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the junction, a negative ionic charge is left. It is the ionic charges in a semiconductor diode that produce the
barrier voltage.
ionize. The process of making positive ions by freeing electrons from balanced atoms or molecules. The
electrons freed from the atoms travel to other atoms or molecules and become part of them.
ionosphere. A region of electrically charged particles (ions) extending upward from about 30 miles above
the surface of the earth. The ionosphere is composed of a number of layers that vary in the amount of
ionization, with the lower layers not ionized as heavily as the higher layers. The names of the layers of the
ionosphere, beginning at the lower layer, are:
D region
E region (the Kennelly Heaviside layer)
F1 region
F2 region
Heliosphere
Protonosphere
Magnetosphere
IRAN (aircraft inspection). A type of aircraft inspection in which the aircraft is inspected and any needed
repairs are made. IRAN stands for “inspect and repair as necessary.”
IRBM (intermediate-range ballistic missile). A ballistic missile with a range of between 200 and 1,500
miles. See ballistic missile.
IR drop. The decrease in electrical pressure caused by electrons flowing through a resistance. The amount
of IR drop, expressed in volts, is found by multiplying the current (I), in amps, by the resistance (R), in
ohms.
iridium. A very hard, brittle, highly corrosion-resistant, whitish-yellow, metallic chemical element.
Iridium’s symbol is Ir, its atomic number is 77, and its atomic weight is 192.2.
Iridium is used for electrical contacts and as an alloying agent in high-temperature metals. Spark
plugs having fine-wire electrodes made of iridium are used in engines in which lead-fouling is a problem.
Irish linen (aircraft fabric). A strong organic fabric made from the fibers of the flax plant. Irish linen is
approved as a direct replacement for grade-A cotton fabric as a covering material for aircraft structures.
iron. A silvery-white, lustrous, malleable, ductile, metallic chemical element. Iron’s symbol is Fe, its
atomic number is 26, and its atomic weight is 55.847. Iron is an important structural metal in the form of
steel (iron with controlled amounts of carbon and other elements).
iron carbide. The form in which carbon exists in steel. The amount of iron carbide and its distribution in
the metal determine the hardness of the steel.
iron-constantan thermocouple. A thermocouple combination used to measure temperatures up to about
410°C. Iron forms the positive element, and constantan, the negative. Iron-constantan thermocouples are
generally used to measure cylinder head temperature on reciprocating engines.
iron-core coil. A type of inductor wound around a core of laminated iron sections. Iron has a much higher
permeability than air, and an iron-core coil has a greater inductance than an equivalent size air-core coil.
iron-core transformer (electrical component). A transformer with the coils wound over a core of
laminated soft iron. The soft iron core concentrates the lines of magnetic flux and increases the inductance
of the coil. The laminations decrease the eddy current losses in the core.
iron-vane meter movement (AC ammeter). The mechanism in an AC ammeter whose pointer is attached
to a soft iron vane and restrained by a calibrated hairspring.
The vane is rotated by the magnetic field produced by the unknown current flowing through a
surrounding coil. The greater the current, the stronger the magnetic field and the greater the pointer
deflection.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
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irreversible controls (aircraft controls). Controls in an aircraft operated by a boost system. Cockpit
controls move the control surfaces, but aerodynamic forces on the control surfaces cannot feed back into
the cockpit controls.
isobar (meteorology). A line on a weather map that shows equal or constant barometric pressure.
isobaric range of cabin pressurization. The range of cabin pressurization in which cabin pressure is
maintained at a constant level (a constant cabin altitude), rather than allowing it to change with flight
altitude.
Most cabin pressurization systems operate in the isobaric range up to the altitude at which the
cabin differential pressure is the maximum allowed by the aircraft structure. Then the system automatically
shifts to the constant-differential mode, which keeps a constant pressure difference between the pressure of
the air inside the cabin and the air outside the cabin.
isoecho (radar meteorology). A circuit in a weather radar that reverses signal strength above a specified
intensity level. An iso echo circuit causes a void on the scope in the most intense portion of an echo when
　
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