Glossary - Letter D

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D |abbreviation| danger area


DA |abbreviation|  1. danger area  2.decision altitude


DAAIS |abbreviation| danger area activity information service


DACS |abbreviation| danger area crossing service


DADC |abbreviation| digital air data computer


DADS |abbreviation| digital air data system


DALR |abbreviation| dry adiabatic lapse rate


damage |noun| harm that is caused to something / If the temperature rises it can cause serious damage to the engine. / - |verb| to cause harm to something / Small stones around the run-up area may damage propellers. /


damage tolerance |noun| the ability of a material or structure to withstand or resist damage / The structural efficiency of bonded and machined structure is not achieved at the expense of damage tolerance. /


dampen |verb|  1. to decrease or reduce / An accumulator is fitted to store hydraulic fluid under pressure and dampen pressure fluctuations. /  2. to make slightly wet


damper |noun| a device to decrease or reduce something / A yaw damper is used for rudder control. /


D & D |abbreviation| distress and diversion cell


danger area |noun| airspace of a particular length, width and depth, within which at particular times there may be activities which are dangerous to the flight of the aircraft. Abbreviation: D, DA


danger zone |noun| an area where danger exists


Dash 8 |noun| a twin engine turboprop regional transport aircraft


data |noun|  1. information made up of numbers, characters and symbols often stored on a computer in such a way that it can be processed / Airspeed information is supplied from an air data computer. / - meteorological data > information about weather conditions stored on a computer  2. information.


datum |noun| a reference or base point of a scale or measurement, e.g. mean sea level


datum shift trim system |noun| a trim system which varies the incidence of an all-moving tailplane without moving the cockpit controls / In some aircraft, the datum shift is operated automatically. /


dB |abbreviation| decibel


DC |abbreviation| direct current


DCL |abbreviation| departure clearance


de- |prefix| undo, remove or stop


deactivate |verb| to turn off a system or a piece of equipment thus stopping it being ready to operate / On some aircraft nose wheel steering must be deactivated prior to retraction. /


dead reckoning, ded reckoning |noun| navigation using calculations based on airspeed, course, heading, wind direction and speed, ground speed, and time / In the early stages of practical navigation, the student pilot navigates by using dead reckoning. / Abbreviation: DR (NOTE: The term comes from ‘deduced’ reckoning or ‘ded’ reckoning.)


de-aerate |verb| to remove gas, especially carbon dioxide or air, from a liquid such as fuel / The pump helps to de-aerate the fuel before it enters the engine. /


de-aeration |noun| the process of removing gas from a liquid such as fuel / Partial de-aeration of fuel takes place in the pump. /


de-aerator |noun| a device to remove gas from a liquid


de-aerator tray |noun| a device in the lubrication system to remove air bubbles from oil


deal |noun| - a great deal > a large amount of, a lot of / A great deal of damage was done to the aircraft as a result of the fire. / - |verb| to handle or manage / A computer can deal with the constant inputs required to control an unstable aircraft. /


debris |noun| scattered broken pieces / Before running up the engine, check that the aircraft is on firm ground and that the area is free of stones and other debris. The aircraft exploded in mid-air, spreading debris over a wide area of the countryside. /


decal |noun| picture, letters or digits printed on adhesive paper, which is transferred onto a surface and may be peeled away /A red decal with AVGAS 100LL in white letters indicates the type of fuel to be used./


decelerate |verb| to slow down / Reverse thrust and brakes help to decelerate the aircraft after landing. / Opposite: accelerate


deceleration |noun| slowing down / Anti-skid braking systems units are designed to prevent the brakes locking the wheels during landing, thus reducing the possibility of wheel skid caused by the sudden deceleration of the wheel. / Opposite: acceleration


decibel |noun| a unit for measuring the loudness of a sound. Abbreviation: dB


decimal |noun| a decimal fraction - |adjective| - decimal fraction > a fraction as expressed in the decimal system / 0.50 is a decimal fraction that is equal to 1/2. / - correct to three places of decimal or to three decimal places > correct to three figures after the decimal point / 2.754 is correct to three decimal places, 2.7 is correct to one decimal place. /


decimal notation |noun| the method of writing a number in the decimal system / The fraction 3/4 can be written as 0.75 in decimal notation. Prices and number are normally written using decimal notation. He finds it difficult to understand how the computer works because it uses binary not decimal notation. /


decimal place |noun| the position of a number to the right of the decimal point


decimal point |noun| the dot (.) used to separate a whole number from a decimal fraction


decimal system |noun| system of counting based on the number 10 and using the digits 0 – 9


decision |noun| the act of deciding or of making up one’s mind - to make a decision > to choose a course of action / The decision to evacuate the aircraft was made by the captain. /


decision altitude |noun| the altitude at which the flight crew must decide to land or go around. Abbreviation: DA


decision height |noun| the altitude at which, during an ILS landing approach, a pilot must decide whether to land or carry out a missed approach / The pilot waited until she was at decision height before initiating the missed approach procedure. / Abbreviation: DH (NOTE: An ILS approach generally has a decision height of 200 ft (60 m) above ground level.)


