Pilot Aviation Abreviations

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

AAIB - Air Accident Investigation Branch of the Department of Transport.

aae - above aerodrome elevation. Also aal above aerodrome level.

ab initio - literally 'from the beginning', elementary flying training.

abm abeam - an aircraft is abeam a point when that point is at ninety degrees left or right of the aircraft's track, but term usually used to indicate a general position rather than a specific point.

a/c - aircraft.

ACARS - Aircraft Communication Adressing and Reporting System.

ACC - Area Control Centre. And ACZ Aerodrome Control Zone. See ATZ, below.

ACR - aerodrome control radar.

accelerate-stop distance - calculated distance required for an aircraft to accelerate to V1 (which see), reject take-off and brake safely to a halt.

A/D - aerodrome.

AD - Airworthiness Directive, issued by airworthiness authorities to correct a defect found in an aircraft type after certification. Compliance is mandatory and may be required immediately and before further flight, within a specified period of time or number of flying hours, or when next due for routine maintenance.

ADF - automatic direction finder/finding. Radio compass which gives a relative bearing to the non-directional radio beacon to which it is tuned.

ADI - attitude deviation indicator. An advanced type of artificial horizon, part of a flight director system providing pitch and roll information and commands.

ADIZ - Air Defence Identification Zone. An area of airspace extending upwards from the surface, usually along a national boundary, within which identification of all aircraft is required in the interests of national security.

ADR - Accident Data Recorder.

ADS - Automatic Dependence Surveillance

AEF - Aerodromes Environmental Federation.

Aerad - UK-published flight guide and navigational chart system.

aerodrome/airport elevation - highest point of an aerodrome's usable runway(s) expressed in feet above mean sea level (amsl).

AFB - air force base, usually U.S. or Canadian.

AFCS - automatic flight control system, an advanced autopilot. Also IFCS, integrated flight control system.

AFI - assistant flying instructor. Also AFIC, assistant flying instructor course, FIC, flying instructor course.

AFIS - Aerodrome Flight Information Service, providing information to, but not control of, aircraft using that aerodrome. Also AFISO, AFIS officer, and AGO, air-to-ground operator.

AFS - Aerodrome Fire Service.

AFTN - Aeronautical Fixed Telecommunications Network. A ground- based teleprinter network transmitting flight plans, weather information etc.

A/G - air-to-ground.

agl - above ground level.

AHRS - attitude-heading reference system. A sensor deriving aircraft attitude and heading information from gyros and accelerometers.

AIAA - area of intense aerial activity, usually military.

AIC - Aeronautical Information Circular. Bulletins issued at intervals by the CAA relating to matters of airworthiness, administration, operating procedures, safety etc. AICs are colour-coded according to subject, e.g. safety circulars are pink, thus `Pink 12' issued in December 1990.

AIP - Aeronautical Information Publication. The UK Air Pilot, statutory bible of aeronautical information published by the CAA.

Airep - form for reporting position and Met conditions in flight.

Airmet - CAA's telephone aviation weather service, covering Southern, Northern and Scottish regions of the UK. Also Metdial and Metplan, privately-operated aviation weather services. For full details see Pilot, February 1991.

AIS - Aeronautical Information Service. CAA unit based at London-Heathrow Airport, providing flight-planning services and information for pilots. Publishes AICs, above.

AIZ - Aerodrome Information Zone. See ATZ below.

ALERFA - alert phase of search-and-rescue procedure.

alternate - aerodrome specified on a flight plan to which an aircraft chooses to divert if a landing at its intended destination is not possible (for reasons of poor weather, for example).

altimeter setting - barometric pressure reading in millibars, hectopascals or (in USA) inches of mercury (Hg) used to set a pressure altimeter's sub-scale to QFE or QNH (which see).

AME - authorized medical examiner. A doctor approved by the CAA (or foreign licensing authority) to conduct examinations for the issue or renewal of aircrew medical certificates.

amsl - above mean sea level (sometimes asl in USA).

ANO - Air Navigation Order. Statutory legal instrument defining the laws of air navigation, pilot licensing etc, in the UK. Other aviation legislation includes the Rules of the Air and Air Traffic regulations and the Air Navigation (General) Regulations

AoA - angle of attack. Also alpha, thus 'high alpha', high angle of attack.

AOA - Airport Operators' Association.

AOC - Air Operator's Certificate, issued by the CAA and required by aircraft operators flying scheduled or charter public transport flights, including cargo, air-taxi and pleasure-flying work. Also Air Officer Commanding in RAF parlance.

AoE - airport of entry (usually in USA).

AOG - aircraft on ground, a term used to denote urgency when requesting spares or service from suppliers or manufacturers, meaning that the aircraft cannot fly again until the parts have been supplied.

AOPA - Aircraft Owners and Pilots Association.

A/P - airport or autopilot.

A & P - Airframe and Powerplant Mechanic (USA).

APA - Aerodromes Protection Agency, the federation of associations for the advancement of British general aviation facilities.

APHAZ - Aircraft Proximity Hazards Assessment Panel, which investigates near-miss reports filed by air traffic controllers (see also JAWG).

APP - Approach (control).

APU - auxiliary power unit. Large transport aircraft and some business jets have an APU, typically a small turbine, to provide power for engine-starting and for running systems when on the ground, obviating the need for external power or ground power unit, GPU.

ARB - Airworthiness Review Board.

ARCAL - aircraft radio control of aerodrome lighting.

Aresti - key shorthand notation system devised by Spaniard José L Aresti whereby aerobatic display routines or competition sequences can be drawn up on paper like a musical score. Now largely superseded by the simpler FAI Aerobatic Catalogue system.

ARINC - Aeronautical Radio Incorporated. A non-profit corporation owned by airlines to set standards for airline avionics and provide communications services.

ARP - aerodrome reference point.

ASDA - accelerate-stop distance available.

ASI - airspeed indicator, a flight instrument which measures the speed of an aircraft through the air.

ASR - altimeter setting region, a geographical area for which the lowest value of QNH (which see) is forecast hourly and relayed by air traffic control centres. Also airport surveillance radar and air-sea rescue.

ATA - actual time of arrival. Also Air Training Association.

ATC - air traffic control.

