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Aviation Glossary

ADF A ground based navigation beacon system. Very old, dating back to WW1. The actual transmitter is called an NDB (non-directional beacon) and transmits a plain simple signal in all directions in the LW band. The receiver in the aircraft is called an ADF (automatic direction finder) and provides a needle that points to the NDB beacon. Accurate when flying directly overhead (the needle flips from pointing forward to pointing backwards); accurate over open flat country and variously poor the rest of the time – affected by coasts, weather, terrain, but still widely used. Retired RAF navigators and ex British Airways captains like them.

Airways Predefined routes used by commercial traffic. In the UK they are mostly in Class A airspace and the pilot needs the full IR. Elsewhere, fairly freely available to VFR traffic.

Altitude Defined as the height above mean sea level (MSL). Measured with an altimeter. See also QNH.

AOC Air Operators Certificate. A piece of paper which the CAA charges a lot of money for, and which is required for most classes of commercial flying. Essentially it is an approval system which an operator needs to carry out various forms of public transport. A major moneyspinner for every national CAA, so in the name of “safety” they vigorously prosecute anybody who is doing such work without an AOC.

ATC Air Traffic Control. These can be those you talk to at the airfield, or those you talk to while en-route.

Avgas The type of petrol used by piston powered aircraft. It is basically low-lead high-octane (100 octane) petrol, supposedly quality controlled so it doesn’t contain water or dirt. Heavily taxed and slightly more expensive than motor vehicle petrol. Diesel engines can burn turbine fuel (called avtur or kerosene) which is much more widely available and is currently tax free by international treaty (in most countries) but diesels are still very rare in aviation.

CAA Civil Aviation Authority. The UK CAA regulates aviation in UK airspace. However, under aviation law, broadly speaking, restrictions placed on a UK registered pilot or aircraft remain valid outside its jurisdiction. Every country has its own CAA, just like even the poorest country has its national airline. A great gravy train which uses the word “safety” to answer to nobody and acts as an employment agency for navigators retiring from the RAF.

CAS Controlled Airspace. See Class A-G. In Europe, flight in CAS requires an explicit clearance from ATC and unless one files a fully IFR flight plan in the airways, one cannot rely on getting the clearance, so the route needs to be planned outside CAS.

CB Cumulonimbus. The worst type of cloud. Very tall, reaching to perhaps 40,000ft and often have an anvil shaped top. Thunderstorms come from them. One can pick up the most dangerous type of ice in them (clear ice). In extreme cases and if going too fast, can break the wings off an aircraft – any aircraft! Fortunately CBs are usually scattered and easily avoided by flying between them (keeping a good few miles away) – unless you are flying in cloud in which case you can just fly straight into one!

CDI Course Deviation Indicator. The device in the aircraft traditionally used to display the information from a VOR. An RMI is much better but costs more money.

Circuit Traditionally, one arrives at an airfield by joining a rectangular pattern around it, which is flown in a specified direction (usually LH because the pilot sits on the left so can see where he is going). One can land “straight in” and if there was no other traffic that would always be done but usually it upsets somebody; straight in joins are also disliked by the old farts in aviation because they are simpler.

Class A-G Airspace classes. A,B,C,D are controlled airspace if VFR. A,B,C,D,E are controlled airspace if IFR. All flight in Class A must be under IFR. Class G is the lowest category, where you can do more or less anything you like and nobody cares. The explanations get more horrid after this.

DCT A direct-to. Most instrument flight takes place on a pre-filed routing comprising of straight line tracks between a series of waypoints. Often, the routing is not very efficient, with doglegs taking one away from what would be the best route. Therefore, during the flight, one works with ATC to shortcuts whenever one can. Usually this works because ATC manage the traffic tactically and can allow flight through airspace through which one could not fly if one did it by the book. Most such shortcuts are given as “direct to [waypoint name]”.