deck |noun| the floor of a ship or aircraft


decode |verb| to change coded information into readable form / Incorrectly spaced information pulses can result in failure by the ground station to decode the aircraft information. /


decoder |noun| a device used to decode signals from the air traffic control radar beacon system / The aircraft receiver is set to the required frequency and linked to a selective call system decoder which has a 4-letter code. /


decrab |verb| to re-align the aircraft on the runway center-line in crosswind conditions immediately before touchdown


decrease |noun| a lessening or reduction / A decrease in power results in the aircraft descending. / - |verb| to become less, to fall / Air density and pressure decrease with an increase in altitude. / Opposite: increase


deduce |verb| to work something out in the mind using information provided / Sometimes, it is possible to estimate the depth of the layer of mist or fog from the ground observations and hence to deduce the ground range from any height. /


defect |noun| a fault or error / Low oil pressure or excessive temperature indicate the development of a possible defect. /


defective |adjective| faulty or not operating correctly / Loss of supply pressure is caused by either a defective booster pump or lack of fuel. /


define |verb|  1. to give an exact explanation, as in a dictionary - it is not easy to define the word > it is difficult to say exactly what the word means  2. to set the limits of something / Cloud tops are very difficult to define. /


definite |adjective| referring to something which is not in doubt, which is certain / Using a time scale on the track, the pilot should be prepared to look for a definite feature at a definite time. / Opposite: indefinite


definition |noun| an exact explanation of what a word or expression means / The definition of a year is the time taken for a planet to describe one orbit around the sun. / - by definition > understood by the use of the word itself / A sphere is, by definition, round. /


deflate |verb| to allow air to escape from something, so that it becomes smaller or collapses. Opposite: inflate - to deflate a tire > to allow the air to escape from a tire


deflation |noun| the act of allowing air to escape from something, so that it becomes smaller or collapses / Deflation of a tire is done by depressing the valve. /


deflect |verb|  1. to cause an object to move away from a neutral or central position / During an out-of-balance turn, the ball in the slip indicator will be deflected to the left or right. /  2. to move a moving object, gas or liquid away from its intended path / In an open-cockpit aircraft, the windshield deflects the airflow over the pilot’s head. /


deflection |noun|  1. movement away from a central or neutral position / Full deflection of the ailerons is sometimes needed on take-off to counteract a crosswind. /  2. the movement of a moving object, gas or liquid away from its intended path / In the southern hemisphere the deflection of wind at the equator is to the left. /


deformation |noun| a change of the correct shape caused by stress / Deformation of wing panels may be an indication of serious structural damage. /


deg |abbreviation| degree


degradation |noun| a decrease in quality / Degradation of the radio signal sometimes makes it impossible to understand the message. /


degrade |verb| to decrease the quality of something / Interfering signals degrade VOR performance. /


degree |noun|  1. a level, amount or quantity - the degree of compression > the amount of compression - a high degree of safety > a high level of safety - to a greater degree > more than - to a lesser degree > less than  2. a unit of temperature  3. a unit of measurement of an angle equal to 1/360th of a circle – each degree is divided into 60 minutes and each minutes into 60 seconds / Make a turn to the right at a bank angle of 30°. / - an angle of 90° > a right angle  4. a unit of direction as measured on a compass > east = 090° west = 270°


degrees true |noun| degrees of direction measured from true north, not magnetic north. Also called: true degrees. Symbol °T


dehydration |noun| an unwanted and sometimes dangerous loss of water from the body / Dehydration can be avoided by drinking plenty of water. /


deice |verb| to remove ice / The ground crew deiced the aircraft prior to take-off. /


deicer |noun| a device or substance used to remove ice / Deicer spray should be checked to make sure it is not harmful to light aircraft windscreens. /


deicing |noun| the removal of ice - |adjective| referring to the removal of ice - deicing station > a designated location where aircraft are deiced in cold weather before departure - deicing truck > vehicle with tank and hydraulic platform for spraying aircraft


delay |noun| a period after the expected time that you have to wait before something happens, the length of time by which something is late / By day, the presence of cloud can cause a delay in clearance of fog. / - |verb|  1. to make late, to cause to be late / Take-off was delayed because of fog. /  2. to put something off until later / He delayed telling her the news until they had landed. /


delayed-action |adjective| in which there is an unusual passing of time between stimulus and response / The door is fitted with a delayed-action lock which operates one minute after the power has been switched off. /


deliver |verb| to provide, to give / The motor will continue to run but will deliver only one-third the rated power. The pump can deliver fuel at the rate of 2,000gph. /


delivery |noun| the act of providing or giving / On some pumps, a depressurizing valve is used to block delivery to the system. /


delivery pressure |noun| the pressure normally expected when fuel is being pumped


deluge |noun| - fire deluge system


demand |noun|  1. a need or use caused by necessity - high current demand on a generator > a situation requiring the generator to produce a lot of electricity  2. a request which is made firmly - on demand > when asked for or ordered / A computer will produce, on demand, a flight plan giving the optimum route, levels and fuel. / - |verb|  1. to require as a necessity / Higher operating weights of modern aircraft demand an increase in the number of wheels fitted to the landing gear. /  2. to ask firmly / He demanded an explanation. /


demonstrate |verb| to show by clear example or explanation / Torricelli first demonstrated that the atmosphere has weight. It will be demonstrated in chapter 12 that turbulence is associated with strong winds. /


demonstration |noun| a clear, often visual, description or explanation / Your instructor will give a demonstration of the stall-recovery technique. /


dense |adjective|  1. referring to a substance which is closely compacted - dense fog > thick fog  2. referring to the amount of mass of a substance for a given unit of volume / Air which contains water vapor is less dense than air which does not. /


density |noun| a quantity of mass for a given unit of volume


density altitude |noun| the pressure altitude corrected for non-ISA temperature (NOTE: Density altitude is a very important factor in calculating aircraft performance because of its effect on engine performance, time to reach takeoff speed (and therefore length of take-off run) and rate of climb.)