ATCO - air traffic control officer. Also ATCA, air traffic control assistant; ATCC, air traffic control centre; SATCO, senior air traffic control officer.

ATIS - automatic terminal information service, a continuous recorded broadcast of routine non-control airport information, usually at large airports.

ATPL - Air Transport Pilot's Licence, needed to act as pilot-in- command of a commercial air transport aircraft exceeding 20,000 kg all-up weight.

ATS - air traffic service. Also ATSU, ATS Unit.

ATSORA - air traffic services outside regulated airspace.

ATZ - Aerodrome Traffic Zone. An area of protected airspace surrounding an aerodrome bounded by a circle of 2 nm or 2.5 nm radius (depending on runway length) centred on the mid-point of the longest runway. Permission is required for entry into and movement in an ATZ.

AUW - all-up weight, a term for the total loaded weight of an aircraft, made up of empty weight plus useful load; maximum auw is the maximum allowable weight, including fuel and payload, specified in an aircraft's Certificate of Airworthiness. Sometimes referred to (in USA especially) as gross weight and maximum gross weight respectively. Also MTWA, maximum total weight authorised; BOW Basic operating weight, the weight of an aircraft with all equipment, lubricants, fuel and operating crew, but without payload; MLW, maximum landing weight, above which fuel must be burned off or jettisoned before landing or there may be risk of structural damage.

avgas - aviation gasoline, usually followed by the octane rating. Used by piston-engined aircraft. Also LL, low lead.

avtur - aviation turbine fuel (kerosene). Used by turboprops and jets.

Awy - airway.

---

B ---------- back to the Top

BAA - British Airports Authority.

BAeA - British Aerobatic Association.

BALPA - British Airline Pilots Association.

BAUA - Business Aircraft Users Association.

BCAR - British Civil Air Requirements. Airworthiness standards laid down by the CAA for certification of aircraft on the UK Civil Aircraft Register.

BCP - break cloud procedure.

BCPL - Basic Commercial Pilot's Licence, the minimum qualification necessary in the UK to receive payment for acting as a pilot. Also CPL, Commercial Pilot's Licence, and SCPL, Senior Commercial Pilot's Licence (no longer issued).

beta mode - manually-controlled mode for CS propellers on turboprop aircraft enabling reverse pitch to be selected for braking or to aid ground manoeuvring.

BFR - Biennial Flight Review (USA). Holders of U.S. FAA Private Pilot Licences must undergo a flight check with an FAA-approved examiner every two years. No UK equivalent.

BGA - British Gliding Association.

BHAB - British Helicopter Advisory Board

BHPA - British Hang-Gliding and Paragliding Association.

BMAA - British Microlight Aircraft Association.

Bottlang - Loose-leaf Euopean airfields manual for VFR operations.

BRG - bearing, the horizontal direction to or from any point expressed in degrees of the compass.

BWPA - British Women Pilots Association.

---

C ---------- back to the Top

C - Celsius (temperature) or compass.

CAA - Civil Aviation Authority.

CAD/CAM - computer-aided design/manufacture.

CAAFU - Civil Aviation Authority Flying Unit, based at Stansted Airport, which performs such tasks as navaid checking and calibration and also examines candidates for instrument ratings and commercial pilot's licences.

CANP - Civil Aviation Notification Procedure. A voluntary system whereby civil operators notify their intention to fly at low level (at or below 1,000 feet agl, when crop-spraying or powerline inspecting for example), aimed at avoiding conflict with low-flying military aircraft.

CAP - Civil Air Publication. Information booklets issued by the CAA, e.g. CAP 53 The Private Pilot Licence.

CAS - calibrated airspeed -- indicated airspeed corrected for air density and compressibility.

CAT - clear-air turbulence. Also CATegory when referring to certain instrument landing systems which require special aircraft instrumentation, certification and pilot qualification beyond those needed for standard instrument approaches (e.g. a CAT IIIC ILS permits operation down to the surface of the runway without external visual reference, true zero-zero operation).

CAVOK - pronounced CAV-okay (ceiling and visibility OK), visibility at least ten kilometres, with no cloud below 5,000 feet, with no Cbs, precipitation, thunderstorms, shallow fog or low drifting snow.

CAVU - ceiling and visibility unlimited. Cloudless (or scattered cloud) conditions with visibility in excess of ten kilometres.

CDI - course deviation indicator. The vertical needle of a VOR indicator which shows the aircraft's position relative to the selected VOR radial.

CDU - control display unit.

ceiling - height above ground or water of the base of the lowest layer of cloud below 20,000 feet which covers more than half of the sky. An aircraft's service ceiling is the density altitude (which see) at which its maximum rate of climb is no greater than 100 feet per minute. Its absolute ceiling is the highest altitude at which it can maintain level flight.

CFI - chief flying instructor (certified flying instructor in USA).

CH - compass heading.

CHIRP - Confidential Human Factors Incident Reporting system, whereby professional pilots and ATC staff may report in confidence incidents arising from human errors for analysis by the RAF Institute of Aviation Medicine at Farnborough.

CHT - cylinder head temperature (gauge). A device which, by means of a probe(s) gives a cockpit readout of the temperature of one or more of an aircraft engine's cylinder heads.

circuit - pattern around which aircraft fly when arriving at an airfield, usually rectangular in UK but not necessarily elsewhere. The circuit (pattern in USA) is aligned with the active runway and may be either left- or right-handed. Dead side is the opposite side of the circuit pattern in operation from which arriving aircraft join for landing. See also finals, below.

C/L - centre-line (of a runway, for example).

Clearance - authorization from air traffic control to proceed as requested or instructed. Used for ground and air manoeuvring, thus "cleared for take-off", "cleared flight-planned route", "cleared to descend" etc.

Clouds - commonly-used abbrevitaions for cloud types :-

C of A - Certificate of Airworthiness issued by the CAA indicating that an aircraft meets the Authority's airworthiness standards. Cs of A are issued to individual aircraft, and also to generic aircraft types (Type Certification in the USA) when the first example of a type is registered. Cs of A on individual aircraft are granted in several categories, e.g. Private, Public Transport, Aerial Work etc. and much be renewed at intervals. Permits to Fly are authorisations granted to aircraft such as homebuilts, vintage aeroplanes, warbirds and some simple classic light aircraft which are not required to meet the standards demanded for a full C of A, and are accordingly restricted in the kinds of operation for which they may be used.

