This glossary is written for virtual airline and flight simulation use within Virtual Air Canada Airline. It is not intended as a real-world aviation reference.
IMC
Instrument Meteorological Conditions
Instrument Meteorological Conditions describes weather conditions in which visibility, cloud ceiling, or other factors are below the minimums required for Visual Flight Rules (VFR) flight. When conditions deteriorate into IMC, pilots must operate under Instrument Flight Rules (IFR), relying on cockpit instruments and air traffic control rather than outside visual reference to navigate and maintain safe separation from terrain and other aircraft. The specific thresholds that define IMC vary by airspace class, but in general terms, flight visibility below 5 statute miles or ceilings below 1,000 feet above the terrain will push conditions into IMC territory.
Commercial air transport operations are conducted under IFR at all times, meaning airline flight crews fly in both VMC and IMC according to the same instrument-based procedures regardless of the weather. The distinction is more operationally significant for departure, approach, and landing minima - an approach that is permitted in VMC may require special equipment or crew qualifications in IMC. Flights departing into IMC must have filed an instrument flight plan, and the crew must be instrument-rated and current. Extended IMC flight in icing conditions adds additional considerations related to aircraft certification and anti-ice systems.