density error |noun| a correction to airspeed to give true airspeed


DEP |abbreviation| departure message


depart |verb| to leave / The flight departs at 0200 GMT. / Opposite: arrive


department |noun| a separate part of a complex whole, especially of an organization


departure |noun|  1. the act of leaving - departure time > the time when an aircraft becomes airborne  2. the distance between two meridians at any given latitude


departure lounge |noun| a room at an airport where passengers wait to board their aircraft


departure point |noun| a place on the map representing the place from which a flight begins


departures |noun| the part of an airport that deals with passengers who are leaving


depend |verb|  1. to be controlled or affected entirely by something / Whether or not an object can be seen by aircrew at a given distance will depend on factors such as size, shape and color of the object. If an aircraft ditches in the sea, early rescue depends on rapid location of survivors. /  2. to rely on / Pilots depend on air traffic controllers to help them conduct a safe flight. /


dependable |adjective| reliable, trustworthy / Mercury barometers have largely been replaced by precision aneroid barometers which are smaller, simpler to use, and more dependable. /


dependent |adjective| relying on or unable to do without something / The height indicated by an altimeter is dependent on the pressure which is set on the sub-scale. /


deploy |verb| to come into action, to become ready to be used / Slide rafts are door-mounted and automatically deploy and inflate when the door is opened in the armed position. /


deposit |noun| a layer of collected matter on a surface / A deposit of ice crystals causes the aircraft surfaces to change their aerodynamic characteristics. Wheel brakes should be inspected for snow or ice deposits. /


depreciate |verb| to decrease in value / The aircraft depreciated by 100% over the 5 year period. / Opposite: appreciate


depreciation |noun| a decrease in value / There was a depreciation of 100% in the value of the aircraft over the 5 year period. / Opposite: appreciation


depress |verb| to push down /Switches on the control columns instantly disengage the autopilot when depressed./


depression |noun|  1. an area of low atmospheric pressure / In the northern hemisphere, the wind blows anticlockwise round a depression and clockwise round an anticyclone and vice versa in the southern hemisphere. / - deep depression > area of very low relative atmospheric pressure  2. a lower area on a surface, which is often difficult to see / A depression on the wing surface must be investigated in case it is an indication of more serious structural damage. /


depressurization |noun| a loss, especially sudden, of cabin pressure /Emergency oxygen must be available in the event of depressurization./


depressurize |verb| to lose pressure suddenly, or to cause to lose pressure / The aircraft began to depressurize at 20,000 feet. /


depth |noun| the distance from the top surface of something to the bottom / The troposphere’s depth is variable in temperate latitudes. /


derive |verb| to get or to obtain / Performance data is derived from flight tests. Kepler derived the laws which relate to the motion of planets in their orbits. /


descend |verb| to lose altitude, usually in a planned maneuver - the aircraft descended to 10,000 feet > the pilot reduced altitude until the aircraft was at 10,000 feet. Opposite: climb,ascend


descent |noun| a planned loss of altitude / The descent from cruise altitude took 40 minutes. / - in the descent > during planned loss of altitude, usually in preparation for landing


describe |verb|  1. to give the particular features of something - to describe what happened > put into words exactly what happened  2. to draw a geometric figure or to move in a line that forms a geometric figure / The definition of a year is the time taken for a planet to describe one orbit around the sun. / - to describe an arc > to draw or move in an arc


description |noun|  1. the act of giving the particular features of something  2. the drawing or making of a geometric figure


desert |noun| a large area of dry often sandy country / Over desert areas the lack of water vapor produces cold nights. /


design |noun| a plan or drawing of something before it is made / The design and testing of aircraft are important stages in the development program. / - |verb| to draw plans using accurate information in preparation for constructing something - to design an aircraft > to have the idea, make drawings, calculate data, etc., with the intention of producing an aircraft


designate |verb| to choose for a special purpose / This region is designated as a fire zone. /


designator |noun| a group of letters and/or numbers that identify something


designer |noun| a person who has the idea for, and makes plans to produce, something / Rutan is a designer of unusual-looking aircraft. /


desirable |adjective| preferred or wanted / Equalization of the air pressure across the eardrum is more difficult to achieve during descents than ascents, and a minimum rate of pressure change is desirable. /


despite |preposition| in spite of / Many beacons and aids which are provided for low operations are left out to keep the chart clear – despite this, the charts still look very difficult to understand. / - despite the weather, we took off > although the weather was bad, we took off