C of E - Certificate of Experience, valid for private pilot licence holders for thirteen months, and renewed by flight test or evidence of completing the minimum required flying experience in the preceding thirteen-month period. Also C of T, certificate of test.

c of g - centre of gravity. The point on an aircraft through which the entire aircraft's weight may be assumed to act (i.e. around which the aircraft, if suspended, would balance). C of G limits are the most forward and rearward positions of the C of G permitted for safe operation. An aircraft loaded outside its C of G limits can be difficult or impossible to control.

com(m) - communication(s)

C of P - centre of pressure, the point through which the total effect of lift may be said to act on an aeroplane.

CPL - Commercial Pilot's Licence

CR or C/R - counter-rotating. Usually in general aviation referring to twin-engined aircraft with `handed' engines whose propellers turn in opposite directions to eliminate propeller torque effect.

CRP - compulsory reporting point.

CRT - cathode ray (television) tube. Used in flight deck displays of new-generation airliners, business aircraft and military jets instead of conventional instruments. See also EFIS, below.

critical altitude - the highest density altitude at which it is possible to maintain the maximum continuous rated power or manifold pressure of an aero engine.

critical engine - the engine on a multi-engined aircraft whose failure would most seriously effect performance or handling of the aircraft, through asymmetric effects or loss of power to systems such as hydraulics.

CRS course - the intended direction of flight in the horizontal plane expressed in degrees of the compass.

c/s - call sign.

CS - constant-speed (propeller). A variable-pitch propeller which maintains constant rpm by automatically changing blade angle. Also CSU, constant-speed unit.

CTA - Control Area. An area of controlled airspace extending upwards from specified limit agl.

CTR - Control Zone. An area of controlled airspace extending upwards from ground level to a specified upper limit.

CVR - cockpit voice recorder. A tape recorder installed on the flight decks of commercial transport aircraft and helicopters and some business aeroplanes to record crew conversation, RT transmissions and cockpit background noises (e.g. trim-wheel operation, flap motor running) in case required for incident or accident investigation.

CW - carrier wave or continuous wave.

CZ - Control Zone (USA).

---

D ---------- back to the Top

DA - Danger Area. Also DACS, Danger Area Crossing Service, and
DAAIS, Danger Area Activity Information Service.

D & D - Distress & Diversion Cells at Air Traffic Control Centres. RAF units which provide a 24-hour listening watch on VHF and UHF emergency frequencies and can locate and assist pilots who are lost or in emergency situations.

deadstick - descent and landing with engine(s) shut down and propeller(s) stopped.

DCT - direct

Decca - hyperbolic area navigation system, originally devel,oped for maritime use, based on signals received from a chain of master and slave ground stations located in northern Europe.

density altitude - pressure altitude corrected for air temperature.

DETRESFA - distress phase of search-and-rescue operation.

DF - direction-finding. A DF bearing can be provided by airfields or other facilities such as D & D cells (above) having suitable direction-finding equipment to locate an aircraft.

DH - decision height. The height on a precision approach at which a pilot must have the runway approach lights in sight to continue the descent, or if not, must initiate a go-around.

DI - direction indicator. A gyro instrument which indicates the magnetic heading of an aircraft. The DI, also known as the directional gyro (DG), is free of the turning errors associated with magnetic compasses but is prone to precession (wander) and must be reset against the magnetic compass at intervals. ALSO -
DI - is also used to refer to the daily inspection -- a thorough pre-flight check of an aircraft prior to the first flight of the day.

DME - distance-measuring equipment. A combination of ground and airborne equipment which gives a continuous slant range distance-from-station readout by measuring time-lapse of a signal transmitted by the aircraft to the station and responded back. DMEs can also provide groundspeed and time-to-station readouts by differentiation.

Doppler - Doppler effect (or shift) is the change in frequency of light, radio or sound waves when source and receiver are in relative motion.

DoT - Department of Transport.

DP - dew point

DR - dead (deduced) reckoning. Plotting position by calculating the effect of speed, course, time and wind against last known position.

dry - when referring to aircraft hire charges means `without fuel', as opposed to wet, with fuel.

DZ - dropping zone, for parachuting etc.

---

E ---------- back to the Top

EAA - Experimental Aircraft Association, the American homebuilders' organisation.

EADI - electronic attitude director indicator. An ADI with CRT cockpit display forming part of an EFIS, below.

EAT - estimated approach time.

EBAA - European Business Aircraft Association.

ECAC - European Civil Aviation Conference.

ECOGAS - European Council of General Aviation Support.

ECU - environmental control unit.

EET - estimated elapsed time.

EFAS - electronic flash approach light system.

EFATO - engine failure at (or after) take-off.

EFIS - electronic flight instrument system, in which multi- function CRT displays replace traditional instruments for providing flight, navigation and aircraft systems information, forming a so-called `glass cockpit'. Now common in commercial transports, corporate aircraft and helicopters, military fighters and some GA piston singles and twins.

EGT - exhaust gas temperature (gauge). A device which provides a cockpit readout of the exhaust gas temperature of an aircraft's (piston) engine(s), enabling the pilot to lean the mixture for maximum fuel efficiency.

EHSI - electronic horizontal situation indicator. CRT-based HSI forming part of an EFIS.

EICAS - engine indicating and crew alerting system. CRT display which monitors engine performance and alerts the crew to system or airframe failure. Found in new-generation transports and business jets.

ELT - emergency locator transmitter. A small radio transmitter fixed to an aircraft's structure which is automatically activated by impact or water immersion and transmits a code on emergency frequencies enabling SAR satellites or search units equipped with DF to locate the crash or ditching site. Carriage mandatory in the USA, but not in UK. Sometimes styled ADELT, automatically deployable ELT, or ELB, emergency locator beacon.

empty weight - weight of the basic aeroplane including all fixed equipment, plus unusable fuel, oil, hydraulic and other fluids.

encoding altimeter - an altimeter which gives a digital output to the transponder (which see) for automatic transmission of the aircraft's pressure altitude to ATC.

EOBT - estimated off-blocks time.

