EASA  Part-66      Terms
Term    Description
Abradable Seal    A general description of 'knife edge’ seals which can wear away slightly and still function
Abrasion    A roughened area caused by the presence of fine foreign material between moving surfaces
Acceleration    Rate of change of velocity Velocity + Time or Distance + Time²
Accessories    Components driven from the engine to assist the function of the aircraft
Adaptor    A connection for joining two components through which a fluid or electricity can be transferred from one section or component to the other

Adiabatic    A process in which no heat is received or lost. This is the ideal compression process
Advance per Revolution    The effective pitch or the actual distance moved forwards by a propeller in one complete revolution taking account of slip. It is the geometric pitch less slip.
Adverse Yaw    A tendency of an aircraft to yaw in a direction that is opposite to the direction of a controlled turn as a result of an imbalance in induced drag created by deflection of the ailerons.
Aeration    The introduction of air into a liquid
Aerodynamic Balance    Achieved when the thrust forces produced by each of the blades are equal and the torque forces produced by each of the blades are equal. No residual periodic out of balance forces will be produced when the propeller is rotating.
Aerodynamic Centre    A reference point on the chord-line of a wing, about which the pitching moment remains constant regardless of the angle of attack
Aerodynamic Correction Factor (AOF)    The amount in degrees and minutes to be added or subtracted from the basic blade angle setting when installing each blade on a variable pitch propeller. The procedure is described as individual 'blade indexing' and is carried out to achieve the correct aerodynamic balance. An AOF value preceded by the letter T indicates thrust balance, when preceded by the letter Q it indicates torque balance.
Aerodynamic Turning Moment (ATM)    A turning moment that tries to turn a propeller blade towards coarse pitch. The force is the result of the aerodynamic reaction acting on the blade centre of pressure. The moment arm is the distance between the centre of pressure position and the blade pitch change axis. It is weaker than the Centrifugal Turning Moment (CTM). During the 'windmill' condition, the ATM reverses to try and turn the blade towards fine pitch.
Aerofoil    A body shaped so as to produce an aerodynamic reaction called lift that acts perpendicular to its direction of motion or line of flight
Aerofoil    Shape of blade section formed to give lift
Aerofoil    A body shaped so as to produce an aerodynamic reaction normal to its direction of travel.
Aileron    The control surface used for lateral control
Air Density    The mass of air per unit volume. Normally expressed in kilograms per cubic metre. An increase in air density will increase thrust and torque.
Airspeed    The speed of an aircraft relative to the air
Allowance    Permitted difference in dimensions to allow for various fits
Aluminised    Coated with aluminium to resist corrosion
Ambient Pressure    The pressure of the air immediately surrounding an object
Aneroid capsule    A metal Container from which most of the air has been exhausted. It is sensitive to the variations of outside pressure
Angle of Advance    The angle formed between the plane of rotation and the relative airflow. The angle of advance subtends the effective pitch. It is also referred to as the helix angle.
Angle of Attack    The angle formed between the chord line of an aerofoil and the relative airflow direction
Angle of Attack    The angle that the chord line makes with the direction of airflow
Angle of Attack    The angle formed between the chord line of a blade and the relative airflow. The ideal effective angle of attack is between 3 and 4°. The angle of attack subtends the propeller's slip. If there were no slip, there would be no angle of attack.
Angle of Incidence    The constructed angle formed between the longitudinal datum of the fuselage and the chord line of the wing
Anhedral    The downward slope of a wing relative to the horizontal plane designed to reduce lateral stability
Anti-icing    The prevention of ice formation on a surface.
Anticipator    A system incorporated in the control run to the Propeller Control Unit and the Fuel Control Unit on some turbo-propeller engines. The system instigates a slight reduction in blade angle during engine acceleration and a slight increase in blade angle during engine deceleration. The objective is to improve the engine acceleration and deceleration times respectively.
Annular    Circular, ring formation round a component
Annulus    Opening between two concentric rings
Approach Minimum    Minimum engine speed for landing aircraft
Aspect Ratio    The ratio of the wing span to the average or mean chord. Can also be the ratio of the square of the wing span to the total wing planform area.
Aspect Ratio    The ratio between the mean chord width of a blade and its span. A high aspect ratio produces a long, narrow blade. The same ratio can be calculated by dividing the square of the blade length by the area of the face side.
Atomisation    The formation of a liquid into a spray
Attenuator    A means of stopping fuel flow fluctuation
Automatic    Self-operating
Auto-coarsen    A system that will automatically coarsen the propeller to prevent over-speeding if the hub switch closes in flight. Closure of the hub switch in flight will only occur if the FFPS has failed to prevent the propeller entering the ground range. The hub switch energises the 'valve lift solenoid', otherwise known as the 'increase pitch solenoid valve'. The pilot valve in the PCU is raised to direct governor oil pressure to increase pitch. The system deactivates again as soon as the hub switch breaks. The Below Stop Warning Light will flash ON and OFF when the system is activated. Otherwise known as the Electro-Hydraulic Pitch Stop.
Auto-feather    A system that uses the engine torque indication system to sense an engine failure and then automatically feathers the propeller. If low torque is sensed, a low torque switch will close and the system energises the "valve lift solenoid', otherwise known as the 'increase pitch solenoid valve', in the PCU and also energises the feathering motor. The pilot valve in the PCU governor is raised to direct feathering oil to the pitch change mechanism. The system is only active when the power lever has advanced to the cruise position. This is necessary because the system would pose a risk at low power settings on approach. Otherwise known as the 'automatic drag limiting system' (ADLS).
Auxiliary    Something, which is helpful to something else
Axial Flow Engine    An engine in which the gas flow travels along the centre line of the engine from front to rear
Axis    A line about which the aircraft rotates
Axis of Rotation    The centre line around which the propeller rotates. It is at right angles to the plane of rotation.
   