DEST |abbreviation| destination


destination |noun| the place to which somebody or something is going / Aerodrome forecasts are normally given in code form for destination and alternates. /


destroy |verb| to damage so much as to make useless / The aircraft was destroyed in the accident. /


destruction |noun| an act or instance of making completely useless by breaking / By testing selected parts to destruction, a safe life can be assessed for all structures and components. /


destructive |adjective| referring to something which destroys - the winds of a tornado are extremely destructive > tornadoes cause a lot of serious damage


detach |verb| to remove a part from something, or to be removed / A fuselage panel became detached and had to be replaced. The parachute flare is a device which is fired to a height of 1,200 ft where a red flare and parachute detach. /


detachable |adjective| referring to something which can be unfixed and removed


detachable wheel spats |plural noun| streamlined coverings for the wheels of light aircraft which can be taken off to allow inspection and repairs of tires


detail |noun| the important and less important facts about something / The amount of detail which appears on a topographical chart depends upon the scale. /


detect |verb| to discover the presence of something / Apart from sensing the abnormal rate of descent of a false glide slope, the pilot can detect an error by comparing height with distance to go. /


detection |noun| the discovery of the presence of something


detector |noun| a device for discovering the existence of something - ice detector > a device for detecting the presence of ice on the airframe / When ice forms on the vibrating rod ice detector head, the probe frequency decreases. /


deteriorate |verb| to become or make bad or worse / The electrolyte in the cells of a nickel-cadmium battery does not chemically react with the plates and so the plates do not deteriorate. / - deteriorating weather > worsening weather


deterioration |noun| worsening - a deterioration in the situation > a worsening of the situation


determination |noun|  1. the act of finding out by calculation / Structure design for a given safe life has led to the determination of the minimum number of flying hours which should pass before major failure occurs. /  2. the strength of mind to do what is required / Determination was a major factor in the trainee passing his exams. /


determine |verb|  1. to find out by calculation / To determine the average age, divide the total number of years by the number of people. When we wish to fly from one place to another, it is first necessary to determine the direction of the destination from the departure point. /  2. to set or to fix precisely / On a large transport aircraft, the safety of hundreds of passengers is involved, and regulations determine the minimum crew that must be carried. /


detonation |noun| a sudden, explosive burning of the air/fuel mixture / Prior to the accident, engine detonation could be heard by people on the ground. /


develop |verb|  1. to come into being / Carburetor icing may develop in any type of carburetor in relatively warm air temperatures. Vertical motion and therefore turbulence suggest that thunderstorms may develop. /  2. to get bigger, to grow and change / During the day, light breezes may develop into strong winds. /


development |noun|  1. something new, made as an improvement on something older / Satellite navigation aids for light aircraft are a recent development. /  2. growth and change / To study weather and its development, the meteorologist has to be aware of the horizontal changes in atmospheric pressure both in space and time. /


deviate |verb| to move away from the normal position or path / If the aircraft deviates beyond the normal ILS glide slope, the flight crew are alerted. /


deviation |noun|  1. the process of moving away from the normal position or path / On final approach, any deviation from the extended center-line of the runway should be corrected immediately. /  2. a magnetic compass error in a particular aircraft caused by magnetic influences in the structure and equipment of the aircraft itself / Deviation is not a constant value but varies from one aircraft to another. /


device |noun| an object, especially mechanical or electrical, which has been made for a particular purpose / A capacitor is a device with the ability to temporarily store an electric charge. /


dew |noun| drops of condensed moisture left on the ground overnight in cool places


dew point |noun| the temperature at which air is saturated with water vapor and condensation begins (NOTE: Weather reports usually include the air temperature and dew point temperature. When the difference between temperature and dew point is small, there is a strong possibility of fog, clouds, or precipitation.)


DF |abbreviation| direction finding


DFDR |abbreviation| digital flight data recorder


DFR |abbreviation| departure flow regulation


DFTI |abbreviation| Distance from touchdown indicator


DH |abbreviation| decision height


DI |abbreviation| direction indicator


diagonal |adjective|  1. joining two opposite corners of a rectangle  2. sloping halfway between the vertical and horizontal / Early aircraft were of the wire braced type of construction, the wire being superseded by tubular diagonal struts. / - |noun| a line joining two opposite corners of a rectangle


diagram |noun| an often simplified drawing showing the structure or workings of something / The diagram shows a simple open-circuit system. /


diagrammatic |adjective| referring to something which is shown as a drawing of a system or structure - diagrammatic format > in the form of a diagram


dial |noun| the face of an instrument showing a scale / A cup anemometer is connected to an instrument with a dial showing wind speed in knots. /


diameter |noun| the distance from one side of a circle to the other, passing through the center - equatorial diameter > the distance from the equator, through the center of the Earth to the equator on the opposite side of the globe


diaphragm |noun| a thin sheet of material used to separate parts or chambers / Some switches are operated by a diaphragm which flexes under fluid or air pressure. /


differ |verb| to be unlike / Track and heading differ by the amount of drift. Because the chart time and the departure/arrival times differ, it is necessary to consider the movement of any weather system which might affect the route. /


differential |adjective| referring to things which react differently when measured against a norm or standard - differential heating of the atmosphere > the heating of the atmosphere to varying temperatures depending on the relative warmth of the land at the equator and the poles


differential expansion switch |noun| a switch which operates on the principle that the coefficients of expansion of dissimilar metals are different


differentiate |verb| to recognize the difference between two things; to show two things to be different / Some types of color blindness make the sufferer unable to differentiate between blue and red. /


diffraction |noun| the breaking down of a beam of radiation / Diffraction produces a surface wave which follows the curvature of the earth. /


diffuse |adjective| spread out in every direction / Glare caused by diffuse reflection of sunlight from the top of a layer of fog or haze can seriously reduce air-to-ground visibility. / - |verb| to spread out in every direction / Light diffuses as it passes through fog. /


diffuser |noun| a device in a jet engine that alters the direction of flow of the air entering the engine as part of the process of compressing it before it reaches the combustion chamber


