相关题目
（ ） is a circle on the surface of the earth, the plane of which passes though the centre of the earth.
（ ） is air traffic control service for controlled flights in control areas.
（ ） is a service provided for the purpose of giving advice and information useful for the safe and efficient conduct of flights.
（ ） is a service provided to notify appropriate organizations regarding aircraft in need of search and rescue aid, and assist such organizations as required.
Air traffic control service for arriving and departing controlled flights. It is （ ） .
Air traffic services include （ ） and （ ） services.
Visibility is the ability as determined by atmospheric conditions and expressed in unit （ ） .
Takeoff distance available is the length of the takeoff run available plus the length of （ ） , if provided.
Special VFR flight is a VFR flight cleared by air traffic control to operate within class B, C, D and E surface areas in meteorological conditions below （ ） .
SIGMET is a weather advisory issued concerning weather （ ） to the safety of all aircraft.
