Wikipedia 说，这么飞是为了当飞机的一个引擎出毛病时，可以找地儿就近降落：

In the North Pacific, ETOPS-180 (180 minutes) is satisfied by the availability of airports in the Aleutians Islands and Midway Atoll. As the Aleutians airports are prone to adverse weather conditions and volcanic activities, Boeing subsidised construction of the Midway Atoll diversion airport to enable the 777 to fly the North Pacific routes. After a petition from Boeing and United Airlines, in 2001, the FAA allowed a 15% extension to the ETOPS-180 rules bringing them to ETOPS-207. The approval is granted only to the 777. This approval is granted only if Northern Pacific route diversion airports are closed.

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