ADF A ground based navigation beacon system. Very old, dating back to WW1. The actual transmitter is called an NDB and transmits a plain simple signal in all directions in the LW band. The receiver in the aircraft is called an ADF 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 pretty 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. A major moneyspinner for the 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) 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 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 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 30,000ft 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 though, 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 too easy.

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.

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 occassionally 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 ground based navigation beacon. A transmitter in the aircraft sends a pulse to the beacon and the beacon responds; 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.

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

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.

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 always tries to fly a GC route.

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.

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 but somewhat controversial in private aviation because the old farts who make up a lot of the aviation scene regard it as sacrilege.

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 is a remotely mounted gizmo 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 mechanical 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 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 ICAO 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 a lot of the data and its pricing policies are very much based on that.

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.

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

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.

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.

PPL Private Pilot License. The lowest form of pilot licence. Internationally valid if country of issue matches the country of aircraft registration; occassionally 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 and the requirement for it was a part of the planning deal struck with the local authority; almost every airfield is surrounded by NIMBYs (Not In My Back Yard) who want to close the airfield and who keep protesting about noise etc.

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 (see IAS) and correct that for altitude and temperature, and there are other corrections for high speeds.

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.