Backlash    The working clearance, measured at the pitch circles of any two gears in mesh
Baffles    Plates fitted to prevent or control the movement of a fluid in the direction, which it would otherwise flow
Balancing    Adjusting the size and position of weights to bring a rotating assembly into static and dynamic balance
Bank    The attitude adopted by an aircraft in a turn where the lift supplies a component towards the centre of the turn called centripetal force
Barometric Pressure    A pressure created by the weight of the atmosphere. It is reduced with an increase of altitude
Bearing    The part of a mechanism in which a rotating shaft revolves and is supported
Below Stop Warning Light    A warning light activated by the hub switch that comes ON whenever the blade angle reduces below Flight Fine Pitch. The objective is to give warning that the propeller is in the ground range. The light remains on throughout the Beta range. Otherwise known as the 'Beta Warning Light'.
Bending Forces    The bending forces acting on a blade. The thrust bending force tries to bend the blade forward. The torque bending force tries to bend the blade against its direction of rotation.
Bernoulli's Theorem    States that the total energy contained in an incompressible flow of air through a venturi will remain constant
Beta Range    The ground range of a propeller's operation. The blade angle range below the flight fine pitch angle, reserved for ground operations such as: engine starting, taxiing and braking. The range cannot be used in flight. The blade angle movement is under the direct control of the power lever whilst in the ground range. The range consists of a maximum in beta positive angle limit, moving back through zero pitch to a maximum negative blade angle limit at Full Reverse.
Bevel    An angle other than a right angle
Bit    A bit is the smallest unit of information in computer and communication theory, representing a logic '1' or logic '0'. It is derived from contracting the terms binary bit.
Blade Angle    The angle between the chord line and the plane of rotation
Blade Angle    The angle formed between the chord line of a blade and the plane of rotation measured at the master station. The angle is the sum of the angle of attack and the angle of advance or helix angle. The blade angle subtends the geometric pitch.
Blade Cuff    An extended fairing incorporated onto the trailing edge of the thickened blade root to improve efficiency and direct more air over the engine, giving better reciprocating engine cooling and improved turbo-propeller intake airflow.
Blade Element    A chord-wise cross-section of a propeller blade taken at any point.
Blade Flutter    A cyclic movement of a blade in its hub fixture that results from excessive play in the blade bearings. The oscillating movement is caused by variations in the aerodynamic and centrifugal forces. The condition is avoided by blade bearing pre-loading during assembly.
Blade Path    The actual helical path of a propeller blade in flight, resolved from its radial and axial movements.
Blade Pre-load    A fixed loading that is placed on to the blade bearings during assembly. The load is set by use of pre-load shims and torque loading on the blade retention nut. The objective of 'pre-loading' is to avoid 'blade flutter".
Blade Station    A position on a blade measured in inches from the blade root. Used as a reference when measuring blade angles or identifying repair areas. The blade angle is usually measured at a 'Master Station' that is positioned about 0.75 of the blade length from the root.
Blade Track    The path followed by a blade when the aircraft is stationary. All parts of the blade should take up a path that is aligned within the plane of rotation.
Blade Twist    The blade angle is gradually reduced from the blade root to its tip to maintain the same angle of attack along the length of the blade. The rotational speed of a blade increases with its distance from the axis of rotation but the forward speed of all parts of the blade is the same so the direction of the relative airflow will van/ along the blade. This makes it necessary to reduce the blade angle from the root to the tip.
Bleed    The removal of air, or air contaminated liquid, from a System
Blending Out    The process where minor damage on a blade is removed to leave a smooth, crack free depression that will avoid any build up of stress. It is a process of converting rough or sharp edges into smooth rounded depressions.
Blisk    A forged one-piece blade and disk
Blueing Check    A pre-installation check that is carried out to confirm the location cone seating area contact. The indicator used is described as being either 'Prussian' or "Engineer's" Blue. The minimum acceptable contact area is 80%.
Bonding    Linking together all the metal parts of an aircraft to obtain positive electrical continuity
Bore    The internal diameter of a cylindrical part
Borescope    An optical tool with which a Visual inspection can be made inside an area which is otherwise impossible to see
Boss    The centre region of a fixed pitch propeller.
Boundary Layer    The layer of air flowing over a surface where shearing action takes place between the surface and the free stream air velocity that retards the flow as the surface is approached
Brake Horse Power    The actual horsepower delivered to the propeller shaft of the engine
Brazing    Uniting two metals by means of molten brass
Breather    A duct connecting the crankcase to the atmosphere that prevents the build-up of crankcase pressure
Brinelling (false)    A satin finish or a series of shallow depressions in the surface of ball or roller bearing braces
Brinelling (true)    A shadow indentation sometimes found at one location on the surface of ball or roller bearing braces
British Thermal Unit (Btu)    The amount of heat required to raise the temperature of one pound of water by 1°F
Brittleness    The liability of a metal to fracture on receiving a blow or shock
Brush Gear Housing    A component mounted at the rear of the propeller, or on the engine casing, that holds the carbon brushes in contact with the slip rings that transmit electrical power for propeller and spinner ice protection. It is a requirement that the brushes have a minimum contact area with the slip ring of 80%.
Bulkhead    A transverse partition which separates one compartment from another
Burning    An injury to the surface caused by excessive heat
Burnishing    A mechanical smoothing of a metal surface by rubbing
Burr    A rough, sharp ridge or projection at the edges of a part after it has been worked or machined
Bush    A hollow cylinder one piece bearing usually phosphor bronze or cast iron
By pass Ratio    Mass airflow flowing through the by-pass duct divided by that passing through the core engine
Byte    The basic unit of information used in practical computer storage and processing. A byte consists of eight (S) adjacent binary bits and this string of bits is processed as one unit by a computer; they are the smallest operable units of storage in computer technology. An individual byte can represent the equivalent of a single character, such as the letter B, a comma, or a percentage sign (%); or it can represent a number from 0 to 255. Because a byte contains so little information, the processing and storage capacities of computer hardware are usually given in kilobytes (1,024 bytes) or megabytes (1,048,576 bytes).
   