diffusion |noun| the process of spreading out / Gas from the turbine enters the exhaust system at high velocities but, because of high friction losses, the speed of flow is decreased by diffusion. /


digit |noun| any number from 0 to 9 / Information is provided in a four-digit group. /


digital |adjective| referring to a system or device which uses signals or information in the form of numbers


digital flight data recorder |noun| a device for automatically recording information on aircraft operation (altitude, airspeed, vertical acceleration, heading, elapsed time, attitude, flight control surface position and engine speed). Such recorders are designed to survive crash accelerations, impacts, crushing and fire and often carry underwater transponders or beacons. Abbreviation: DFDR


dihedral |noun| the angle between an upward sloping aircraft wing and a horizontal line


diluted |adjective| made weaker by adding water or some other fluid / Spillage from a lead acid battery may be neutralized by washing with a diluted solution of sodium bicarbonate. /


diluter |noun| a device for decreasing the strength or concentration of a liquid or gas / Most flight decks use the diluter demand system in which the oxygen is diluted with cabin air. /


dim |adjective| (of light) with low intensity / The dials and displays are easier to see if the ambient lighting is dim. /


dimension |noun| a measurable distance such as height, length, etc., or a measurement of height, length, etc. / Variations of atmospheric pressure produce changes in the dimension of the capsule chamber. /


diminish |verb| to decrease or to reduce in size or importance / Friction is greatest near the ground and diminishes with height. At higher altitudes, ground objects are less easily seen because of diminished size. /


diode |noun| an electronic component that allows an electrical current to pass in one direction and not the other


dioxide |noun| an oxide containing two atoms of oxygen.


dip |verb| to move e.g. the wing or nose of an aircraft so that it points downwards


direct |adjective|  1. in a straight line; by the shortest route  2. complete - |verb| to guide or control the movement of something / Clamshell doors are hydraulically or pneumatically opened, and direct the exhaust gases forwards to produce reverse thrust. /


direct current |noun| an electric current flowing in one direction only / An electric starter is usually a direct current electric motor coupled to the engine, which automatically disengages after the engine starts. / Abbreviation: DC


direction |noun| the course taken by somebody or something / The Earth rotates about its own axis in an anticlockwise direction. /


directional |adjective| referring to the course taken by somebody or something


directional gyro |noun| a gyroscopic instrument which indicates direction but does not have a north-seeking magnet / The directional gyro should be set to correspond with the magnetic compass. /


directional radar beam |noun| a signal from a directional beacon enabling the pilot to determine a bearing from the beacon with a communications receiver


direction indicator |noun| an instrument which gives direction information. Abbreviation: DI


directive |adjective| referring to the ability of a device to send or receive signals in straight lines / The antenna is highly directive in transmission and reception. / - |noun| general or detailed instructions from management to staff to guide them in their work / According to the management directive, all late arrivals should be logged. /


director |noun|  1. a device with a central controlling function / EFIS is a highly sophisticated type of flight director system. /  2. a person who is a member of the board that controls the activities of a company


disadvantage |noun| an unwanted situation or condition, or a factor which makes somebody or something less likely to succeed / The disadvantage of a booster pump is that the output is constant so that when engine demand is high, fuel pressure tends to be low and vice versa. / Opposite: advantage


disadvantaged |adjective| - physically disadvantaged (person) > a person who has a physical disability


disappear |verb| 1. to vanish / If air blew at right angles to isobars, the horizontal pressure differences would eventually disappear. / 2. to pass out of sight / The aircraft took off, climbed out and soon disappeared from view. /


disarm |verb|  1. to switch off an active or live system / On the ground approaching the terminal, the flight deck will instruct the cabin crew to disarm the escape devices. /  2. to forcibly remove a weapon from somebody / The hijacker was disarmed by security forces. /


disc |noun| a circular flat plate / A turbine consists of a disc on which is mounted a number of blades. /


discharge |noun| a release of power from a source such as a battery / A lightning flash is a large-scale example of an electrical spark, or discharge. / - battery discharge > the loss or release of electrical supply from a battery - |verb| to release electrical supply from a source such as a battery / The battery discharged overnight. /


disconnect |verb| to separate two things attached to one another / The electrical supply can be disconnected by pulling out the plug. /


discrimination |noun| the ability to know or see the difference between two similar things / Targets on the same bearing which are separated radially by less than half a pulse length distance will appear at the receiver as one echo, so good target discrimination requires short pulses. /


discuss |verb| to write about or talk about a subject / This chapter will discuss HF and VHF voice communications. /


disembark |verb| to leave the aircraft after landing / The passengers finally disembarked at 20.00 hours. /


disembarkation |noun| the act of leaving the aircraft after landing / The exits are used as conventional doors for disembarkation. /


disengage |verb| to switch off a system or device / Switches on the control columns instantly disengage the autopilot when depressed. /


dish |noun| a shallow container for food


dish antenna |noun| a circular antenna with a shape like a shallow bowl


disintegration |noun| the falling apart or destruction of something / Electromagnetic radiations resulting from the disintegration of radioactive materials are known as gamma rays. /


dismantle |verb| to take apart into single components / One type of inspection is able to reveal fatigue cracks, corrosion, internal damage, the presence of loose articles and mercury spillage without the need to dismantle the aircraft. / Opposite: assemble (NOTE: The verb ‘mantle’ is not used.)


