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。
2
Get clearance before making major design changes.
在进行主要的设计变更之前进行清理。
3
We decide the cross and span clearance by over-voltage times and electrical insulation strength (air spacing) from the system major in places that the public not easy to arrive.