Calibrate    To measure or check against a known true master tool or instument
Cam     An eccentric projection on a revolving shaft designed to change rotary motion into reciprocating motion
Camber    The curvature of an aerofoil represented by a line drawn equidistant from the upper and lower surfaces
Camber Side    The curved side of the aerofoil cross-section of a blade that forms the blade back.
Capillary    A tube processing a hair-like bore
Casting    Pouring molten metal into a mould in which it is allowed to solidify. The resultant shape is a casting
Catalyst    A material used to bring about a change, but does not actually enter into a change itself
Centistoke    A unit of viscosity measurement of both fuels and oils
Centre of Gravity    The point through which the weight of a body acts whatever position the body may be in
Centre of Pressure    The position on the chord at which the resultant of all the lift forces acts
Centre of Pressure (CP)    The point on the chord line of a blade where the resultant of all the thrust forces on the blade is said to act.
    The point is normally forward of the blade s pitch change axis.
Centrifugal    The throwing out action of a revolving mass
Centrifugal Force    A force that acts on a blade as the result of the blade mass rotating. The force creates maximum stress in the blade root sections.
Centrifugal Latch    A latch that mechanically locks the pitch change piston in a low pitch position when the propeller is not rotating. The latch is incorporated in to single-acting counterweight propellers to prevent them from being forced into feather by the 'feathering spring' after engine shut down. The latches are engaged by spring pressure and are disengaged by centrifugal force.
Centrifugal Turning Moment (CTM)    A powerful turning moment that tries to turn a blade towards fine pitch. The force is centrifugal. The moment is the distance that the mass of the blade leading and trailing sections project forward or to the rear of the plane of rotation. CTM is always greater than ATM.
Centripetal Force    The continuous force that keeps a body travelling in a circular path
Ceramic    Clay-like material composed primarily of magnesium and aluminium oxide, which may have been moulded and fired to produce an excellent insulating material
Chafing    A rubbing action between two parts, which have a limited relative motion
Check    To make a comparison of a measurement of time, temperature, size, pressure or any other quantity with the correct figure for that measurement
Chipping    Breaking out small pieces
Choked    A condition where airflow from a convergent nozzle is at Mach 1 and cannot be further accelerated regardless of pressure applied.
Choo-Choo    A mild compressor surge condition
Chord    A straight line that joins the centre of curvature of the leading edge of an aerofoil section to the centre of curvature or apex of the trailing edge
Chord    An imaginary straight line which passes through an aerofoil or wing section from the leading edge to the trailing edge
Chord    A straight line that joins the centre of curvature of a blade aerofoil section leading edge to the centre of curvature of its trailing edge. On many propeller blades, the face side is flat and its width is coincident with the chord of the blade.
Chrome-Alumel    Bimetallic metal used in the exhaust temperature indicating system
Chugging    Low frequency oscillations of airflow within the engine
Clearance    The space provided between two working parts to allow for freedom of movement
Clutch    A device by which to shafts or rotating members may be connected or disconnected while at rest or in relative motion
Co-axial    Shafts or gears having the same centre line
Co-axial Propellers    Two Counter-rotating propellers that are mounted on the same rotational axis but are driven by separate engines.
Coarse Pitch    Largest blade angle normally used
Coarse Pitch    A high blade angle.
Coefficient    A numerical constant that is used as a multiplier when calculating a variable quantity such as lift or drag
Combustible    Liable to burn, to spring suddenly into flame
Combustion    A chemical process in which a material is united which oxygen at such a rate that light and heat are released
Combustion Chamber    Section of the engine in which fuel is injected and burned
Comparator    A unit incorporated into an engine synchronisation or synchrophase system that compares signals from a "master" engine to those from   slave" engines and then transmits signals to the 'slave' engines to correct their RPM or phase.
Compound Engine    A configuration of turbo-prop engine that has two shafts, a Gas Generator shaft that drives a high-pressure compressor, and a Power Turbine shaft that drives a low-pressure compressor and the propeller. The engine has the flexibility and ease of starting features of a twin-spool engine but does not have the flexibility of a free-turbine engine.
Compression Ratio    The ratio of volume before compression to the volume after compression
Compressibility Effect    The loss of efficiency caused by the formation of a shock wave when a blade tip reaches sonic airflow velocity. Torque increases, thrust reduces and a lot of noise is generated and the tip loss component of propeller efficiency increases. Caused by high blade tip velocities.
Compressor Stall    The abrupt loss of the efficiency of the axial flow compressor in a turbine engine when the angle of attack of the compressor blades becomes excessive
Concentric    Having a common centre
Conductivity    The ease with which a substance transmits electricity
Cone    Conical seating used to locate and centralise a propeller on to its drive shaft. Two cones are used. A two-piece, steel front cone and a split bronze rear cone. The conical section permits a greater contact area. Minimum permissible contact area is 80% as proven by a pre-installation "blueing check".
Constant Speed    Pitch varies to maintain a constant propeller rotational speed. The reason for doing this is to maintain an effective blade angle of attack throughout all aircraft and engine speed variations.
Constant Speed Unit (CSU)    An automatic governor that is sensitive to engine rpm and will alter the angle of the blades to maintain a pre-selected engine RPM and blade angle of attack when changes in aircraft speed and engine RPM occur. The unit takes its drive from the engine reduction gearing so that the governor speed is directly related to propeller speed. The name CSU is used to describe the unit fitted to a reciprocating engine propeller control system. When related to a turbo-propeller system, the unit is more complex and is described as a Propeller Control Unit (PCU).
Contra-rotating Propeller    Two propellers mounted on the same shaft but rotating in opposite directions. The rear propeller is driven directly by the shaft whilst the front propeller is driven through a gearbox. It is a means of improving solidity and absorbing engine power.
Convection Cooling    Refers to internal cooling air which escapes through small holes and slots
Convection Current    The movement set-up in heated air or a fluid due to a part of the substance moving away from the source of heat and taking the heat with it
Convergent    A convergent duct is one that has a gradual reduetion in the size of the bore
Convergent-Divergent Duct    Afterburner design, a supersonic exhaust duct
Corrected RPM    An rpm obtained by correcting an observed rpm
Corrosion    Breakdown of the surface by chemical action
Corrugated    Having a ridged or wrinkled surface
Counterweight    A weight that is attached to the blade root of each blade on a single-acting variable pitch propeller to overcome CTM and use centrifugal force to turn the blades towards coarse pitch. Propeller control oil pressure can then be used to move the blades to fine pitch. A failure in propeller control oil pressure will result in the blades moving coarse towards feather rather than fine pitch.
Couple    Two equal and opposite forces that act on a body that are not in the same straight line but whose moment about any point in the plane is always the same. A single force cannot balance a couple. It can only be balanced by an opposing couple of equal magnitude.
Creep    A condition of permanent elongation in rotating airfoils from thermal stress and centrifugal loadings
Critical Range    The range of engine RPM where vibration is known to occur. It is normally displayed with a red arc on the engine RPM indicator. Sustained operation in this range is normally prohibited.
Cropping    The removal of a section of a blade tip within specified limits. Done to remove damage. To preserve balance, all blades on a propeller must be cropped by a similar amount. The maximum amount a blade can be cropped is normally one inch.
Cruise    A moderate speed of travel at optimum speed for sustained flight
Cruise Control    Engine operation procedures that allow the best efficiency for power and fuel consumption during flight
Cruise Power    Used for fuel economy and engine life during cruising flight
Cruise Stops    Stops that are used to prevent blades fining off when the aircraft is travelling at high speed. The steps on all the propellers are automatically engaged by solenoids that are signalled by hub switches at cruise RPM. They will automatically withdraw if all engine power levers are pulled back together. If one propeller control system fails, the related step will prevent the propeller fining off. The cruise stop warning light extinguishes when the stops are engaged.
Curvic Coupling    A circular set of gear-like teeth on each of two mating flanges which provide engagement when meshed together and bolted
Cut-off    To sever an object or step flow
Cyclic Timer    The timer used in conjunction with electrical ice protection systems. The timer has two speeds - FAST and SLOW. The fast cycle is selected in the higher ambient temperature range -6°C to +10°C. The slow cycle is selected in the lower ambient temperature range below -6°C.
   