EPNdB - effective perceived noise decibel. Unit of measurement of aircraft noise levels.

ETA - estimated time of arrival. Also ETD, estimated time of departure; ETE, estimated time en route.

EROPS - extended range operations, usually long over-water flights by twin-jet airliners.

ETPS - Empire Test Pilots School, based at the Aircraft & Armament Experimental Establishment, Boscombe Down.

Eurocontrol - organisation, headquartered in The Netherlands, comprising Belgium, France, Germany, Ireland, Luxembourg, Netherlands and UK for coordinating enroute air traffic control in Europe.

---

F ---------- back to the Top

FAA - Federal Aviation Administration, USA, their equivalent of our CAA.

FADEC - full-authority digital engine control.

FAF - final approach fix, the point at which a published instrument approach begins.

FAI - Fédération Aéronautique Internationale, the international body for verification of aeronautical record attempts and sporting regulations.

FAR - Federal Aviation Regulations (USA).

FARA - Formula Air Racing Association.

FBO - fixed-base operator, an American term for commercial operators supplying fuel, maintenance, aircraft sales, rental, flight training, handling and other general aviation services at an airport. (So-called because the the first FBOs were early barnstormers who chose to settle at one field.)

FBW - fly-by-wire. Also FBL, fly-by-light. Aircraft control systems in which pilots' control inputs are transmitted to control surfaces electronically or via fibre optics rather than by mechanical linkage.

FCL - Flight Crew Licensing (Division), a CAA department handling all aspects of private and professional pilot, flight engineer and navigator licensing.

fcst - forecast.

FDR - flight data recorder, popularly known as a `black box' (actually painted bright orange), by which various parameters of an aircraft's flight performance are recorded for analysis in the event of an incident or accident.

feather (of a propeller) - to set the angle of CS or VP propeller edge-on to the airflow to minimise drag and rotation following engine failure on multi-engined aircraft. Also applies to motor gliders which have feathering propellers to enhance engine-off soaring performance.

final(s) - final approach. The part of a landing sequence or aerodrome circuit procedure in which the aircraft has made its final turn and is inbound to the active runway. Downwind is the segment of the circuit paralleling the runway and flown on a reciprocal heading. Base leg is the crosswind segment bringing the aircraft from the downwind leg to final approach. The leg before downwind is called the Crosswind leg.

FIC - Flight Information Centre.

FIR - Flight Information Region. UK airspace is divided into two FIRs, London and Scottish.

FIS - Flight Information Service, providing a variety of services and information (but not control) to air traffic in the two FIRs above.

FJ - fast jet.

FL - flight level, a level of constant atmospheric pressure shown by an altimeter set to a standard 1013.2 millibars, expressed in rounds hundreds of feet, thus FL330 is 33,000 feet.

flag - warning signal incorporated in certain navigation and flight instruments indicating that the instrument is not operating satisfactorily or that the strength of signals being received from ground stations is below acceptable limits.

flat rating - throttling or other restriction of engine power ouput (usually in turboprops and turboshafts) at sea level to enable it to give constant predictable power at higher operating altitudes.

flameout - combustion failure in a turbine engine resulting in power loss.

flicker effect - nausea, dizziness or vertigo which can be brought on by flickering at certain frequencies of a bright light source such as sunlight or strobe when viewed through a rotating propeller or rotor blades.

FMS - flight management system.

FOD - foreign object damage, usually to turbine engines through ingestion of runway debris etc.

fpm - feet per minute, a measure of an aircraft's rate of climb or descent. Similarly m/s or mps, metres per second.

FSS - Flight Service Station (USA).

FTO - flying training organisation.

---

G ---------- back to the Top

g - the acceleration force of gravity, normally 1g on earth. Zero g (0g) is weightlessness, as experienced by orbiting astronauts. g is expressed as positive (+) and negative (-) values, During a normal loop a pilot experiences positive g, tending to force him down in his seat. In an outside loop, with the pilot's head on the outside of the vertical circle, negative g forces him up against his straps. Aircraft structural load limits are expressed in positive and negative values, the positive limit usually greater than negative, except in specialist aerobatic types.

g-loc - g-induced loss of consciousness. Pilot blackouts caused by excessive g or by too-rapid onset of g-forces. Experienced mostly by pilots of high-performance military jets and competition aerobatic aircraft, has led to fatal crashes.

GA - general aviation, all flying other than airlines and the military.

GAAC - General Aviation Awareness Campaign.

GAFOR - General Aviation Visual Flight Forecast. Met briefing service in operation in France, Germany, the Netherlands and some other European countries.

GAMA - General Aviation Manufacturers Association, an American trade organisation.

GAMTA - General Aviation Manufacturers & Traders Association, UK.

GAPAN - Guild of Air Pilots and Air Navigators.

GASIL - General Aviation Safety Information Leaflet. Monthly safety and accident prevention bulletin for pilots and engineers published by the CAA.

GCA - ground-controlled approach. A landing approach in which a ground controller gives verbal guidance in azimuth and elevation to a pilot using precision approach radar (PAR) to monitor the aircraft's approach path. Still used by the military, but defunct in civil aviation.

GFT - General Flying Test, taken by student pilots to qualify for the PPL, and also by candidates for the BCPL and CPL. Also NFT, navigation flight test, which is part of the practical examination for the PPL.

Glonass - Russian equivalent of GPS/Navstar satellite navigation system.

gnd - ground

GNSS - Global Navigation Satellite Systems. go-around to climb away from a runway after making an approach, either to make a further attempt at landing or to divert to an alternate airport (formerly `overshoot').

Goxio - VHF direction-finding (France)

GP - glidepath

gph - gallons per hour, an expression of fuel consumption or fuel flow (FF) in either imperial or U.S. gallons. Usually lb/hr for turbine-powered aircraft.

GPS - Global Positioning System (Navstar). A U.S. developed satellite-based high-precision navigation system, intended primarily for military use but now in widespread use by commercial and private operators, though with reduced accuracy compared with military versions.

GPWS - ground proximity warning system. A radar-based flight- deck system to give pilots audible warning by means of horns, hooters, taped or synthetic voices of terrain close beneath an aircraft's flight path.

GRADU - gradual (term used in Met reports).

