Indicated altitude. The altitude read directly from the altimeter (uncorrected) when it is set to the current altimeter setting.
Indirect indication. A reflection of aircraft pitch-and-bank attitude by instruments other than the attitude indicator.
Induced drag. Drag caused by the same factors that produce lift; its amount varies inversely with airspeed. As airspeed decreases, the angle of attack must increase, in turn increasing induced drag.
Induction icing. A type of ice in the induction system that reduces the amount of air available for combustion. The most commonly found induction icing is carburetor icing.
Inertial navigation system (INS). A computer-based navigation system that tracks the movement of an aircraft via signals produced by onboard accelerometers. The initial location of the aircraft is entered into the computer, and all subsequent movement of the aircraft is sensed and used to keep the position updated. An INS does not require any inputs from outside signals.
Initial approach fix (IAF). The fix depicted on IAP charts where the instrument approach procedure (IAP) begins unless otherwise authorized by ATC.
Inoperative components. Higher minimums are prescribed when the specified visual aids are not functioning; this information is listed in the Inoperative Components Table found in the United States Terminal Procedures Publications.
INS. See inertial navigation system.
Instantaneous vertical speed indicator (IVSI). Assists in interpretation by instantaneously indicating the rate of climb or descent at a given moment with little or no lag as displayed in a vertical speed indicator (VSI).
Instrument approach procedures (IAP). A series of predetermined maneuvers for the orderly transfer of an aircraft under IFR from the beginning of the initial approach to a landing or to a point from which a landing may be made visually.
Instrument flight rules (IFR). Rules and regulations established by the Federal Aviation Administration to govern flight under conditions in which flight by outside visual reference is not safe. IFR flight depends upon flying by reference to instruments in the flight deck, and navigation is accomplished by reference to electronic signals.
Instrument landing system (ILS). An electronic system that provides both horizontal and vertical guidance to a specific runway, used to execute a precision instrument approach procedure.
Instrument meteorological conditions (IMC). Meteorological conditions expressed in terms of visibility, distance from clouds, and ceiling less than the minimums specified 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.
Intercooler. A device used to reduce the temperatures of the compressed air before it enters the fuel metering device. The resulting cooler air has a higher density, which permits the engine to be operated with a higher power setting.
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.
Interpolation. The estimation of an intermediate value of a quantity that falls between marked values in a series. Example: In a measurement of length, with a rule that is marked in eighths of an inch, the value falls between 3/8 inch and 1/2 inch. The estimated (interpolated) value might then be said to be 7/16 inch.
Inversion. An increase in temperature with altitude.
Inversion illusion. The feeling that the aircraft is tumbling backwards, caused by an abrupt change from climb to straight-and-level flight 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.
Isobars. Lines which connect points of equal barometric pressure.
Isogonic lines. Lines drawn across aeronautical charts to connect points having the same magnetic variation.
IVSI. See instantaneous vertical speed indicator.
J
Jet route. A route designated to serve flight 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 flows generally from west to east.
Judgment. The mental process of recognizing and analyzing all pertinent information in a particular situation, a rational evaluation of alternative actions in response to it, and a timely decision on which action to take.
K
KIAS. Knots indicated airspeed.
Knot. The knot is a unit of speed equal to one nautical mile (1.852 km) per hour, approximately 1.151 mph.
Kollsman window. A barometric scale window of a sensitive altimeter used to adjust the altitude for the altimeter setting.
L
LAAS. See local area augmentation system.
Lag. The delay that occurs before an instrument needle attains a stable indication.
Land breeze. A coastal breeze flowing from land to sea caused by temperature differences when the sea surface is warmer than the adjacent land. The land breeze usually occurs at night and alternates with the sea breeze that blows in the opposite direction by day.
Land as soon as possible. Land without delay at the nearest suitable area, such as an open field, at which a safe approach and landing is assured.
Land as soon as practical. The landing site and duration of flight are at the discretion of the pilot. Extended flight beyond the nearest approved landing area is not recommended.
Land immediately. The urgency of the landing is paramount. The primary consideration is to ensure the survival of the occupants. Landing in trees, water, or other unsafe areas should be considered only as a last resort.
Lateral axis. An imaginary line passing through the center of gravity of an airplane and extending across the airplane from wingtip to wingtip.
Lateral stability (rolling). The stability about the longitudinal axis of an aircraft. Rolling stability or the ability of an airplane to return to level flight due to a disturbance that causes one of the wings to drop.
Latitude. Measurement north or south of the equator in degrees, minutes, and seconds. Lines of latitude are also referred to as parallels.
LDA. See localizer-type directional aid.
Lead radial. The radial at which the turn from the DME arc to the inbound course is started.
Leading edge. The part of an airfoil that meets the airflow first.