Dashpot    A mechanical damper used to cushion or slow down movement by restricting the flow of a viscous liquid
Datum    A quantity, condition, fact, or other premise, given to be admitted, from which other things or results may be found
De-icing    Application of a liquid or heat to a component to break up and/or prevent the formation of ice
De-icing    The removal of ice from a surface after it has formed.
De-Inhibit    To remove the inhibitor from an item before being put into service
Dead Throttle Movement    Movement of the throttle lever with no corresponding response from the engine
Deceleration    The rate of decrease of velocity
Defect    Any imperfection, fault, flaw or blemish which may require repair or replacement
Density    The mass per unit volume of a material; expressed as grams/cm³ or kilograms/m³
Density    Mass per unit volume
Density Altitude    That altitude in standard air, which compares with existing air density
Depression    A pressure below standard atmospheric pressure
Detent    A pin, catch or lever forming a check to the mechanism
Diaphragm    A flexible partition used to transmit force
Differential Pressure Switch    A diaphragm and electrical micro switch arrangement, which receives two pressure senses
Diffuser    1. A duct used on a centrifugal-flow turbine engine to reduce the velocity of the air and increase its pressure
    2. The divergent section of the gas turbine engine used to convert energy of the compressor discharge air to pressure energy
Dihedral (Lateral)    The upward slope of a wing relative to the horizontal plane designed to increase lateral stability
Dihedral (Longitudinal)    The angle between the chord of the tail plane and the chord of the main plane designed to give longitudinal stability
Direct-coupled Engine    A configuration off turbo-prop engine that has a single shaft. The turbine shaft drives the compressor and the propeller. The principle is described as 'gear coupled’ because of the mechanical connection between the compressor and propeller drive. The engine offers good acceleration and deceleration characteristics but does not have the flexibility or ease of starting offered by twin spool compound and free-turbine engines.
Direction of Rotation    The direction of rotation of a propeller as viewed from the rear. A clockwise rotation is described as being RIGHT HAND.
    An anti-clockwise rotation is described as being LEFT HAND.
Directional Stability    The ability of an aircraft to regain its original position following a divergence in yaw around normal axis. Stability is provided by the keel surface aft of the CG.
Disc Area    The area of a circle circumscribed by a blade tip
Disconnect    Uncouple or detach pipelines, controls, cables, etc
Displacement    This is the total distance travelled by the vibrating body from one peak to rest. It is the amplitude of the perodic movement.
Divergent    A divergent duct is one that has a gradual increase in size of the bore, a passage, which widens in the direction of the flow
Dowel    A small diameter raised plug which fits into a matching hole, used for locating items
Drag    The force that opposes the forward motion of a body through the air
Drag    Air resistance. An aerodynamic force on a body acting parallel and opposite to the relative wind
Drag    The resistance force that is created by a body moving through the air. It acts in direct opposition to the movement of a blade and is referred to as the torque.
Drain    A small hole or pipeline leading from a component to atmosphere, to allow fluid to vent, or to empty a tank, cavity or sump
Duct    A cast or drilled hole forming a passage to convey air, oil, and fuel through engine parts
Dutch Roll    A roll and yaw motion. Occurs when lateral stability is higher than directional stability particularly on swept back wing aircraft
Dynamic Balance    Achieved when the centre of gravity of the propeller lies within the plane of its rotation and no periodic out of balance moments are created by centrifugal force when it is rotating.
Dynamic Stability    The ability of an aircraft to return to its original Position after a disturbance without oscillating around that position
   
Elasticity    The capacity of a material to return to its original dimensions on the removal of distorting forces
Empirical    Relying on observation or experiment, not on theory
End Float    The axial movement of a gear or shaft mounted in bearings
Endurance    The ability to remain in the air for the longest possible time
Energy    The capacity to do work
Engine Pressure Ratio (EPR)    The ratio of turbine discharge pressure divided by compressor inlet pressure
Epicyclic Gear    Where a gear or train of gears revolve round the circumference of a larger gear
Epicyclic Reduction Gear    The reduction gearing used on turbo-prop engines and some types of reciprocating engine. Consists of a sun and planet gears that rotate within a fixed or rotating annulus depending on type. The gear reduces the rotational speed of the propeller drive shaft relative to the engine speed, producing high torque and reducing propeller blade tip speeds. Compound epicyclic reduction gearing used on turbo-prop engines may have engine/propeller speed ratios in the order of 22:1.
EPR Rated Engine    A gas turbine engine whose "rated thrust" is guaranteed to occur at a certain engine pressure ratio setting
Equilibrium    The State of even balance where all opposing forces and turning moments neutralise each other
Equivalent Airspeed (EAS)    The airspeed calculated from the measured pressure difference between ambient and sea level ISA
Equivalent Shaft Horsepower (ESHP)    A turbo-prop engine power rating that is the sum of the shaft horsepower (SHP) and the engine residual jet thrust horsepower (THP). Residual jet thrust is converted to thrust horsepower by assuming that 2.51b of residual jet thrust is equivalent to one shaft horsepower.
Equivalent specific fuel consumption (ESFC)    A means of comparison for turboprops and some turbo shafts where: ESFC = Wf ÷ ESHP
Evacuated bellows    A set of bellows from which all of the air has been removed and the bellows sealed
Examine    To make a visual survey of the condition of an item
Extrusion    Forcing plastic metal through a die of required shape by means of hydraulic pressure
   