disorientation |noun| a state of confusion in which there is loss of understanding of where one is or which direction one is facing, etc. / When the cabin is rapidly and completely filled by smoke and fumes passengers will suffer from disorientation. /


dispatch |noun|  1. the airport service which is responsible for liaising with the crew about operational matters / Ask Dispatch for an updated weight and balance sheet. /  2. the Flight Ops department of an airline which plans and monitors each flight and provides operational information in abnormal situations / When the flight crew encountered volcanic ash they contacted their company dispatch. /


dispatcher |noun| the airport agent who provides the weight and balance sheet which must be checked and signed by the captain. The weight and balance sheet contains updated information about the aircraft payload (passengers, baggage, cargo and fuel) and its location. This allows the aircraft’s center of gravity, which must be within certain limits for safe take-off and flight, to be calculated.


dispensation |noun| permission not to have to do something / At very high altitudes the flying pilot must be on oxygen at all times, unless an aircraft dispensation has been obtained. /


dispense, dispense with |verb| not to include or not to use something / In some cases the rivets are dispensed with and the skin is fixed to the internal members by the redux process. /


dispersal |noun|  1. the act of leaving an area and going in different directions - the dispersal of a crowd > the disappearance of a crowd  2. the clearing away of something such as mist, e.g. by the wind / Dispersal of cloud takes place when surface heating lifts the cloud base or drier air is advected. /


disperse |verb|  1. to leave an area going in different directions - the crowd dispersed > the people in the crowd left the area, going in different directions, so that eventually the crowd disappeared  2. to clear away / The fluorescent green dye will disperse slowly in a calm sea but quickly in a moderate to rough sea. /


displace |verb| to move something out of its normal position / The atmosphere is said to be stable if, when a parcel of air is displaced vertically, it tends to return to its original level. /


displacement |noun| movement away from the normal position / The ILS is a cross-pointer indicator which shows the aircraft horizontal displacement from the localiser and vertical displacement from the glide path. /


display |noun|  1. the appearance of information on a monitor screen or on the panel of an instrument or of an indicator / There are three different types of electronic display systems: EFIS, EICAS and ECAM. / - digital display > information shown as numbers / The clock uses a digital display to show the time of 12:33. /  2. a show or demonstration - |verb| to show, e.g. on a panel or a screen / Alerting and warning information is displayed. /


disregard |verb| to decide not to comply with or ignore an instruction, information or recommendation etc. / Flight crews may disregard controller instructions in certain situations, for example to resolve a TCAS Resolution Advisory. /


disseminate |verb| to send out or spread / Meteorological stations make routine weather observations at fixed intervals and disseminate this information locally. /


dissimilar |adjective| referring to something which is not the same as, or is unlike, something else / Differential expansion switches operate on the principle that the coefficients of expansion of dissimilar metals are different. /


dissipate |verb| to spread out and lose power or strength, or to cause something to do this / Tropical storms often dissipate as they pass from sea to land. /


dissipation |noun| the process of spreading out and losing power or strength / The rubber used on nose or tail wheels is usually constructed to form a good electrical conductor for the safe dissipation of static electricity. /


dissolve |verb| to become or to cause to become part of a liquid and form a solution / Sugar dissolves in water. There is a possibility that in some types of accumulator, gas may be dissolved into the fluid and thus introduced into the system. /


dissolved |adjective| that has melted and become of a liquid - dissolved water > water in solution in fuel


distance |noun| a space between two places or points, or the measurement of such a space / The distance from point A to point B is 100 nautical miles. The distance from point A to point B on the diagram is 2 cm. The height of the aircraft is the vertical distance, measured in feet, of the aircraft above the surface of the Earth. /


distance measuring equipment |noun| an airborne secondary radar whose signal is converted into distance / It is quite common to find a VOR located together with DME (Distance Measuring Equipment) to give simultaneous range and bearing from the same point on the ground. / Abbreviation: DME (NOTE: DME equipment is usually located in a VOR station. Other equipment in the aircraft transmits a signal to the VOR station, which replies. The equipment in the aircraft converts the signal into distance and also calculates ground speed and the time needed to reach the station.)


distillation |noun| the process by which a liquid is heated and the resulting vapor is then condensed and collected / With kerosene-type fuels, the volatility is controlled by distillation. /


distinct |adjective| clear and easily seen or understood / When a lead-acid battery is fully charged, each cell displays three distinct indications. /


distinction |noun| something which makes one thing different from another / A clear distinction is made between showers and general precipitation. /


distinctive |adjective| easily recognized because of particular features or characteristics / Concorde is a very distinctive-looking airplane. /


distinguish |verb| to know or to see the difference between things / A receiver antenna would be unable to distinguish between signals unless they had some differing characteristics. /


distinguishable |adjective| easily recognized as different from /Useful ground features must be easily distinguishable from their surroundings./


distort |verb|  1. to put out of shape / Stress could cause the body of the aircraft to distort or change its shape. /  2. to produce a bad radio signal / The sound of the transmission is distorted if the volume is set too high. /


distortion |noun|  1. the bending or twisting of something so that it is out of shape / Difficulty in closing a door may be caused by distortion of the airframe. /  2. alteration of the electrical signal that makes a transmission unclear / Distortion of the signal made it difficult for the controller to understand what the pilot said. /


distraction |noun| something which disturbs mental concentration and attention / A cabin attendant entering the flight deck when crew workload is high will be a distraction. /


