Aviation Glossary - I

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i/c - intercom

IAS - indicated airspeed. An aircraft's speed through the air as indicated by the ASI, without correction for position error, altitude or outside air temperature. Also see CAS, RAS and TAS.

IATA - International Air Transport Association.

ICAO - International Civil Aviation Organisation.

IF - instrument flying. Also intermediate frequency.

IFF - identification friend or foe.

IFR - instrument flight rules prescribed for the operation of aircraft in instrument meteorological conditions (see below). Flight in most controlled airspace in the UK is conducted under IFR or Special VFR. Also used by military to denote in-flight refuelling.

IGE - in ground effect. Helicopter performance with an earth surface immediately below. Also OGE, out of ground effect. Helicopters can hover at a greater maximum altitude IGE (above a mountain slope, for example) than they can in free air, OGE.

IGS - instrument guidance system.

ILS - instrument landing system. The approach aid employing two radio beams to provide pilots with vertical and horizontal guidance during the landing approach. The localiser provides azimuth guidance, while the glide

IMC - instrument meteorological conditions: weather below VMC minima, see below. Also associated rating used in conjunction with UK PPL.

INCERFA - uncertainty phase of search-and-rescue procedure.

INS - inertial navigation system. A gyroscope-based system which senses acceleration and deceleration and computes an aircraft's position in latitude and longitude with great accuracy. Used mostly by long-haul airliners, military aircraft and sophisticated business jets. Also IRS, inertial reference system.

INTER - intermittent or fluctuating, term used in Met reports.

IR(E) - Instrument Rating (Examiner). Also infra-red.

ISA - International Standard Atmosphere - a set of standard conditions or temperature and pressure which serve as a basis for comparison. ISA = pressure 1013.2 millibars, temperature 15 degrees C. Aircraft performance figures quoted by manufacturers are often based on such a 'standard day'.

ITT - inter-turbine temperature. Also TGT, turbine gas temperature TIT, turbine inlet temperature.