To enter Class B airspace, such as an approach to a major metropolitan airport, an explicit ATC clearance is required.

需要持有明确的空中交通管制许可才能进入B类空域，如接近一个大都会机场。

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For example, all a VFR pilot has to do to enter Class C airspace is establish two-way radio contact with ATC.

例如，VFR 飞行员进入 C 类空域所要做的就是与空中交通管制建立双向无线电联系。

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Uncontrolled airspace is designated Class F, while controlled airspace below 5,490m above sea level and not in the vicinity of an airport is Class E.

无交通管制的空域被定为等级 F，而低于海拔5490米、不在机场附近的管制空域是等级 E。

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If ATC is busy and clearance cannot be obtained to fly through the Class Bravo airspace, it is still possible to fly the Bay Tour: execute a left Dumbarton departure heading toward Half Moon Bay, follow the coastline all the way to the Golden Gate Bridge, then east to the Bay Bridge and retrace your path back to PAO staying under Class Bravo airspace throughout.

FORBES: San Francisco: Up Close And Personal

Any trepidation I had of traversing the Class Bravo airspace was long forgotten.

FORBES: San Francisco: Up Close And Personal

Informing ground control on initial contact and indicating an intent to fly the Bay Tour paves the way smoothly for obtaining the Class Bravo clearance to transit through the San Francisco airspace.

FORBES: San Francisco: Up Close And Personal