Leading edge devices. High lift devices which are found on the leading edge of the airfoil. The most common types are fixed slots, movable slats, and leading edge flaps.
Leading-edge flap. A portion of the leading edge of an airplane wing that folds downward to increase the camber, lift, and drag of the wing. The leading-edge flaps are extended for takeoffs and landings to increase the amount of aerodynamic lift that is produced at any given airspeed.
Leans, the. A physical sensation caused by an abrupt correction of a banked attitude entered too slowly to stimulate the motion sensing system in the inner ear. The abrupt correction can create the illusion of banking in the opposite direction.
Licensed empty weight. The empty weight that consists of the airframe, engine(s), unusable fuel, and undrainable oil plus standard and optional equipment as specified in the equipment list. Some manufacturers used this term prior to GAMA standardization.
Lift. A component of the total aerodynamic force on an airfoil and acts perpendicular to the relative wind.
Limit load factor. Amount of stress, or load factor, that an aircraft can withstand before structural damage or failure occurs.
Lines of flux. Invisible lines of magnetic force passing between the poles of a magnet.
L/MF. See low or medium frequency.
LMM. See locator middle marker.
Load factor. The ratio of a specified load to the total weight of the aircraft. The specified load is expressed in terms of any of the following: aerodynamic forces, inertial forces, or ground or water reactions.
Loadmeter. A type of ammeter installed between the generator output and the main bus in an aircraft electrical system.
LOC. See localizer.
Local area augmentation system (LAAS). A differential global positioning system (DGPS) that improves the accuracy of the system by determining position error from the GPS satellites, then transmitting the error, or corrective factors, to the airborne GPS receiver.
Localizer (LOC). The portion of an ILS that gives left/right guidance information down the centerline of the instrument runway for final approach.
Localizer-type directional aid (LDA). A NAVAID used for nonprecision instrument approaches with utility and accuracy comparable to a localizer but which is not a part of a complete ILS and is not aligned with the runway. Some LDAs are equipped with a glideslope.
Locator middle marker (LMM). Nondirectional radio beacon (NDB) compass locator, collocated with a middle marker (MM).
Locator outer marker (LOM). NDB compass locator, collocated with an outer marker (OM).
LOM. See locator outer marker.
Longitude. Measurement east or west of the Prime Meridian in degrees, minutes, and seconds. The Prime Meridian is 0° longitude and runs through Greenwich, England. Lines of longitude are also referred to as meridians.
Longitudinal axis. An imaginary line through an aircraft from nose to tail, passing through its center of gravity. The longitudinal axis is also called the roll axis of the aircraft. Movement of the ailerons rotates an airplane about its longitudinal axis.
Longitudinal stability (pitching). Stability about the lateral axis. A desirable characteristic of an airplane whereby it tends to return to its trimmed angle of attack after displacement.
Low or medium frequency. A frequency range between 190 and 535 kHz with the medium frequency above 300 kHz. Generally associated with nondirectional beacons transmitting a continuous carrier with either a 400 or 1,020 Hz modulation.
Lubber line. The reference line used in a magnetic compass or heading indicator.
M
MAA. See maximum authorized altitude.
MAC. See mean aerodynamic chord.
Mach number. The ratio of the true airspeed of the aircraft to the speed of sound in the same atmospheric conditions, named in honor of Ernst Mach, late 19th century physicist.
Mach meter. The instrument that displays the ratio of the speed of sound to the true airspeed an aircraft is flying.
Magnetic bearing (MB). The direction to or from a radio transmitting station measured relative to magnetic north.
Magnetic compass. A device for determining direction measured from magnetic north.
Magnetic dip. A vertical attraction between a compass needle and the magnetic poles. The closer the aircraft is to a pole, the more severe the effect.
Magnetic heading (MH). The direction an aircraft is pointed with respect to magnetic north.
Magneto. A self-contained, engine-driven unit that supplies electrical current to the spark plugs; completely independent of the airplane’s electrical system. Normally there are two magnetos per engine.
Magnus effect. Lifting force produced when a rotating cylinder produces a pressure differential. This is the same effect that makes a baseball curve or a golf ball slice.
Mandatory altitude. An altitude depicted on an instrument approach chart with the altitude value both underscored and overscored. Aircraft are required to maintain altitude at the depicted value.
Mandatory block altitude. An altitude depicted on an instrument approach chart with two underscored and overscored altitude values between which aircraft are required to maintain altitude.
Maneuverability. Ability of an aircraft to change directions along a flight path and withstand the stresses imposed upon it.
Maneuvering speed (VA). The design maneuvering speed. Operating at or below design maneuvering speed does not provide structural protection against multiple full control inputs in one axis or full control inputs in more than one axis at the same time.
Manifold absolute pressure. The absolute pressure of the fuel/air mixture within the intake manifold, usually indicated in inches of mercury.