De-Iced Through a quirk of physics it is possible for water to remain liquid, in tiny droplets suspended in the air, at temperatures below freezing (0C) and down to about -15C and occasionally even lower. These droplets are visible as cloud. So, if one is flying through cloud and the temperature is between 0C and -15C these droplets will splatter against the aircraft surface (which will obviously be at or just slightly above the actual air temperature) and the aircraft gets covered with ice, whose thickness will just keep building up and up and if nothing is done about it wing lift is eventually lost and the aircraft plummets! Various anti-ice and de-ice devices exist: electric heating of propellers, antifreeze liquid oozing out of tiny holes in the wing, inflatable rubber covers which break off the ice, etc. Icing doesn’t always happen though; it depends on the type of cloud. This is very much a brief summary!

DME Distance Measuring Equipment. A transmitter in the aircraft sends a pulse to a ground based beacon which returns a response; the delay is used to compute the distance between the two. Range up to 200 miles, depending on aircraft altitude, terrain etc. No directional information is provided. Often used in conjunction with a VOR, and used in instrument approaches to see how far you have to run before the runway.

EGT Exhaust Gas Temperature. See LOP. The EGT is the parameter used to set the desired engine operating point. Optimal efficiency is obtained at or around peak EGT (stochiometric combustion).

FAA Federal Aviation Administration. The American version of the UK CAA.

FIS Flight Information Service. Under the ICAO treaty, each country is supposed to provide a basic level of radio service to airborne aircraft. The FIS is generally just a “radio contact”, with very few obligations beyond an “alerting service” which means initiating a search and rescue operation if something goes wrong. In many countries, an air traffic controller providing the FIS can in fact see you on radar, and might then provide a higher level of service e.g. information on nearby aircraft.

Flight Level The height shown by the altimeter when it is set to 1013 millibars. FL is used above a certain altitude, usually. FL is expressed in hundreds of feet (FL150=15,000ft). One needs to be careful with FL when terrain clearance is an issue; one would not want to fly at say FL150 over mountains reaching to 14,000 feet when the actual pressure is 975mb because one’s altimeter will be optimistic by (1013-975)*30 = 1140ft and one’s actual altitude will be 13,860ft!!! Each millibar equals approximately 30 feet in height. See also Altitude and QNH.

Flight Plan All flight crossing national frontiers requires a flight plan, as do internal flights in many countries (not in the UK). Also, filing an FP can usefully trigger search and rescue if you don’t turn up at the other end. An FP contains the rules one is flying under (VFR or IFR), details of the aircraft, fuel carried, emergncy equipment, and the planned route.

FOB Fuel on Board. This is the actual fuel in the tanks at any particular moment. One can see it on fuel gauges although those are rarely accurate enough to be of much use. A much better system is a “totaliser” which measures the actual fuel flow very accurately (within about 2%) but you have to tell it how much is in the tanks to start with.

GC Great Circle. This is the shortest route between two points on the earth. If you stretch a string between two points on a globe, that is the GC route. Obviously one would always try to fly a GC route but controlled airspace and other things get in the way.

GPRS A way of transferring data (as opposed to voice or text messages which use plain GSM) over the mobile phone network. Coverage pretty well matches GSM coverage. Can be very expensive, depending on the tarriff. 3G is a faster version of GPRS and comes in various flavours e.g. UMTS.

GPS Global Positioning System A system run by the US military, comprising of a network of satellites which gives you your position to within a few metres and thus enables accurate navigation anywhere on the earth. A brilliant system which has transformed navigation but somewhat controversial in private aviation (well, in airport bars, anyway) because the old farts who make up a lot of the aviation scenery regard it as sacrilege.

