Aviation glossary
IMC
Instrument Meteorological Conditions — weather that does not meet VMC. Flight is only possible under IFR with the appropriate rating.
IMC (Instrument Meteorological Conditions) are conditions in which the pilot loses external visual references (cloud, fog, heavy precipitation, dark night over unlit terrain). A VFR pilot without an instrument rating must actively avoid IMC — inadvertent flight from VFR into IMC is one of the most common causes of fatal accidents in general aviation. The rule for VFR helicopter pilots is simple: as the weather deteriorates, slow down, descend, turn back, land.
Related terms
- VMCVisual Meteorological Conditions — the visibility and cloud-clearance minima that must be met for a VFR flight.
- VFRVisual Flight Rules — the pilot navigates by outside visual reference and must meet VMC minima.
- IFRInstrument Flight Rules — the pilot navigates by radio-navigation instruments independently of outside visibility.
- Marginal VMCMarginal visual meteorological conditions — weather that still meets VFR minima, but with very little margin. Demands heightened attention and an escape plan.