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ILS (Instrument Landing System): Electronic approach aid which enables a pilot to carry out an approach for landing when weather conditions preclude visual contact with the ground.

IFR (Instrument Flight Rules): Stipulated procedures for navigating aircraft by reference to cockpit instruments and radio navigation aids alone. Enables flight regardless of visibility. Normal operating procedure for airline flights.

IMC (Instrument Meteorological Conditions): Weather conditions in which visibility is less than specified for visual flying, and in which flight is legally possible only under IFR.

Knot: One nautical mile per hour. *****alent to 1.853km/h.

Lifeguard: A term attached to an airliner's radio callsign when the aircraft is transporting time sensitive medical materials/supplies, such as blood plasma, organs for transplant, etc. (Example: TWA Flight 800 was known as "Lifeguard TWA800")

Localizer: The horizontal guidance portion of an Instrument Landing System. Indicates to the pilot whether he is to the left of, to the right of, or on course.

LOG: Localised VHF radio beacon providing accurate horizontal guidance.

Lowest Safe Altitude (LSA): Designated minimum altitude for particular air route, providing minimum of 1,000 feet clearance above underlying terrain.

MAC: Mean Aerodynamic Chord.

MSA (Minimum Safe Altitude): Altitude below which IFR aircraft may not descend unless specifically authorised to do so by ATC. Takes into account high terrain underlying an air route.

Mach number: Figure expressing relationship between true airspeed of aircraft and speed of sound.

Mayday (repeated three times): Radio telephony version of former morse code "SOS" distress call. Derived from the French "m'aidez" - "help me".

N1: RPM of Stage 1 fan of turbofan jet engine, expressed as a percentage of normal maximum fan speed.

N2: RPM of gas turbines of turbofan jet engine, expressed as a percentage of normal maximum turbine speed.

Nautical mile (nm): Measure of distance used for navigation in the air and at sea. Equal to one minute of an arc of latitude on the earth's surface. Is 800 feet longer than a statute mile and *****alent to 1.853km.

Nav: Navigation.

Navaid: Radio navigation aid.

ND: Navigation Display.

NDB: Non directional beacon. Ground based medium frequency radio transmitter sending continuous signals in all directions for use by aircraft fitted with ADF (radio compass).

NOTAM (Notice to Airmen): Message concerning changes to serviceability of aerodromes, radio and navigation facilities.

NTSB: National Transportation Safety Board.

Octas ("eighths"): Expression of cloud amount. Eight octas (or eighths) represents a completely overcast sky; four octas a half clouded sky.

PFD: Primary Flight Display.

Pitot-static system: System of instruments, connecting tubes and air sensors for measuring altitude, airspeed, and rate of climb or descent.

Precipitation: (Meteorological) Rain, hail, sleet or snow in or falling from cloud.

Preflight (inspection): "Walk around" inspection of aircraft by pilot, usually immediately prior to flight.

QFE: Code expression designating altimeter setting in millibars for particular airport. When set on subscale of altimeter, instrument reads aircraft's height above that airport.

QNH: Code expression designating altimeter setting in millibars - when set on subscale of aircraft's altimeter, instrument reads aircraft's height above mean sea level.

Radial: Bearing to or from VOR radio range.

Radio Compass: See ADF.

Radio Range: Type of radio beacon providing defined aircraft tracks to or from that navigation aid.

Rate One turn: Shallow standard rate turn used in instrument flight conditions.

RMI: Radio Magnetic Indicator.

RMP: Radio Management Panel.

RPM (rpm): Measure of engine speed expressed in revolutions per minute.

Rudder(s): Control surface(s) at rear of vertical tail (fin) controlling yawing movement of aircraft.

SAR: Search and Rescue.

SEC: Spoiler/Elevator Computer.

SFCC: Slat/Flap Control Computer.

Sigmet: Warning signal issued by Aviation Meteorological Service when weather conditions suddenly deteriorate.

Slats: Aerodynamic device fitted to leading edge of wings to delay onset of stall.

Spot height: Height noted on chart showing elevation of prominent mountain peak.

SSC: Side Stick Controller. Replaces a conventional flight yoke on Airbus aircraft.

Stalling speed: Low airspeed at which aircraft wings suddenly lose lift. No connection with engine "stall". Is absolute minimum airspeed at which aircraft can maintain flight.

Stick-shaker: Stall warning device which shakes aircraft's control columns as stalling speed is approached.

Tailplane: Horizontal aerofoil member of an aeroplane's tail assembly or empennage. Provides longitudinal stability in flight. Known as the stabilizer in US aviation parlance (see also elevators).

Threshold: The point at which a runway begins.

Transponder: Radio device fitted to aircraft which, when triggered off by certain radar wavelengths, emits a signal visible on ground radar screens. Signal usually includes additional information such as altitude of the aircraft.

Trim: Adjusting control of aircraft in climb, level flight and descent, so pilot is not required to maintain continuous pressure on elevators, ailerons or rudder.

T-VASIS: T Visual Approach Slope Indicator System.

UHF (Ultra High (radio) Frequency): Frequency band of 300 to 3000 MHZ. Aviation use confined mainly to military aircraft.

V (code): Schedule of indicated airspeeds stipulated for different phases of flight (see following).

V-1: Decision speed during takeoff. Aircraft is committed to fly when this speed is passed.

Vr: Rotation speed. Speed at which aircraft is "rotated" into liftoff attitude by raising the nosewheel off the runway.

V-2: Takeoff safety speed. Minimum control speed plus safety margin to allow for engine failure and other contingencies.

Vne: Never exceed speed.

Vref: Flap reference speed. Landing speed for stipulated number of degrees of flap extension.

VASIS: Visual approach slope indicator. System of lights located on ground on either side of runway to indicate correct angle of descent to approaching aircraft.

VSI (Vertical Speed Indicator): Instrument displaying rate of climb or descent in feet per minute.

VHF (Very High (radio) Frequency): In general use for inflight radio communications on air routes. Its frequency band from 30 to 300 MHZ is largely free from interference and static, but range is limited to "line of sight".

VFR (Visual Flight Rules): Stipulated flight procedure for navigating aircraft visually, clear of cloud, in Visual Meteorological Conditions.

VMC (Visual Meteorological Conditions): Weather providing specified range of visibility, making it possible for pilots to use visual means to avoid obstructing terrain and other aircraft.

VOR: Very High Frequency Omnidirectional Radio Range