Face Side    The flat side of the aerofoil cross-section of a blade. It is also referred to as the pressure side
Fatigue    The diminishing resistance to fracture caused by fluctuating stresses
Feather    The blade coarse pitch position where the blade chord line is aligned with the direction of flight. The selection is used to prevent a windmill condition following an engine shut down.
Feather Stop    An internal fixed stop that limits the travel of the pitch changing mechanism at its maximum coarse position. On feathering propellers, this step coincides with the blade's feathered position.
Feathering Pump    A mechanical engine oil pump driven by a heavy duty 25V DC electrical motor that can be selected on to draw oil from a feathering oil reserve well in the engine oil tank and then to supply the oil under pressure to the propeller governor when the normal engine oil pressure supply fails.
Feathering Reserve    A volume of engine oil contained in a well in the engine oil tank that cannot enter the normal engine oil circulation system. It has a single cutlet to the feathering pump.
Feathering Spring    A double spring incorporated into single acting variable pitch propellers that applies a force to move the blade towards coarse pitch against propeller control oil pressure. If propeller control oil pressure fails, the spring will force the blades onto the feather stop.
Fences    Vanes fitted chord-wise across a swept back wing to check the span-wise boundary layer outflow
FFPS Withdrawal Solenoid Valve    A solenoid valve that opens to pass third oil line pressure to the FFPS withdrawal piston in the pitch change mechanism. The solenoid receives an electrical signal from either a FFPS stop lever or a switch on the flight deck. The solenoid is also in circuit with the aircraft weight on ground switch and an FFPS withdrawn warning light. The FFPS cannot be withdrawn in flight.
Fin    A vertical symmetrical aerofoil section structural member designed to provide directional stability. Alternative name is vertical stabiliser.
Fine Pitch    A low blade angle.
Fineness Ratio    The length of a streamlined shape divided by its maximum thickness
Fillet    A radius formed at an intersection
Filtered    The process in which the solids are separated from the fluids
Fit    Correctly attach one item to another
Fits    There are four types of fits:
    1. Force fit, requiring hydraulic pressure or heat to mate the parts
    2. Driving fit, requiring pressure of hammering to mate the parts
    3. Push fit, requiring to be pushed into position to mate the parts; parts are net free to rotate
    4. Running fit, where the parts are free to rotate
Fixed Pitch Propeller    A propeller that has only one blade angle setting. Fixed pitch propellers are classified by their diameter and pitch, the pitch being related to the blade angle at 0.75 of the propeller radius or a nominated master station.
Flame Propagation    The spread of the flame from the point of ignition
Flange    A projecting rim
Flanged Shaft    A propeller shaft that has a flat propeller mounting plate at right angles to the shaft centreline.
Flap    A lift augmentation surface designed to increase the coefficient of lift throughout the normal range of the angle of attack
Flash Point    The temperature at which a vapour will ignite if brought into contact with a flame
Flexible Coupling    Used to connect two shafts in which perfect rigid alignment is impossible
Flight Fine Pitch    The minimum safe blade angle that can be set in flight. Blade angles below this in flight will produce a windmill condition, over-speeding and negative torque.
Flight Fine Pitch Stop (FFPS)    A removable stop in the pitch changing mechanism. It limits the travel of the mechanism towards fine to the flight fine pitch position. The step is engaged during flight and can only be withdrawn by selection when the aircraft is on the ground. When the stop is withdrawn, the pitch change mechanism can be moved into the Beta range. The step is mechanically engaged under spring force and is hydraulically withdrawn by third oil line pressure. The FFPS warning light is ON whenever the stop is withdrawn.
Flight Range    The blade angle range used when in flight that includes flight fine pitch at the lower end and feather at the higher end. When a propeller is selected in the flight range, access to the ground range is denied by the flight fine pitch stop.
Fluctuate    To continually change or vary in an irregular way
Fluidity    The ability of a fluid to flow easily and smoothly
Flux    1. A substance used to clean the surface of a job, prevent oxidisation and aid the flow of the material in such processes as fusion, soldering, brazing and welding
    2. Magnetic line of force
Force    Energy brought to bear, which tends to cause a motion or change
Forced Vibration    If an external disturbing force is continuously applied to a body it will continue to vibrate until the forces are removed. This is forced vibration. If a tuning fork is struck it will vibrate at its resonant frequency. If its stem is then pressed onto a tabletop, for example, the tabletop will be forced to vibrate at the same frequency.
Forging    Shaping the metal by hammer blows
Form Drag    Boundary layer normal pressure drag resulting from the adverse pressure gradient over the back of an aerofoil creating a higher-pressure region behind the Iow pressure region at the front. It forms a part of the profile drag.
Fowler Flap    A flap that moves rearwards initially increasing area and then downwards to increase the camber of a wing
Free Stream Air    The air that is undisturbed by the passage of a body through it
Free Turbine    A turbine wheel which rather than driving a compressor rotor, drives a propeller or helicopter transmission through a reduction gearbox
Free Turbine    A configuration of turbo-prop engine that incorporates a Gas Generator shaft and a separate Power Turbine shaft. There is no mechanical interconnection between the shafts leaving them free to rotate independent of each ether. The principle is described as 'gas coupled1 because it is only the gas flow that connects the gas generator and power turbine shafts. This configuration permits the propeller and the engine to operate at its own optimum rotational speeds. Ease of starting is also an important feature.
Frequency Generator    An engine RPM signal generator used in engine synchronisation systems. A pulse generator.
Fretting    Decolourisation on surfaces, which are pressed or bolted together under high pressure
Frise Ailerons    A configuration where the up-going aileron projects a 'beak' into the airflow under the wing to create additional profile drag to counter adverse yaw
Frequency    The number of complete cycles in one second is the frequency and is measured in Hertz (Hz).
Fuel/Air Ratio    The proportion of fuel to air in a combustible mixture
Fulcrum    A point on which a lever is supported
   
Gag    To prevent movement (especially rotation)
Galling    The transfer of metal from one surface to another, caused by chafing
Gas Generator    The section of a turbo-prop engine that produces the gas flow to drive the power turbines. Includes the compressor, combustion section and the compressor turbines.
Gasket    A thin sheet of material sandwiched between surfaces to make a gas tight joint
Gaspath    The airflow or open portion of the engine from to back where air is compressed, combusted, and exhausted
Gear Pump    A form of constant displacement pump in which two spur gears mesh and rotate within a close fitting housing
Gear Ratio    The ratio between the output and input speeds of a train of gears
Geared Fan    A design which allows the fan to rotate at a different speed than the compressor rotor. The fan being geared down allows for higher tip speeds in the compressor
Generate    To produce, for example electrical energy
Geometric Pitch    The theoretical distance that a propeller moves forward in one revolution at zero degrees angle of attack without slip. The distance is the product of the circumference of a circle traced out by the master station in one revolution and the tangent of the blade angle at the master station. The formula for geometric pitch is 2πR tan8, where R is the radius of the disc at the master station and 8 is the blade angle at the master station. The geometric pitch will be the same for all blade stations.
Gouging    The displacement of materials from one surface by cutting, tearing, or displacement effect
Governor    A speed controlling unit
Governor Oil Pressure    Normal engine oil system pressure that has been boosted by a spur gear oil pump in the propeller governor. The increase in oil pressure is required to operate the propeller pitch change mechanism.
Grooving    Smooth rounded furrows, such as score marks, where the sharp edges have been polished off
Ground Adjustable Propeller    A propeller that is designed so that its blade angle can only be adjusted manually when the aircraft is on the ground and the engine is stopped.
Ground Fine Pitch    A low blade angle that is set for engine starting and idling. It produces low torque and thus low resistance to the propeller's rotation. High idling percentage RPM can be maintained with relatively low turbine loading.
Ground Idling Consumption    The amount of fuel used by the engine at, its lowest RPM on the ground
Guttering    A deep concentrated erosion, caused by overheating or burning
   