distress |noun|  1. serious danger or difficulty  2. a personal worry or anxiety / Some passengers were in distress after the incident. /


distress and diversion cell |noun| a unit at an air traffic control center that provides immediate assistance to aircraft in difficulty


distress signal |noun| a signal transmitted by an aircraft in danger


distribute |verb| 1. to give or send out / There are two basic configurations which are used to distribute electrical power, the parallel system and the split bus system. /  2. to spread over a wide area / Multiple wheel undercarriage units distribute the weight of the aircraft. /


distribution |noun|  1. the act of giving or sending out / Parallel AC and DC power distribution systems are found on commercial aircraft containing three or more engines. / 2. the fact of being spread over an area / There is a high distribution of used and disused airfields in the south of England. /


distributor |noun| a device which sends an electrical charge to each spark plug in turn / The distributor directs the high voltage impulses to the cylinders in turn as they reach their ignition point. /


disturb |verb| to upset the normal condition of something / Small hills can disturb the flow of air. /


disturbance |noun| something that upsets the normal condition of something / In general, the higher the mountain and the faster the air flow the greater is the resulting disturbance. /


ditch |verb| to land a plane in the sea, in an emergency / Even though aircraft have ditched successfully, lives have been lost because life rafts were not launched in time. /


ditching |noun| the act of landing a plane in the sea, in an emergency / After all four engines stopped, the captain had to seriously consider the possibility of a ditching in the Indian Ocean. /


diurnal |adjective| referring to the 24-hour cycle of day and night / Diurnal changes in surface temperature over the sea are small. /


dive |noun| a steep nose-down attitude of an aircraft - to pull out of/from a dive > to return the aircraft to level flight after a nose-down flight path / During maneuvering of an aircraft, when banking, turning and pulling out from a dive, stresses on the airframe are increased. / - |verb| to put the aircraft into a steep nose-down attitude / The aircraft dived to avoid the other aircraft. / (NOTE: diving – dived )


diverge |verb| to move further apart from something else / Air diverges at low levels and converges at high levels, causing a sinking or subsiding effect in the atmosphere. / Opposite: converge


divergence |noun| the act of moving apart / Divergence of air at high levels leads to rising air at low levels with a consequent pressure fall. / Opposite: convergence


divergent |adjective| referring to something which moves further apart from something else


divergent duct |noun| a duct which has an inlet area which is smaller than the outlet area


diversion |noun| a change in route or destination caused by bad weather, technical problem, etc. / The aircraft had to make a diversion to another airport due to fog. /


divert |verb| to turn away from a course or a destination / An automatic cut-out valve is fitted to divert pump output to the reservoir when pressure has built up to normal operating pressure. The aircraft was diverted to Manchester airport because of fog. /


divide |verb|  1. to separate into parts / Air masses are divided into two types according to source region and these are known as polar and tropical air masses. /  2. to calculate how many times a number is contained in another number / Eight divided by four equals two (8 ÷ 4 = 2). /


division |noun|  1. separation into parts - the division of the lower atmosphere > the separation of the atmosphere into its component layers  2. the calculation of how may times a number is contained in another number / The division sign is ÷. /


DME |abbreviation| distance measuring equipment


document |noun| a piece of writing, e.g. a memo, letter or report / The flight crew route flight plan is a composite document which also serves as a navigation log. /


documentation |noun| a collection of letters, memos, reports, etc. / Flight crews are provided with a full meteorological briefing, backed by documentation, a short time before ETD. /


do-list |noun| a series of actions to be performed in the form of a procedure; it may be performed by one crew member, technician or controller. It is often used for routine actions.


domestic |adjective| referring or belonging to inside a country / Domestic flights usually leave from Terminal 1. /


dominant |adjective| main or most influential / Both pressure and temperature decrease with height but the pressure change is the dominant one and so, as pressure decreases with height, so does density. /


dominate |verb| to have the most effect or influence on / Because the chart time and the departure/arrival times differ, it is necessary to consider the movement of any weather system which will dominate the route. /


Doppler radar |noun| radar which can distinguish between fixed and moving targets or provide ground speed and track information from an airborne installation


Doppler VOR |noun| an adaptation of VOR to reduce errors caused by location


dot |noun| a small circular mark on paper / The highest point in a locality is marked by a dot with the elevation marked alongside. /


downdraft |noun|  1. cool air which flows downwards as a rainstorm approaches. Opposite: updraft  2. air which flows rapidly down the lee side of a building, mountain, etc.


downstream |adverb| in the direction of flow, or further along the line of flow / Internally driven superchargers are generally used on medium and high powered engines and are fitted downstream of the throttle valve. /


downward |adjective| moving to a lower level / When flying in turbulent air conditions, an aircraft is subjected to upward and downward gust loads. /


downwards |adverb| to a lower level, towards the bottom / Pull the toggles downwards to inflate the life jacket. / Opposite: upwards (NOTE: In US English, downward  is used as an adverb and as an adjective.)


downwind |adjective|,|adverb| in the same direction as the wind is blowing - turn downwind > turn the aircraft so that it is flying in the same direction as the wind is blowing. Opposite: upwind


downwind leg |noun| part of the airfield traffic circuit which runs parallel to, but in the opposite direction to, the approach to land which is made into wind