GRP - glassfibre-reinforced plastic; also CFRP, carbon-fibre reinforced plastic. Composite materials seeing increasing use in entire airframes for GA aircraft (e.g. Beech Starship) and for components for helicopters, airliners and military aircraft.

GS - glideslope. The vertical guidance part of an instrument landing system which establishes a safe glidepath (usually three degrees) to a runway.

G/S - groundspeed. The speed an aircraft makes over the ground, a product of its airspeed and wind speed.

---

H ---------- back to the Top

H24 - continuous round-the-clock operation. Also HJ operates during day; HN operates during night; HO operates during times to meet operational requirements, and HX no specific operational hours.

HAA - Historic Aircraft Association.

HAI - Helicopter Association International.

half-mill(ion) - 1:500,000 scale ICAO aeronautical chart.

Hdg - heading. The direction in which an aircraft's nose points in flight in the horizontal plane, expressed in compass degrees.

Heavy - suffix used in RT callsigns to indicate that the aircraft is a large transport, alerting controllers and following aircraft to the possibility of wake turbulence (which see).

Hectopascal (hPa) - unit of pressure measurement, equivalent to one millibar (which see), now the ICAO-standard for altimeter setting.

HEMS - helicopter emergency medical services.

Hertz - standard radio equivalent of frequency in cycles per second. See also kHz and MHz.

HF - high-frequency band, used for long-range radio communications in the 3-30 MHz range.

Hg - inches of mercury, a unit of pressure measurement.

HIAL - high intensity approach lighting.

HIRF - high intensity radiated (electromagnetic) fields.

HIRL - high intensity runway lighting.

HISL - high intensity strobe light.

holding pattern - racetrack-shaped manoeuvre which keeps aircraft within a specified airspace while awaiting further clearance from air traffic control.

hot-and-high - airfield conditions of high altitude and high ambient temperatures that can severely limit aircraft performance. See also density altitude.

HOTAS - hands on throttle and stick. Ergonomic cockpit design technology, originally developed for military combat aircraft, enabling a pilot to fly the aircraft and manage all navigation, weapons and other systems from control column/throttle lever hand grips.

HRA - Highlands Restricted Area

HSI - horizontal situation indicator. A cockpit navigation display, usually part of a flight-director system, which combines navigation and heading.

HUD - head-up display. A method of projecting instrument readouts or data which enables a pilot to see them while looking through the aircraft's windscreen. Mostly used on military aircraft, but now in service on some commercial airliners.

---

I ---------- back to the Top

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. (see also CAS, RAS and TAS.

IATA - International Air Transport Association.

i/c - intercom

ICAO - International Civil Aviation Organisation.

IF - instrument flying.

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-slope defines the correct vertical descent profile. Marker beacons and high intensity runways lights are also part of the ILS.

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 - instrument rating.

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 15øC. 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.

---

J ---------- back to the Top

JAA - Joint Aviation Authority.

JAR - Joint Aviation Requirements, being drawn up by eighteen European states, aimed at unifying airworthiness, flight crew licensing and other criteria among signatory nations.

JAWG - Joint Airmiss Working Group. A civilian/military committee which reviews and reports on all airmisses which occur in UK airspace.

Jeppesen - U.S.-developed navigational/approach chart system with worldwide coverage, similar to British Aerad system.

JPATS - Joint Primary Aircraft Training System. A USAF/U.S. Navy programme to select a common aircraft design as a basic pilot trainer for the two services, due to be decided next year.

---

K ---------- back to the Top

kHz - kilohertz, the frequency of a radio carrier wave measured in thousands of cycles per second. 1 kHz = 1,000 Hertz.

knot (kt) - one nautical mile per hour (never one knot per hour), the standard unit of aviation speed measurement. One knot equals 1.1515 mph; one nautical mile equals 6,080 feet.

kW - kilowatt.

---

L ---------- back to the Top

LARS - Lower Airspace Radar Advisory service, available to all aircraft flying in uncontrolled UK airspace from 3,000 feet amsl to FL95. See also MMARS, RAS and RIS, below.

LAMS - Light Aircraft Maintenance Schedule, the CAA-approved schedule for fixed-wing light aircraft below 2,730 kg auw whose Cs of A are valid for three years subject to compliance with LAMS.

lat - latitude.

LATCC - London Air Traffic Control Centre at West Drayton, near Heathrow.

lczt - (ILS) localizer (USA)

LDA - landing distance available.

LF - low-frequency radio waves with frequencies in the 30-300 kHz band.

LITAS - low-intensity two-colour approach system.

LLTV - low light level television.

LLZ - localizer ((USA).

LOC - localiser. The azimuth guidance portion of an instrument landing system.

locator - medium-frequency non-directional radio beacon used as an aid to establishing yourself on final approach during an instrument landing procedure. Also LOM, locator outer marker.

lon(g) - longitude

Loran - low-frequency hyperbolic radio long-range navigation system which measures time difference between reception of synchronised signals transmitted from ground transmitters. Loran-C, operates in the 100-110 kHz frequency band with an operating range of 600-1,500 nm independent of line-of-sight, and is becoming very popular among GA aircraft operators in the USA.

---

M ---------- back to the Top

M or mag - magnetic

Mach number - ratio of true airspeed to the speed of sound. Mach 1 is the speed of sound at sea level, ISA, approximately 1,100 feet per second or 760 mph.

MAP - missed approach point. The point on a published ILS approach expressed in time or distance from the final approach fix, or as an altitude on the glideslope, at which the missed approach procedure must be initiated if the runway or approach lights are not clearly in sight.

marker beacons (mkrs) - part of an instrument landing system using 75 MHz transmitters emitting fan-shaped or elliptical signal patterns vertically upwards, defining specific points along the glideslope. The outer marker OM is situated at or near the glideslope intercept altitude of the ILS localiser, the middle marker (MM) defines a point on the glideslope at or near decision height (DH). Markers provide aural and visual indications on a cockpit marker beacon receiver.