Half-Ball Valve    In the shape of a half-ball, positioned over the end of a duct and capable of controlling fluid flow
Helix Angle    Name given to the angle of advance that subtends the effective pitch.
Horizontal Stabiliser    The tail plane. A structural member fitted to give longitudinal stability.
Horsepower HP    The standard unit of power used for mechanical measurement. It is equal to 33,000 foot pounds of work done in one minute
Horse Shoe Vortex    The shape derived when the wing tip trailing vortices are bridged in the aircraft wake by the wing bound vortex so forming the three sides of a closed figure
Hot Start    Condition developing when starting a turbine engine in which the gas temperature exceeds the allowable limit
Housing    That part of a mechanism which carries a bearing
Hub    A high tensile steel, hollow casing, often produced in two parts, that holds the blades within its flanged apertures and absorbs the centrifugal forces they produce.
Hub Switch    A mechanical switch that is moved by a cam on the foot of one blade to make or break contact with a slip ring. Contact is made when the blade is in the ground range. This illuminates the Below Stop Warning Light on the flight deck. The switch is also in circuit with the auto-coarsen system through the 'Valve lift solenoid', otherwise known as the 'increase pitch solenoid valve'.
Hung Start    A condition when starting a turbine engine in which ignition is achieved, but the engine refuses to accelerate to a self-sustaining speed
Hydraulic Pitch Lock    A spring leaded ball valve in the coarse pitch oil line that automatically closes to prevent oil returning through the coarse pitch line following a failure of the propeller control oil pressure. The valve traps oil in the pitch change mechanism to form a hydraulic lock that prevents the propeller from running to fine pitch after a failure of control oil pressure. The valve is held open by fine pitch oil line pressure and is mechanically closed by spring pressure.
Hydromatic Propeller    A propeller that incorporates an oil-operated geared pitch change mechanism. The pitch change mechanism changes the axial movement of its piston into rotary movement of a bevel gear that is engaged with a gear segment on each blade root.
Hydromechanical    Combination of hydraulic and mechanical operation
Hysteresis    A lag, delay of differential in a function or operating point in a system
   
Idler Gear    A gear in a train of gears which reverses the direction of motion but does not change the overall ratio of the gear train
Impeller    The rotating member of a centrifugal pump or blower that imparts kinetic energy
Inclusion    Foreign material enclosed in the metal
Inconel    A propriety trademark of the International Nickel Company for a chromium-iron alloy similar to stainless steel, but which cannot be hardened by heat treatment
Increase Pitch Solenoid Valve    More commonly called the 'valve lift solenoid'. It receives an electrical signal from either the hub switch or the low torque switch. When open, it allows oil pressure to act on the pilot valve lift piston in the governor head. This lifts the pilot valve to direct governor oil pressure to auto-coarsen or auto-feather the propeller dependent on which system is activated.
Indexing    The act of applying the Aerodynamic Correction Factor (AOF) when assembling a propeller.
Indicated Airspeed (IAS)    The airspeed displayed on the air speed indicator that due to changes in air density will not always correspond with the true air speed or the equivalent Iower than true airspeed at altitude.
Induced Drag    A type of drag that is generated as a by-product of lift and is inversely proportional to the Square af airspeed. If airspeed doubles, induced drag reduces by a factor of four.
Induced Drag    The portion of the total drag that is thrust dependent. It is created by the blade tip vortices and increases as thrust increases.
Inertia    The opposition which a body offers to a change of motion
Inertia    The tendency of a body to remain at rest or if moving to continue moving in a straight line. The greater the mass of the body the greater its inertia will be.
Inhibit    To cover temporarily with a protective film in order to resist corrosion
Inspect    To examine, and where necessary test, equipment; the review by a supervisor of the work or tradesmen
Insulate    To separate by means of a non-conductor
Insulation Resistance Check    Checks carried cut to determine whether there is any breakdown of the insulation between heater elements and the blades or spinner. During service, the insulation resistance of heater elements may vary due to moisture absorption caused by atmospheric conditions.
Interference Drag    A part of Profile Drag that is produced by the effect of the airframe component joints where changes in section create interference to the local boundary layer airflows. Wing and tail to fuselage etc.
International Standard Atmosphere (ISA)    A table of Standard values for atmospheric conditions at different altitudes that is used internationally to establish and compare the Performance of equipment that is reliant on air density.
International Standard Atmosphere (ISA)    A set of standard values for ambient pressure, air temperature and density at various altitudes. The standard was produced to enable meaningful performance checking of equipment that relies on air density for its operation. The standard values for sea level ISA are: air pressure - 14.7psi (1013mb), temperature 15°C.
Isolating    To separate one system from another
Isothermal    At constant temperature
   
Jet pump    A pump which operates by directing a motive fluid through a venturi for the purpose of carry surrounding fluid along with it or for removing vapours
Jet silencer    A device used to reduce and change the lower frequency sound waves emitting from the engine's exhaust nozzle, and thus reducing the noise factor
Jointing    A thin paper gasket
Journal    The part of a rotating shaft that is supported in a bearing
   
Keel Surface    The area presented by the aircraft in side elevation. The area to the rear of the C of G gives directional stability.
Kelvin Temperature scale    Absolute temperature scale with minus 273° Celsius as absolute zero
Kinetic Energy    Energy contained in a body due to motion
   
Labyrinth    Seal formed by a series of passages
Laminar Flow    The boundary layer flow where the streamlines remain separated and the varying speed sub-layers slide over each other without intermingling
Laminated    Consisting of thin plates, one upon the other
Lap    To polish using fine abrasive, and produce a flat surface
Lateral Axis    A line running span-wise through the aircraft centre of gravity at right angles to the longitudinal and normal axes
Lateral Stability    The ability of the aircraft to return to its original lateral axis around the longitudinal axis called roll. Stability is provided by dihedral and sweep back.
Leading Edge    The front edge of an aerofoil or streamlined shape
Lift    An upward force whose line of action is at right angles to the relative airflow direction and acts on the Centre of Pressure. Lift = CL x 1/2pv²  x S
Lift    Caused by pressure differences on blade surfaces
Lift    The component of the reaction force en an aerofoil that acts at right angles to the relative airflow. When related to a propeller the forward acting component of the lift force is referred to as the 'thrust'.
Lift Coefficient (CL)    A measure of the lift effectiveness of an aerofoil that takes into account shape and angle of attack. Used as a multiplier to the lift calculation 1/2pv²S
Lift/ Drag Ratio    The ratio of total lift and total drag L/D
Longitudinal Axis    A line running from the aircraft nose to tail that passes through the centre of gravity at right angles to the normal and lateral axes
Low Torque Switch    A switch incorporated into a turbo-prop torque indicating system that closes when it senses a specified low torque signal. It completes a circuit to the Valve lift solenoid', otherwise known as the 'increase pitch solenoid valve’, and also energises the feathering motor to initiate auto-feathering. This can only occur when the power lever has been advanced to the cruise position. The auto-feather system is not active at lower power settings.
Lubricant    A natural or artificial substance used to reduce friction between moving parts or to prevent corrosion on metallic surfaces
Lug    A projection from a structural member used as an attachment point
   
