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。
When an obstacle appears in the region B in Fig. 2, clearance values become zero.
如果一个障碍物呈现在图2中的B区时，间隙值变为零。
2 measure the B dimension of the flange clearance and measure four points on the circumference.
测量法兰间隙b尺寸，在圆周上测四点（均布）。
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