MATZ - military aerodrome traffic zone. An area of protected airspace surrounding certain military airfields which normally extends for a five nautical mile radius around the airfield and upwards from the surface to 3,000 feet above aerodrome level. A 'stub' projection protects the final approach path to the main runway outwards from the MATZ boundary, enclosing an area two nautical miles either side of the extended runway centreline and upwards from 1,000 feet above the surface to 3,000 feet above aerodrome elevation. Although recognition of a MATZ is not mandatory for civil aircraft, all MATZ aerodromes offer a penetration service to civil traffic during operational hours.

Mayday - international radio distress call (from the French, m'aidez -- help me). It signifies imminent danger to life requiring immediate assistance.

mb - millibar.

MDA - minimum descent altitude. The lowest altitude, in feet amsl, to which descent is authorised on final approach during a non-precision instrument landing (i.e. where no glideslope guidance is given) without visual reference to the runway.

MDH - minimum descent height, agl.

MEDA - military emergency diversion airfield.

Met - meteorology, weather.

METAR - coded aerodrome Met report. Also SPECI, special civil aviation weather report, and TAF, terminal aerodrome forecast of weather expected.

MF - medium frequency. Radio waves with frequencies in the 300- 3,000 kHz range.

MFA - military flying area

MFD - multi-function display. An EFIS CRT offering selectable displays of weather radar, navigation maps, checklists and data other than primary flight information.

MH - magnetic heading

MHz - Megahertz, the frequency of radio carrier waves measured in millions of cycles per second.

minimums - weather condition requirements for a particular mode of flight (e.g. for VFR operation, IFR take-offs and landings).

MLS - microwave landing system. A microwave-based instrument approach system intended to replace ILS in the 1990s and claimed to offer a number of advantages such as the ability to fly segmented and curved precision approaches.

MMARS - Military Middle Airspace Radar Service, available to military and civil aircraft operating in UK airspace between FL100 and FL245.

MOA - military operations area.

mogas - car fuel, approved for use in some light aircraft subject to certain conditions.

MPA - man-powered aircraft

MSA - minimum sector altitude or minimum safe altitude.

msl - mean sea level

MTBF - mean time between failures. Also MTTR, mean time to repair

MTMA - military terminal control area.

---

N ---------- back to the Top

NATS - National Air Traffic Services. A division of the CAA providing UK air traffic control.

NBAA - National Business Aircraft Association, USA.

NDB - non-directional beacon. A medium-frequency navigational aid which transmits non-directional signals, superimposed with a Morse code identifier and received by an aircraft's ADF.

nm - nautical mile.

NOE - nap of earth. Low flying, usually by the military, using contour-flying techniques and terrain-masking to avoid being seen.

NORDO - no radio (used on flight plan form).

NOSIG - no significant change, term used on Met reports.

Notam - Notices to Airmen, issued by the CAA (and equivalent authorities elsewhere) to inform pilots of new or changed aeronautical facilities, services, procedures or hazards, temporary or permanent. Also SNOWTAM, a NOTAM concerning runway conditions in snow.

NOTAR - no tail rotor. A system patented by McDonnell Douglas for maintaining directional control of helicopters without use of an anti-torque tail rotor.

NPRM - Notice of Proposed Rule-Making (USA). Advance publication by the FAA of proposed changes or additions to Federal Air Regulations.

NTSB - National Transportation Safety Board. U.S. equivalent of UK's AAIB.

---

O ---------- back to the Top

OAT - outside air temperature. The temperature of the air outside an aircraft measured by a probe with a cockpit gauge readout. OAT affects the measurement of indicated airspeed and its value is needed to calculate true airspeed. At high speeds kinetic heating demands correction to the indicated OAT for true outside air temperature.

OATS - Oxford Air Training School, a large commercial pilot training school at Oxford Airport, UK.

obst - obstruction.

OBS - omni-bearing selector, part of a VOR used to select the radial from a VOR.

OCH - obstacle clearance height. The lowest height above the elevation of the runway threshold or above aerodrome elevation used to establish compliance with obstacle clearance criteria in an instrument approach. Also OCA, obstacle clearance altitude, and OCL, obstacle clearance limit.

OEM - original equipment manufacturer.

okta - a measurement of cloud cover. One okta means one-eighth of the sky is covered.

Omega - high accuracy, very-low frequency (VLF) long-range navigation system of the hyperbolic type, covering the entire earth down to the surface from eight ground-based transmitters. Used principally by airliners, military aircraft and intercontinental business aircraft.

o/r - on request.

o/t - other times.

---

P ---------- back to the Top

Pan - international radio call signalling urgency.

PAPI - precision approach path indicator, a system of coloured lights installed at the approach end of a runway which provides visual guidance to the correct glidepath. A successor to VASI, below.

PAR - precision approach radar. Primary radar equipment showing an air traffic controller the height, track and range of an aircraft on final approach, enabling him to guide it to a landing.

pax - passengers.

PFA - Popular Flying Association, the UK homebuilt and antique aircraft organisation.

PIC - pilot-in-command (also styled P1).

PINS - Pipeline Inspection Notification System.

PIO - pilot-induced oscillation. An undulating flight path brought about by over-controlling.

PLN - flight-plan.

PMS - performance management system.

POB - (number of) persons on board. Also SOB, souls on board.

POC - proof-of-concept.

POH - pilot's operating handbook, an aircraft's `owner's manual'.

Pooley's - annually-published flight guide to United Kingdom and Ireland, named after its creator and publisher Bob Pooley.

PPL - Private Pilot's Licence. Also PPL(H) for helicopters, PPL(SLMG) for self-launched motor gliders.

PPO - prior permission only. Certain airfields or events require advance notification (by telephone, for example) of your intended arrival. Also PNR, prior notice required, and PPR, prior permission required.

PROB - probability percentage, term used in Met reports.

procedure turn - manoeuvre which reverses the direction of an aircraft's flight during an instrument approach procedure to enable it to intercept the final approach course.

psi - pounds per square inch, a measurement of pressure.

PTT - press-to-transmit (switch) on an aircraft's control wheel or stick enabling the pilot to make RT transmission 'hands on' via a headset microphone.

Purple Airspace - special temporary airways created for flights by certain members of the royal family, notified by NOTAM.

---

Q ---------- back to the Top

QFI - Qualified Flying Instructor. Also QHI, qualified helicopter instructor.

Quadrantal Rule - system of cruising altitudes used in UK uncontrolled airspace below FL250.