Mach No.    The true air speed of an aircraft divided by the local speed of sound in air
Mach No.    The ratio of velocity of a body to the local velocity of sound
Magnetic Flux    Lines of magnetic energy given off from a magnet
Major Repairs    Repairs that are considered to be beyond the scope of an operator and should be referred back to the manufacturer or an approved repair organisation.
Mandrel    An accurately ground shaft for supporting or locating a hollow part during measurements or machining, the mandrel accurately fits the hole
Manifold Air Pressure (MAP)    The gauge pressure, in psi, or absolute pressure, in inches Hg, existing in the intake manifold of a reciprocating engine. Otherwise known as 'boost pressure’ on supercharged engines. MAP is used to set the engine power output of reciprocating engines.
Manoeuvre Margin    The distance from the aircraft centre of gravity to the Manoeuvre Point
Manoeuvre point    The aft centre of gravity position where any movement of the elevator would cause instability
Mass    A measure of the amount of material contained in a body
Master Station    The position along the length of a blade where the blade angle is measured. The position is normally measured in inches from the axis of rotation and is approximately three quarters of the blade length from the shank.
Master Switch    Switch in an electrical circuit capable of isolating the whole circuit
Mating    Surfaces that are or will be in contact with others
Matter    Any substance that has weight and occupies space
Mean Chord    The average chord length of a wing
Mechanical Advantage    (Of a machine) the ration of load to effort
Mechanical Pitch Lock    A lock that will automatically engage to lock the pitch change mechanism piston at its last position if the propeller control oil pressure supply fails. The lock prevents the propeller from running to fine pitch in the event of a control oil failure. The lock does not prevent movement towards feather. The lock is held out of engagement by fine pitch oil line pressure during normal operation and is mechanically engaged under spring pressure in the event of an oil pressure failure.
Megger    A test instrument for measuring the serviceability of the insulation of low tension electrical wiring
Meter    An instrument used for measuring
Metering    To measure
Microswitch    Registered trade name for a type of electrical switch, which is used to open or close a circuit with an extremely small movement of the actuator
Minor Repairs    Repairs that are classed as being within the scope of an operator.
Moment    The moment of a force about a point is the product of the force and the perpendicular distance between its line of action and the point
Momentum    The tendency of a body to continue in motion, after being placed in motion
Motoring Cycle    Turning engine through a starting cycle without a light up
   
Natural Vibration    When a body oscillates under the action of its own gravitational or elastic forces with no external forces being present it is described as having a free or natural vibration. The pendulum and spring may be set into oscillation by external forces; but the subsequent oscillations are caused by their internal forces only.
Negative Torque System (NTS)    Similar to the auto-coarsen system, this system uses the engine torque indicating system to sense low torque and will initiate automatic coarsening of the propeller. The NTS system will keep activating each time the torque falls until the engine is either shut down or the fault clears.
Neoprene    A form of rubberised material used for joins and diaphragms
Net Thrust    The effective thrust developed by a jet engine during flight, taking into consideration the initial momentum of the air mass prior to entering the engine
Neutral Point    The aft position of the aircraft centre of gravity where the aircraft is on the point of becoming unstable. It is point where longitudinal stability becomes neutral.
Nick    A sharp indentation caused by striking one part against another metal object
Nimonic    A special heat resisting alloy
Nominal    A figure or value about which is a permitted variation
   
Observed RPM    The rev/min as read directly from the Cockpit Tachometer
On-speed Condition    A propeller governor condition where the speeder spring balances the centrifugal force acting through the governor flyweights. The engine RPM will be equal to that selected by the RPM lever.
Optimum Angle of Attack    The angle of attack where an aerofoil would produce the highest lift/drag ratio, usually 3° to 4°
Orifice    An opening at the end of a tube or pipe
Oscillate    To swing back and forth with a constant force or rhythm
Oscillation    To swing to and fro, like a pendulum
Overswing    Tendency of the engine to temporarily exceed maximum rev/min on full throttle opening
Over Speed Condition    A propeller governor condition where the centrifugal force acting through the governor flyweights is greater than the governor speeder spring force The propeller blades move towards coarse to reduce RPM.
Overtemperature    1. Condition in which a device has reached a temperature above that approved by the manufacturer
    2. Any exhaust temperature that exceeds the maximum allowable for a given operating condition or time limit
Overshoe    A thin strip rubber mat that is bonded with adhesive onto the leading edge of a blade. It may be grooved to assist in the distribution of de-icing fluid so it may carry embedded heating elements as part of an electrical ice protection system. Otherwise known as a 'boot' or 'shoe'.
   
Parameter    A variable quantity, which is measurable and affects other variables, e.g. the parameter of temperature varies mass flow
Parity Bit    In order that the data being transmitted is authentic and free from errors, an addition check bit is used in each storage location. A computer using ASCII code, which is an 8-bit system, will have an additional ninth bit for parity checking. Aircraft data bus systems such as AR.INC 429 will have a parity bit within the 32-bit word.
Particle    A small piece of any substance or matter
Peening    Deformation of the surface, caused by impact
Phase Lag    The angular difference in the rotational positions of similar blades on two or more propellers. Phase lag is adjusted through the synchrophasing system
Phugoid    Otherwise known as 'porpoising'. An oscillatory diving and climbing long period motion
Pigmented    Coloured with pigment, colouring matter or dye
Pilot Valve    A landed valve in the governor body that is raised or lowered under the combined action of the governor flyweights and the speeder spring to direct governor oil pressure to the pitch changing mechanism.
Pinion    The smaller of a pair of high ration gears
Pitch    A nose up or down movement when the longitudinal axis moves around the lateral axis
Pitch    The distance a propeller moves forward in one complete revolution. Pitch may be described as being effective or geometric to describe the actual and theoretical distances respectively.
Pitch Change Axis    The axis about which a blade turns when the blade angle is altered.
Pitch Distribution    The twist in a propeller blade along its length
Pitch Range    The plane in which the propeller rotates. It is at right angles to the axis of rotation.
Pitting    Surfaces are said to be pitted when corrosion, excessive heating, or hammering, has caused shallow irregular depression in the surface
Pivot    Pin or hinge about which rotation may take place
Placard    A notice placed in or on the aircraft depicting pertinent information relating to the aircraft, its operation, particular component limitations, etc.
Place    Flat surfaces
Plane of Rotation    The plane at right angles to the rotation shaft
Plane of Rotation    The plane in which the propeller rotates. It is at right angles to the axis of rotation.
Plastic    1. The property whereby a material is easily deformed
    2. A synthetic resin, capable of being moulded
Play    Relative motion between two parts
Plenum    An enlargement of a duct or an enclosing space in an aircraft engine induction system or air conditioning system
Porosity    The condition of a material having small pores or small cavities throughout the material
Port    An opening for the inlet and/or outlet gases
Potential Energy    That energy possessed by an object because of its position, configuration, or the chemical arrangement of its constituents
Pour Point    1. The lowest temperature at which a fluid will pour without disturbance
    2. Refers to the lowest temperature at which oil will gravity flow
Pressure    Load divided by cross-sectional area. Nm² or Ib in²
Pressure Altitude    Is the altitude at which the air pressure, ISA day, is equal to the local air pressure during a ground run, irrespective of the actual height above sea level
Pressure Ratio    The ratio or pressures at 2 points in an engine, e.g. P1/P2 ratio or P3/P4 ratio
Pressurise    To raise the pressure in a compartment
Primary    First
Priming    To fill completely a system with it's own fluid
Profile Drag    The drag associated with the shape of a body and its surface finish. Includes Form Drag, Skin Friction and Interference Drag. Combines the effects of boundary layer friction drag and surface friction. Profile drag is proportional to the Square of airspeed.
Progressively    To alter by easy stages. Variable
Propeller    A component that is designed to convert engine power output into thrust
Propeller Brake    A disc brake that is operated by the aircraft hydraulic system pressure. The brake may be applied following the shut down of a free turbine engine to reduce the run down time and to prevent the propeller wind milling in the prevailing wind. The brake may only be applied below a given RPM. Fusible plugs in the brake will release the hydraulic pressure if the brake overheats in use.
Propeller Control Unit (PCU)    Description given to the propeller governor unit fitted to a turbo-propeller engine. The unit normally takes its drive from the engine reduction gearing so that its governor speed is directly related to propeller speed.
Propeller Over-speed Limits    Up to 115% RPM no special checks are required, a tracking check may be recommended. 115% to 130% RPM in excess of any specified time limit requires that the propeller be removed for inspection. Over 130% requires that the propeller be returned to the manufacturers for investigation.
Pulsate    To expand and contract rhythmically, yet not to change direction
Pulse Generator    A frequency generator used in engine synchronisation systems to signal engine RPM.
Purge    To cleanse a system by flushing
Pusher    The term used to describe a propeller that is mounted behind the engine and produces a thrust that pushes the aircraft forwards.
Pyormetric    Measurement of heat
   
