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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However, multi-engined aircraft - like the one that crashed - can be provided with ATC clearance to fly elsewhere in the London Control Zone.

BBC: London helicopter crash: What are the rules for pilots?

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