Q-code - code system developed when air-to-ground communication was by wireless telegraphy, enabling many routine phrases and questions to be reduced to three letters. Now largely redundant, except these:

---

R ---------- back to the Top

rabbit lights - colloquialism for sequentially flashing lead-in runway approach lights.

ramp weight - maximum permissible weight of an aircraft, which exceeds maximum take-off weight by an allowance for fuel burned during engine-start and taxi.

RAPID - change expected to take place in thirty minutes or less, term used in mer reports.

RAS (1) - rectified airspeed. Indicated airspeed corrected for instrument position error.

RAS (2) - Radar Advisory Service. Provided outside regulated airspace to notify pilots of conflicting traffic and to advise suitable avoiding action. Also RASA Radar Advisory Service Area.

rating - add-on qualification to a pilot's licence, e.g. Night Rating, Multi-engine Rating, Instrument Rating, Seaplane Rating etc. Individual Type Ratings are necessary to fly aircraft over 12,500 pounds MTWA.

RCL - runway centre-line.

RBI - relative bearing indicator, displaying information from the ADF.

RDO - radio.

RIS - Radar Information Service. Provided to notify pilots of conflicting traffic outisde regulated airspace, but offering no avoiding action.

RMI - radio magnetic indicator. A navigation aid which combines DI, VOR and/or ADF display and will indicate bearings to stations, together with aircraft heading.

RMK - remark(s).

RMU - radio management unit.

Rnav - area navigation. A system of radio navigation which permits direct point-to-point off-airways navigation by means of an on-board computer creating phantom VOR/DME transmitters termed waypoints.

RON - remain over night (night-stop).

RT - radio telephony. Voice communications, as opposed to WT, wireless telegraphy. Also styled RTF.

RVR - runway visual range, a horizontal measurement of visibility along a runway.

rwy - runway.

Rx - receiver.

---

S ---------- back to the Top

SACP - (CAA) Standing Advisory Committee on Pilot Licensing.

SAR - search-and-rescue. Also Sarsat, SAR satellite.

SAS - stability augmentation system. An automatic flight control system employed in many helicopters and some fixed-wing aircraft to enhance their stability and handling qualities.

satcoms - satellite communications, now being introduced on intercontinental airliners and business jets for (non- operational) air-to-ground voice communications via ground relay stations.

SB - Service Bulletin. Advisory notices issued by aircraft, engine and equipment manufacturers alerting owners and engineers to faults or problems requiring preventitive or remedial maintenance or modification. Often termed `mandatory', but do not have the legal force of Airworthiness Directives (which see).

SBAC - Society of British Aerospace Companies. The UK aerospace manufacturers' trade association.

SDAU - Safety Data Analysis Unit of the CAA.

`second pilot' - unofficial term used to describe short (usually 8-10 hours) flying courses designed to enable non-pilot light aircraft passengers to take control and land in an emergency such as pilot incapacitation. Also standby or safety pilot and pinch-hitter (U.S.)

Sectional - (U.S.) VFR navigation chart, equivalent to our 1:500,000 or `half-million'.

Semi-circular - system of cruising altitudes.

SELCAL - selective calling. A high-frequency system enabling air traffic control to alert a particular aircraft, by means of flashing light or aural signal in the cockpit, for receipt of a message without the crew having to maintain a listening watch. Used on long-haul over-ocean airline routes and by intercontinental bizjets.

sfc - specific fuel consumption of an engine, expressed in pounds of fuel consumed for each unit of power (hp, shp, lb/st) produced. Also surface.

SID - standard instrument departure. A standard IFR departure route enabling air traffic controllers to issue abbreviated clearances and thus speed the flow of traffic.

SIGMET - warning of severe weather conditions (active thunderstorms, hail, severe turbulence, icing etc.) issued my Met offices.

sl - sea level.

SMOH - since major overhaul. Term used in aircraft for sale advertisements where engine hours are quoted (see TBO). Also STOH, since top overhaul, TTSN, total time since new; TTAF/E, total time airframe/engine,

SMR - surface movement radar.

SOB - souls on board, the number of persons on board an aircraft. Also POB.

socked-in - A colloquialism referring to an airport closed to air traffic by bad weather, similarly clamped.

SOP - standard operating procedure.

specific range - measure of an aircraft's fuel efficiency, expressed as nautical miles flown per pound of fuel burned (nm/lb)

SPL - Student Pilot's Licence. No longer issued in the UK, where a CAA medical certificate serves as an SPL.

squawk - to transmit an assigned code via a transponder (see SSR below).

SR - sunrise.

SRZ - Special Rules Zone. An area of protected airspace surrounding an airfield and extending from the surface upwards to a specific level which affords safety to air traffic movements in the vicinity of airfields whose traffic level does not warrant the establishment of a Control Zone. Also SRA, Special Rules Area. extending vertically and horizontally from a level above the surface, but not necessarily terminating at the same upper level as the SRZ.

SRA - Surveillance Radar Approach. Also Special Rules Area.

SRE - Surveillance Radar Element of a GCA.

SS - sunset.

SSB - single sideband. Reduction of bandwith by transmitting only one sideband and suppressing the other, and usually also the carrier wave.

SSR - secondary surveillance radar. A radar system comprising a ground-based transmitter/receiver which interrogates a compatible unit in the aircraft (see transponder below), providing instant radar identification without having to manoeuvre. Assigned four-digit transponder codes are referred to as squawk codes.

STAR - Standard Terminal Arrival Route, for inbound IFR traffic.

STC - Supplemental Type Certificate. U.S. system for post-type certification approval of aircraft modifications such as re- engining, STOL kits, etc, where the full certification process is not deemed necessary. Also used by manufacturers to certify (often greatly changed) new models of old types under so-called `grandfather rights'.

STOL - short take-off and landing. Also VTOL, vertical take-off and landing; V/STOL, vertical/short take-off and landing; STOVL, short take-off, vertical landing.

---

T ---------- back to the Top

T - true. Also TH, true heading, and TT, true track.

TACAN - tactical air navigation system. An ultra-high frequency electronic navigation aid which provides suitably-equipped aircraft with a continuous indication of bearing and distance to the selected Tacan station. The distance element can be received by civilian DMF equipment, but otherwise Tacan is principally a military navaid.