GPWS Ground Proximity Warning System. This is a terrain database (nowadays derived from the U.S. Space Shuttle radar topography mission) and the aircraft position and trajectory is continuously compared to this database. If a conflict is predicted along the current trajectory, appropriate vocal or visual warnings are generated. For obvious reasons, the warnings are partially suppressed when the aircraft is arriving at or departing from an airport which is in the GPWS database. GPWS is mandatory on passenger carrying aircraft over a certain number of seats, and has dramatically reduced the number of these flying into mountains.

GS Ground Speed. This is the speed of the aircraft over the ground below. Because the aircraft is flying in air which is itself moving relative to the ground (it’s called “wind”!) the only way to measure GS is with a ground referenced position measurement system; either GPS or INS.

Gyro Short for a Gyroscope. Aeroplanes can roll and pitch during flight and it is important that the pilot has a stable reference telling him which way up the aircraft is when flying in cloud. Also, traditional compasses (a magnet floating on some liquid) don’t work well in turbulence. A gyro can be inside the panel mounted instrument, or it can be a remotely mounted unit which sends stable orientation and aircraft heading signals to various instruments visible to the pilot.

HSI Horizontal Situation Indicator. This is a clever instrument which combines several functions into one unit. It contains a CDI and can be used to track a GPS, VOR, Localiser, Glideslope (ILS), and the whole inside of the instrument rotates according to the aircraft heading so if you are flying away from the beacon you do not need to mentally invert the indication. The HSI was developed in the 1950s to ease specific instrument navigation procedures. The HSI instrument is merely the visible bit of a larger system which includes a fluxgate magnetometer (an electronic compass, in short) mounted in the wingtip, and a remotely mounted gyro. The HSI is a clever instrument full of weird and wonderful mechanical bits, and a fully electronic version (with an LCD screen) is called an EHSI.

IAS Indicated airspeed. For historical reasons (aviation goes back to before WW1) the speed of an aircraft is indicated with a very simple instrument which is basically a pressure gauge fed from a piece of pipe (called the pitot tube) which sticks horizontally into the airflow, so the faster you go the more pressure it measures. This simple system is very reliable but is accurate only at low altitudes. At high altitude there is a large error due to the air being thinner, with the actual speed of the aircraft (called the True Airspeed, or TAS) being higher than what is indicated. Additional correction is needed for the outside temperature. However, while modern aeroplanes have a direct TAS readout of some sort, the old style indicator is retained, partly as a simple backup and partly because key aspects of the aircraft behaviour (for example, the stall speed) are related to the indicated airspeed. See also TAS.

ICAO An international treaty, also known as the Chicago Convention, signed around 1944, which sets certain standards and privileges in international aviation. The general principle is that the holder of an appropriate licence, flying an appropriately registered aircraft, can fly without challenge to the airspace of any signatory and this is the foundation of international air travel. All civilised countries, and most of the uncivilised ones too, have signed it but in fact any member state is free to file “differences” as they feel like. This leads to a complicated maze of regulations which few people fully understand.

ILS Instrument Landing System. This is an instrument approach which is a means of guiding an aircraft to the runway. An ILS system emits two radio beams which the aircraft follows: the Localiser and the Glideslope. The Localiser beam provides lateral (left to right) guidance. The Glideslope beam provides vertical (up and down) guidance. The aircraft can be controlled either manually by the pilot (who has an instrument with two needles which need to be kept centred) or can be controlled by an autopilot. As with all instrument approaches, the aircraft is thus flown all the way to a prescribed “decision height” at which one must either see the runway or one must carry out a “missed approach”. There are various types of instrument approaches but an ILS is the best and safest one by far; at most airports one can fly an ILS down to 200ft above the ground which gets you below most weather.

INS Inertial Navigation System. This is a gyroscopic system which gives you your position anywhere on the earth, potentially to a high accuracy. Not as accurate as GPS but is self contained and immune from interference. Big jets have used this for decades to navigate over oceans etc. It’s very expensive (6 figures) and is not used in light aircraft.