MAP. See missed approach point.
Margin identification. The top and bottom areas on an instrument approach chart that depict information about the procedure, including airport location and procedure identification.
Marker beacon. A low-powered transmitter that directs its signal upward in a small, fan-shaped pattern. Used along the flight path when approaching an airport for landing, marker beacons indicate both aurally and visually when the aircraft is directly over the facility.
Mass. The amount of matter in a body.
Maximum altitude. An altitude depicted on an instrument approach chart with overscored altitude value at which or below aircraft are required to maintain altitude.
Maximum authorized altitude (MAA). A published altitude representing the maximum usable altitude or flight level for an airspace structure or route segment.
Maximum landing weight. The greatest weight that an airplane normally is allowed to have at landing.
Maximum ramp weight. The total weight of a loaded aircraft, including all fuel. It is greater than the takeoff weight due to the fuel that will be burned during the taxi and runup operations. Ramp weight may also be referred to as taxi weight.
Maximum takeoff weight. The maximum allowable weight for takeoff.
Maximum weight. The maximum authorized weight of the aircraft and all of its equipment as specified in the Type Certificate Data Sheets (TCDS) for the aircraft.
Maximum zero fuel weight (GAMA). The maximum weight, exclusive of usable fuel.
MB. See magnetic bearing.
MCA. See minimum crossing altitude.
MDA. See minimum descent altitude.
MEA. See minimum en route altitude.
Mean aerodynamic chord (MAC). The average distance from the leading edge to the trailing edge of the wing.
Mean sea level. The average height of the surface of the sea at a particular location for all stages of the tide over a 19-year period.
MEL. See minimum equipment list.
Meridians. Lines of longitude.
Mesophere. A layer of the atmosphere directly above the stratosphere.
METAR. See Aviation Routine Weather Report.
MFD. See multi-function display.
MH. See magnetic heading.
MHz. Megahertz.
Microburts. A strong downdraft which normally occurs over horizontal distances of 1 NM or less and vertical distances of less than 1,000 feet. In spite of its small horizontal scale, an intense microburst could induce windspeeds greater than 100 knots and downdrafts as strong as 6,000 feet per minute.
Microwave landing system (MLS). A precision instrument approach system operating in the microwave spectrum which normally consists of an azimuth station, elevation station, and precision distance measuring equipment.
Mileage breakdown. A fix indicating a course change that appears on the chart as an “x” at a break between two segments of a federal airway.
Military operations area (MOA). Airspace established for the purpose of separating certain military training activities from IFR traffic.
Military training route (MTR). Airspace of defined vertical and lateral dimensions established for the conduct of military training at airspeeds in excess of 250 knots indicated airspeed (KIAS).
Minimum altitude. An altitude depicted on an instrument approach chart with the altitude value underscored. Aircraft are required to maintain altitude at or above the depicted value.
Minimum cross
ing altitude (MCA). The lowest allowed altitude at certain fixes an aircraft must cross when proceeding in the direction of a higher minimum en route altitude (MEA).
Minimum descent altitude (MDA). The lowest altitude (in feet MSL) to which descent is authorized on final approach, or during circle-to-land maneuvering in execution of a nonprecision approach.
Minimum drag. The point on the total drag curve where the lift-to-drag ratio is the greatest. At this speed, total drag is minimized.
Minimum en route altitude (MEA). The lowest published altitude between radio fixes that ensures acceptable navigational signal coverage and meets obstacle clearance requirements between those fixes.
Minimum equipment list (MEL). A list developed for larger aircraft that outlines equipment that can be inoperative for various types of flight including IFR and icing conditions. This list is based on the master minimum equipment list (MMEL) developed by the FAA and must be approved by the FAA for use. It is specific to an individual aircraft make and model.
Minimum obstruction clearance altitude (MOCA). The lowest published altitude in effect between radio fixes on VOR airways, off-airway routes, or route segments, which meets obstacle clearance requirements for the entire route segment and which ensures acceptable navigational signal coverage only within 25 statute (22 nautical) miles of a VOR.
Minimum reception altitude (MRA). The lowest altitude at which an airway intersection can be determined.
Minimum safe altitude (MSA). The minimum altitude depicted on approach charts which provides at least 1,000 feet of obstacle clearance for emergency use within a specified distance from the listed navigation facility.
Minimum vectoring altitude (MVA). An IFR altitude lower than the minimum en route altitude (MEA) that provides terrain and obstacle clearance.
Minimums section. The area on an IAP chart that displays the lowest altitude and visibility requirements for the approach.
Missed approach. A maneuver conducted by a pilot when an instrument approach cannot be completed to a landing.
Pilot's Handbook of Aeronautical Knowledge (Federal Aviation Administration) Page 83