Instrument meteorological conditions (IMC).
Meteorological conditions expressed in terms of visibility, distance from clouds, and ceiling less than the minimums speci.ed for visual meteorological conditions, requiring operations to be conducted under IFR.

Instrument takeoff. Using the instruments rather than outside visual cues to maintain runway heading and execute a safe takeoff.
Interference drag. Drag generated by the collision of airstreams creating eddy currents, turbulence, or restrictions to smooth flow.
International Civil Aviation Organization (ICAO). The United Nations agency for developing the principles and techniques of international air navigation, and fostering planning and development of international civil air transport.
International standard atmosphere (IAS). A model of standard variation of pressure and temperature.
Inversion illusion. The feeling that the aircraft is tumbling backwards, caused by an abrupt change from climb to straight-and-level .ight while in situations lacking visual reference.
Inverter. A solid-state electronic device that converts D.C. into A.C. current of the proper voltage and frequency to operate A.C. gyro instruments.
Isogonic lines. Lines drawn across aeronautical charts to connect points having the same magnetic variation.
IVSI. See instantaneous vertical speed indicator.
Jet route. A route designated to serve .ight operations from 18,000 feet MSL up to and including FL 450.
Jet stream. A high-velocity narrow stream of winds, usually found near the upper limit of the troposphere, which . ows generally from west to east.
KIAS. Knots indicated airspeed.
Kollsman window. A barometric scale window of a sensitive altimeter used to adjust the altitude for the altimeter setting.