IFR Instrument Flight Rules. Quite simply, the alternative to VFR. No limits on visibility. All flight in cloud (in IMC) has to be under IFR, but one can fly under IFR under visual conditions too. Commercial traffic flies almost exclusively under IFR.

IMC Instrument Meteorological Conditions. Everything that is not VMC is IMC, by definition.

IMC Rating This is a chopped-down version of the IR and is available in the UK only. It removes some of the restrictions which the UK CAA has added to the basic PPL (e.g. the need to be in sight of the surface). It also permits IFR flight in airspace classes D,E,F,G so, basically, one can fly in and above the clouds. Popular in the UK and much easier to get than the full IR.

IR Instrument Rating. This is an international privilege which (broadly speaking, provided the aircraft registration country matches the country of the IR issue) allows the pilot to fly under IFR anywhere. It is very time consuming to obtain, with 1-2 years of study for the European version and perhaps 6 months for the FAA version. Most private pilots in Europe who are doing the IR are doing the FAA version for this reason.

Jeppesen An American company selling flight planning software. Has a virtual monopoly on data outside the USA and its pricing policies are very much based on that.

KT Knot. The unit of speed measurement in aviation. 1kt is 1nm per hour, so 100kt is about 115mph.

LFOB Landing Fuel on Board. See FOB. If one has a fuel totaliser connected to a GPS, one can get a continuously recomputed figure for the fuel which will be in the tanks at the currently loaded destination, based on the current ground speed being maintained. This is a great facility which enables long flights to be done safely.

Licence This is the basic piece of paper a pilot must have. There is the private one (PPL), a commercial one (CPL) and an air transport one (ATPL). There are also various licenses which are valid only within the country of issue. One can add Ratings to a Licence; for example an Instrument Rating (IR). An ATPL includes an IR already.

LOP Lean of Peak. Traditional aircraft engines have manual controls for the throttle and the mixture. The mixture control enables a lightweight air-cooled engine to deliver a high power during climb when the airflow is not so good, while achieving near-optimal efficiency in cruise by setting the mixture to either peak EGT or Lean of Peak.

METAR This is a weather report which refers to the actual weather at the specified airport. See also TAF.

MFD Multifunction Display. This is a panel mounted instrument which can display various things. The basic function is a moving map (for this it uses a separate GPS receiver) and others include the display of nearby traffic (if you pay for the TCAS option), ground proximity data (if you pay for the GPWS option), weather radar (if you have weather radar), lightning data (if you have a stormscope receiver), etc.

MP Manifold Pressure. This is the pressure in the engine air intake. MP is one of the parameters one sets when flying; it is set with the throttle lever. Put simply, and at constant RPM, MP is a direct measure of the engine torque.

NM Nautical Mile. The unit for distance in aviation. 100nm is about 115 statute miles.

NMEA The data stream coming out of a bare GPS receiver. Comprises of latitude, longitude, altitude, time of day, and lots of other stuff. Not much use by itself; typically is used by software to locate the aircraft on a moving map image.

Overhead Join A standard but really stupid way to arrive at an airfield. One generally arrives into a circular pattern 2000ft above the airfield; the idea being that one doesn’t have a radio (in WW1, few did) and one can read a picture on the ground called a signal square which is made up of white-painted bits of wood which tells the pilot the current runway, etc. A concept dating back to WW1 and much loved by old farts, even though many airfields have got rid of their signal square years ago, or it has rotted away.

PC/Anywhere A software program from Symantec which enables one PC to fully control another. The remote PC’s screen, keyboard and mouse are simply copied to your local PC. A very good way to do all sorts of things from far away. Requires a high speed connection (broadband or faster) to be usable. The Microsoft version is called Terminal Services / Remote Desktop.

PPL Private Pilot License. The lowest form of pilot licence. Internationally valid if country of issue matches the country of aircraft registration; occasionally one gets a bit more. The pilot must fly VFR unless holding an instrument rating of some sort.

