instrument flight rule

…all pilots must obey the instrument flight rule; that is, they must depend principally on the information provided by the plane’s instruments for their safety. In poor visibility and at night, instrument flight rules invariably apply. At airports, in control zones, all movements are subject to permission and instruction from…

…instrument operation; these are called instrument flight rules (IFR). Under such conditions, crosswind runways cannot be used simultaneously with main runways, so that the capacities of the crossed and V configurations are equivalent to that of a single runway.