TAF - Terminal Area Forecast.

TAS - true airspeed. Rectified airspeed corrected for altitude and outside air temperature.

TBO - time between overhauls, an engine manufacturer's recommended overhaul interval in hours, a rough and not guaranteed guide to life expectancy of an aero-engine before it will need overhaul.

TCA - Terminal Control Area (USA).

TCAS - traffic alert and collision avoidance system. U.S. developed radar-based airborne collision avoidance system operating independently of ground-based equipment. TCAS-I generates traffic advisories only, TCAS-II provides advisories and collision avoidance instructions in the vertical plane.

TEMPO - temporarily, term used in Met reports.

THR or thld - threshold.

TMA - Terminal Control Area. An area of controlled airspace at the intersection of airways in the vicinity of control zones (CTRs) around major airports.

TO - take-off (sometimes TKOF).

TODA - take-off distance available. Also TODR, take-off distance required, and TORA, take-off run available.

track - actual flight path of an aircraft over the ground.

transponder - airborne receiver/transmitter portion of the SSR system which receives the interrogation signal from the ground and automatically replies according to mode and code selected. Modes A and B are used for identification, using a four-digit number allocated by air traffic control. Mode C gives automatic altitude readout from an encoding altimeter.

transition altitude (TA) - altitude in the vicinity of an aerodrome at or below which the vertical position of an aircraft is controlled by reference to altitude, i.e. with the aerodrome QNH set on its altimeter. Above transition altitude QNE is set and flight levels used. Also transition level (TL) at which a descending aircraft changes from FL to QNH.

trend - Met forecast for the next two hours, added to some METARs.

TSO - Technical Standard Order. A standard established by the U.S. FAA for quality control in avionics, instruments and other airborne equipment. If it complies, equipment is said to be 'TSO'd' and is more expensive than similar non-TSO's equipment.

TVOR - terminal VOR. A low-powered VOR located at or near an airport and used as an approach aid.

TWR - Tower (aerodrome control tower).

TWY - taxiway.

Tx - transmitter.

---

U ---------- back to the Top

UAS - University Air Squadron.

UDF - UHF direction finding.

UFN - until further notice.

UHF - ultra-high frequency. Radio frequencies in the 300-3,000 MHz band.

UHMRA - Upper Heyford Mandatory Radio Area.

UIR - Upper Information Region, covering the same geographic areas as a FIR, but extending vertically upwards from 24,500 feet, within which certain additional operational rules apply. Also UIS, Upper Information Service.

Unicom - privately-operated advisory A/G radio service at uncontrolled airfields (USA). UNL - unlimited

u/s - unserviceable (i.e not working) when applied to an aircraft or its equipment.

UTC - Co-ordinated Universal Time, formerly Greenwich Mean Time (see also Zulu).

---

V ---------- back to the Top

V-speeds - designations for certain velocities relating to aircraft operation, thus:

Vnav - vertical navigation.

VAL - visual approach and landing chart.

var - variation (magnetic)

VASIS - visual approach slope indicator system. A coloured light system providing visual guidance to the glidepath of a runway.

VDF - very-high frequency direction-finding, whereby an aircraft's bearing from a ground receiving station may be determined from its RT transmissions.

VFR - Visual Flight Rules. Prescribed for the operation of aircraft in visual meteorological conditions (VMC). VMC is generally defined as five miles visibility or more and 1,000 feet vertical and one nautical mile horizontal clearance from cloud, but variations apply to aircraft operating below 3,000 feet amsl. Special VFR (SVFR) clearances are granted at the discretion of ATC for VFR flight through some controlled airspace where IFR usually apply. Also CVFR, Controlled VFR Flight.

VHF - very high frequency. Radio frequencies in the 30-300 MHz band, used for most civil air-to-ground communication.

vis - visibility.

VLF - very low frequency. Radio frequencies in the 3-30 kHz band.

VLF/Omega - worldwide system of long-range navigation using VLF radio transmission.

VMC - Visual Meterorological Conditions. See VFR, above.

Volmet - continuous recorded broadcasts of weather conditions at selected airfields.

VOR - very high frequency omnidirectional range. A radio navigation aid operating in the 108-118 MHz band. A VOR ground station transmits a two-phase directional signal through 360ø. the aircraft's VOR receiver enables a pilot to identify his radial or bearing from/to the ground station. VOR is the most commonly used radio navigation aid in private flying. Increased accuracy is available in Doppler VORs (DVOR) which have replaced some VOR is the UK system. Also VORTAC, combined VOR and TACAN, and VOT, VOR test facility.

VP - variable-pitch (propeller), whose blade angle can be altered in flight either automatically or manually.

VRP - visual reporting point. Landmarks used for position reporting by aircraft operating VFR.

VSI - vertical speed indicator. One of the primary flight instruments showing rate of climb or descent. Also IVSI, instantaneous VSI.

---

W ---------- back to the Top

wake turbulence - wingtip vortices generated behind a wing producing lift. Behind a large heavy aircraft they can be powerful enough to roll or even break up a smaller aircraft.

WAT - weight-and-temperature.

w.e.f. - with effect from. Also w.i.e., with immediate effect.

wind shear - localised change in wind speed and/or direction over a short distance, resulting in a tearing or shearing effect, usually at low altitude, that can cause a sudden loss of airspeed with occasionally disastrous results if encountered when taking-off or landing.

WIP - work in progress.

WP - waypoint.

wt - weight

Wx - weather.

WX NIL - no significant weather, term used in Met reports.

---

X ---------- back to the Top

xmsn - transmission.

xpdr - transponder.

---

Y ---------- back to the Top

---

Z ---------- back to the Top

zero-fuel weight - maximum permissible weight of an aircraft beyond which an additional load must be in the form of fuel (i.e. max take-off weight less total usable fuel in applicable aircraft, which are so limited because of the wing-bending moments associated with near-empty wing fuel tanks).

zero-timed - overhauling an aero-engine to 'service limits' (not the same `good as new' or factory remanufactured).

Zulu or Z - used worldwide for times of flight operations, formerly Greenwich Mean Time, now Co-ordinated Universal Time (UTC).