Plain English Meaning

A pilot flying by looking outside accidentally flies into clouds, fog, or weather so poor that the ground and horizon disappear. They now have to fly the aircraft using only the cockpit instruments, which is dangerous if they have not been trained for it.

Full Definition

A flight that began under Visual Flight Rules and subsequently entered Instrument Meteorological Conditions, meaning weather conditions in which the pilot can no longer see well enough outside the aircraft to control it by visual reference to the ground or horizon. This situation is one of the most lethal scenarios in general aviation because the pilot is operating under rules that assume continuous outside visibility, and may not be , , or equipped to fly the aircraft solely by to the instruments.