currently, rules on using these devices are left up to individual airlines． And although some airlines prohibit （禁止）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 is prediction how electromagnetic fields might affect an aircraft’s computers．

　　Experts know that portable device 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．

　　The fact that aircraft may be vulnerable（易受损的）to interference raises the risk that terrorists

　　may use radio systems in order to damage navigation equipment． As worrying though, is the passenger who can’t hear the instructions to turn off his radio because the music is too loud ．

　　68．What is said about the over 100 aircraft incidents in the past 15 years?

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