PPR Prior Permission Required. A lot of airports are “PPR” which means you need to contact them (phone/fax) before departing. Some need 24hrs, some a number of days or even longer. In the UK, PPR is normally just a phone call before the flight. Some smaller airfields are PPR as a conditions of the planning permission, while bigger airports use PPR to create paper-pushing jobs.

QNH The current barometric pressure in millibars which one sets in the adjustment window of an altimeter to make it read the height above mean sea level. An altimeter simply measures air (barometric) pressure. Air pressure varies by a few % either side of 1000 millibars (mb) according to the prevailing weather, and since each millibar is worth about 30 feet in elevation, the altimeter has to be set to the current barometric pressure to read the correct altitude (altitude is defined as the height above mean sea level, MSL). One can then ensure one is flying well above terrain, because the terrain elevations on the charts are always referred to MSL. The problem is that obtaining the QNH can be problematic or impossible (mid Atlantic?) and it varies anyway so above a few thousand feet one sets the altimeter to 1013 mb and then one calls it a “flight level”. See Flight Level.

Rating Additional privileges which can be added to a Licence. See Licence.

RMI Radio Magnetic Indicator. A device to display the information from a VOR. Does what a CDI does but is much easier to use because the needle points directly towards the VOR. In effect it is like an ADF but instead of the needle pointing at an NDB it points at a VOR.

TAF This is a weather forecast for a specified airport, generally for a few hours ahead but Long TAFs are available for major airports, up to 24 hours ahead. See also METAR.

TAS True Airspeed. This is what it says: the actual speed of the aeroplane through the air. Suprisingly, there is no way to measure it directly; one has to measure the indicated airspeed (seeIAS) and correct that for altitude and temperature, and there are other corrections for high speeds.

TCAS Traffic Awaress System. This detects the proximity of nearby aircraft and warns appropriately. It relies on the other aircraft being suitably equipped. It is mandatory on passenger carrying aircraft over a certain number of seats.

TCU Towering Cumulus. A type of cloud, category just below a CB, with a substantial vertical shape, which is usually very rough to fly through and is best avoided.

Turboprop An propeller driven aircraft powered by a jet engine but instead of deriving thrust by the accelleration of air the engine is used to drive a propeller. The great advantages over a piston engine are simplicity, very high reliability, ability to use the much more widely available and cheaper jet fuel. But, like all jet engines, very expensive to buy and maintain!

UTC International standard time for all aviation purposes. Usefully equal to GMT, so the UK is on UTC during the winter.

VFR Visual Flight Rules. Basically, this means clear of cloud. A basic PPL is limited to VFR. Additional VFR requirements are visibility >3000m and continuous visual contact with the surface but these are not uniform across Europe; e.g. many countries permit VFR above a solid cloud layer. VFR is banned in Class A airspace.

VMC Visual Meteorological Conditions. The ICAO definition is basically clear of cloud and horizontal visibility of >1500m. VMC is a requirement for VFR flight.

VOR A ground based navigation beacon. It transmits a rotating beam (like a lighthouse but it’s in the VHF band and rotates at 1800rpm) onto which the beam’s current compass bearing (0…360) is continuously encoded. A VOR receiver in the aircraft tells the pilot which compass bearing (relative to the VOR) he is located on. There is no distance information. Often used in conjunction with a DME.

VPN Virtual Private Network. A secure (encrypted) connection between two computers. Typically, one would use a VPN to access one’s office computer from far away; nobody can eavesdrop on the data. The VPN function is provided in Windows as standard.

Waypoint Just a point on a route. Navigation is done in straight line sections, with a waypoint at each junction. With very few exceptions one doesn’t fly in curves. When planning a flight, one tends to use ground based navigation beacons as waypoints, for obvious reasons! But often there isn’t anything where you want to go so one can make them up.

WIFI A high speed short range (tens of metres at most) data connection, usually used for internet access in cafes and hotels. Also known as 802.11.