DR |abbreviation| dead reckoning


draft |noun| US same as draught


drag |noun| the resistance of the air created by moving the aircraft through the air / To reduce the effect of drag on an aircraft by the fixed under-carriage a retractable type was introduced. If an engine failure occurs, the windmilling propeller may cause considerable drag. / (NOTE: There are two basic types of drag called parasite drag and induced drag. Parasite drag is caused by friction between the air and the aircraft surface, aerials, landing gear, etc. Induced drag is produced by lift.)


drain |noun| a device to allow fluid to escape from its container / When the cabin is pressurized the drains close, preventing loss of pressure. / - |verb| to allow fluid to escape by providing a hole or tube, etc., through which it can pass / The moisture drains in the lower skin of the cabin are open when the cabin is unpressurized, allowing moisture to drain. /


drainage |noun|  1. the act of allowing a fluid to escape from its container / Drainage of water from the fuel system should be carried out before the first flight of the day. /  2. a system of outlets for fluid such as water or fuel to pass out of a closed area


draught |noun| a local current of air / a downdraft or an updraft / (NOTE: This word is written draft in US English.)


draw |verb|  1. to make a picture as with a pencil, on paper, etc. / Because there is a temperature gradient across each front it is possible to draw isotherms which reduce in value from warm to cold air. /  2. to pull or to take / Fluid is drawn into the pump body. /  3. to pull towards oneself (NOTE: drawing – drew – drawn)


drift |noun| movement away from the desired course, created by wind blowing at an angle to the intended direction of flight / If the wind direction is not the same as the aircraft track or its reciprocal, then the aircraft will experience drift. / - |verb| to move away from the desired course / When landing, a cross-wind from the right will cause the aircraft to drift to the left. / - drift correction > the action by which the pilot corrects the horizontal flight path by bringing the aircraft back onto the extended runway center-line or localizer beam - drift-down > losing height gradually


drifting snow |noun| snow that has been blown by the wind to form a deep deposit. Abbreviation: DRSN


drill |noun|  1. a short series of actions carried out in a particular sequence / The starting drill varies between different aircraft types and a starting check procedure is normally used. /  2. a tool, often electrically powered, for making holes in metal, wood, etc.


drive |noun| a series of connected devices that transmit power to the wheels, propellers, etc. / Rotation of the engine for starting is done by an electric starter motor connected to a drive shaft in the accessories gearbox. / - |verb|  1. to make something move or turn - shaft-driven > using a rotating shaft as a means of transmitting power from one part to another, e.g. from a turbine engine to a helicopter rotor  2. to control and guide / He’s learning to drive. / (NOTE: driving – drove – driven)


drizzle |noun| precipitation, often persistent, in the form of very small drops of water / Drizzle is the lightest form of precipitation consisting of fine water droplets. / (NOTE: In weather reports and forecasts, drizzle is abbreviated to DZ.)


drogue parachute |noun| a small parachute used in releasing a larger parachute from its pack


drone |noun| an aircraft whose flight is controlled from the ground


drop |noun|  1. a small amount of liquid that falls / a drop of water, a few drops of rain /  2. a sudden lowering / The passage of a cold front is usually followed by a drop in temperature. A sudden drop in oil pressure is normally an indication of serious engine trouble. / - |verb| to become lower or to decrease suddenly / The temperature dropped by several degrees. /


droplet |noun| a small drop of liquid / Experiments show that smaller droplets of rain can remain super cooled to much lower temperatures than large droplets. /


DRSN |abbreviation| drifting snow


drum |noun| a cylindrical device, often with closed ends


dry |adjective| containing no water or no moisture


dry ice |noun| solidified carbon dioxide


dual |adjective| double, in pair / Most light aircraft with side-by-side seating have dual controls. /


duct |noun| a channel or tube through which fluids or cables can pass / The modern jet engine is basically a duct into which the necessary parts are fitted. /


due |adjective|  1. expected to arrive - the flight is due at 10 o’clock > the flight should arrive at 10 o’clock  2. - due to > because of / Due to daytime heating, the stability decreases and the wind speed increases. / - |adverb| exactly and directly / The aircraft flew due east. /


dump |verb| to offload quickly / Normal operating cabin pressure can be reduced rapidly in the event of emergency landings, by dumping air. The aircraft flew out to sea in order to dump fuel before landing. /


duplication |noun| the act of copying or doubling / Control surfaces are divided into sections operated by a separate control unit, thus providing duplication to guard against failure of a unit. /


durability |noun| the ability of a substance or device to last a long time / High quality components have good durability. /


duration |noun| the length of time for which something continues / The duration of the examination is two hours. The duration of the flight was three hours. /


dust |noun| a fine powdery substance blown by the wind and found on surfaces / Solid particles in the air include dust, sand, volcanic ash and atmospheric pollution. /


Dutch roll |noun| aircraft oscillating from side to side / Our Yaw Damper system is inoperative and we are experiencing some Dutch roll. /


duty |noun|  a period of work - on duty > at work - off duty > not at work - duty time > the time during which a crew is scheduled and authorized to work


dye |noun| a material used to change the color of something / Minute surface cracks which are difficult to detect by visual means may be highlighted by using penetrating dyes. /


dynamic |adjective| referring to something in motion - dynamic pressure > pressure created by the forward movement of the aircraft / If the dynamic pressure increases due to an increase in forward speed, the force required to move the control column will increase. / Opposite: static pressure


dynamic seal |noun| a seal which is part of a moving component, e.g. in a hydraulic system / dynamic seals require lubrication to remain effective /


DZ |abbreviation| drizzle

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