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ILS (instrument landing system). A special type of electronic guidance system used to allow aircraft to
land when the ceiling and visibility are too low for a safe visual approach to the runway. An ILS is made up
of four basic parts: the localizer, glide slope, marker beacons, and approach lights. The localizer produces a
narrow electronic path extending out along the center line of the instrument runway, to direct the pilot
laterally as he approaches the runway.
The glide slope produces a narrow electronic path extending upward from the approach end of the
instrument runway at an angle of approximately three degrees. The glide slope directs the pilot vertically as
the aircraft approaches the end of the runway.
The marker beacons produce highly directional radio signals to identify the position of the aircraft
along the localizer path. When the aircraft is directly over a marker beacon, a distinctive tone is received on
the radio, and a colored light on the instrument panel flashes the proper code.
The approach lights are a series of high-intensity lights located along the approach path to help the
pilot transition from flying by instruments to flying by visual reference as the aircraft breaks out of the
overcast.
ILS categories. Categories of instrument flight allowed at airports equipped with the following types of
instrument landing systems:
ILS Category I An ILS approach procedure which provides for approach to a height above
touchdown of not less than 200 feet and with runway visual range of not less than 1,800 feet.
ILS Category II An ILS approach procedure which provides for approach to a height above
touchdown of not less than 100 feet and with runway visual range of not less than 1,200 feet.
ILS Category IIIA An ILS approach procedure which provides for approach without a decision
height minimum and with runway visual range of not less than 700 feet.
ILS Category IIIB An ILS approach procedure which provides for approach without a decision
height minimum and with runway visual range of not less than 150 feet.
ILS Category IIIC An ILS approach procedure which provides for approach without a decision
height minimum and without runway visual range minimum.
IMC (instrument meteorological conditions) (air traffic control). Meteorological conditions expressed
in terms of visibility, distance from cloud, and ceiling less than the minimums specified for visual
meteorological conditions.
IMEP (indicated mean effective pressure). The average pressure existing inside the cylinder of a
reciprocating engine during its power stroke. IMEP, expressed in pounds per square inch, is a measured
pressure which contrasts with brake mean effective pressure, BMEP, which is a computed pressure. The
horsepower found when IMEP is used in the horsepower formula is indicated horsepower, IHP.
“Immediately” (air traffic control). A term used by ATC when such action compliance is required to
avoid an imminent situation.
immersion heater. An electrical heater used to heat liquids by immersing (submerging) the heater in the
liquid.
immiscible. Not able to mix. Liquids are immiscible if they will not mix with each other. Water and oil are
immiscible. If a bottle containing both water and oil is agitated, the oil and water will form an emulsion.
But when the bottle sits for a while without further agitation, the oil and water will separate, the oil rising to
the top of the water.
impact extrusion. A form of extrusion made by forcing cold metal through a die by striking it a hard blow.
This blow comes from a punch driven at a high velocity.
impact ice. Ice that forms on an aircraft flying through freezing rain or other visible moisture when the air
temperature is in the freezing range (between 0° and –15°C). Impact ice forms on the leading edges of the
wing and tail surfaces and disturbs the smooth flow of air over these surfaces. Impact ice also forms on the
engine inlet air filters and can stop an engine functioning unless inlet air is taken from an alternate source.
Impact ice forming on the inlet air duct of a jet engine can break off and be pulled through the
engine, causing severe damage to the compressor blades.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
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impact pressure. Pressure resulting when moving air is stopped. Impact pressure picked up by a pitot tube
is called pitot pressure.
impact wrench. A power-driven tool used to install and remove nuts, bolts, and screws. The torque
delivered to the fastener by an impact wrench is in a series of sharp blows or impacts.
impedance (Z). The total opposition to the flow of alternating current caused by the combined effect of
resistance, capacitance, and inductance in a circuit. Impedance (Z) is found by adding, as vectors, the
　
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