Quantity    An amount or portion. The exact amount of a particular thing
Quill Drive    A short drive shaft designated to shear at its waisted portion, used to prevent continued driving force being transmitted to seized or partly seized components
   
Race    The surface on which a bearing is supported
Radial    Issuing as rays from a common centre
Radial Inflow Turbine    A turbine wheel, which receives its gases at the blade tips and guides the air inward and outward to the exhaust duct
Rake    A pressure sensor, usually small holes in a stationary engine component, which act as a total pressure probe
Ram Air Pressure    Pressure slightly higher than ambient, caused by the forward motion of the aircraft
Ram Ratio    The ratio of ram pressure to ambient pressure in a jet engine
Ram Recovery    The increase in thrust as a result of ram air pressure and density on the front of the engine caused by air velocity
Ram Recovery Point    The point at which the suction condition in the inlet returns to the value of ambient pressure
Rarefied Air    "Thin Air", such as that found at higher altitudes
Ratchet    A mechanism which consists of a toothed wheel and a bar, or pawl, which allows the wheel to rotate in one direction but prevents its backward motion
Ratio    The relationship between one number and another
Reciprocating    Movements backward and forward in a straight motion line
Recondition    This process consists of dismantling the assembly, renewing worn and unserviceable parts, reassembling and testing
Reconnect    Correctly couple pipelines, controls, cables etc
Reduction Gears    The gear arrangement in an aircraft engine which allows the engine to turn at a faster speed than the propeller
Reference RPM    The specified RPM that should be obtained when a reciprocating engine is set at a given manifold air pressure (MAP), usually this is static (Opsi gauge), at sea level ISA conditions.
Reference Torque    The specified torque that should be obtained by a turbo-propeller engine at sea level ISA conditions at a given power setting, usually take-off power.
Refit    To replace correctly an item that has previously been removed
Reid Vapour Pressure    A measure of the pressure required above a liquid to hold vapours in the liquid at a given temperature
Relative Airflow    The airflow in relation to the aircraft passing through it
Relative Airflow    The airflow that meets a propeller. It is the resultant of the axial and radial airflows encountered by a propeller as a result of its simultaneous forward and rotational movement. The relative airflow direction and velocity is dependent on the direction and velocity of the axial and radial airflow components.
Replenish    To refill a container to a given level, pressure or quantity
Resonant Frequency    Many objects, because of their shape and material, have a natural vibration frequency, which occurs if the object is struck. This natural frequency is the resonant frequency of the object.
Restrictor    An orifice for reducing or restricting the flow of a fluid
Resultant Force    The resultant of the components of thrust and torque that acts at right angles to the line of direction of the relative airflow.
Reverse Pitch    The angular position taken up when a blade passes back through zero pitch to a negative angle. Used in the power on brake condition to provide a braking force after landing. Can only be selected when the aircraft is on the ground. The maximum negative reverse angle is limited by a fixed stop. When in reverse pitch, the angle of attack is negative, the thrust is negative but the torque remains positive.
Reynold's No.    The value where laminar flow becomes turbulent. Depends upon air density, air velocity, chord length and coefficient of air viscosity. pvl/µ.
Rich Mixture    One which has an excess of fuel
Rigid    Fixed, will not move, stiff, unyielding
Roll    The movement of the lateral axis around the longitudinal axis
Root    End of blade nearest hub
Root Loss    The loss in propeller efficiency created by the thickened aerofoil sections at the blade shank and the close proximity of the engine.
Rotor    The revolving part of a component
Rudder    The control surface used for directional control
   
Scalar Quality    That which is considered to have a magnitude only
Scoring    The term applied to scratch like mark found on bearings or on cylinder walls and pistons caused by lack of lubrication or by the ingress of dirt between the bearing surfaces
Screech Liner    A perforated liner within an afterburner, designed to combat destructive vibrations which cause metal fatigue and noise emissions
Secondary    Second
Sediment    Matter that settles to the bottom of the liquid
Semispherical    In the shape of half a sphere. Dome-shaped
Separation    Describes the action where the flow no longer follows the contour of an aerofoil but 'separates' from it to produce a turbulent re-circulating airflow
Servo    The portion of the system which assists in the operation of the main system
Shaft Horsepower (SHP)    The unit used to describe the power output of turbo-prop and reciprocating engines.
Shank    The inner, thickened region of a propeller blade that is exposed to maximum stress. Otherwise known as the 'root'. No repairs are permitted in this region.
Shim    A thin piece of metal cut to shape, used between two surfaces to adjust accurately their distance apart
Shock Load Check    A concentricity check of the propeller shaft carried out after a heavy landing or impact. Shaft eccentricity is HALF the DTI reading.
Shock Wave    A compression wave formed when a body moves through the air at a speed greater than the speed of sound
Short Period Pitching Oscillation    A heavily damped oscillation that incurs little change in aircraft height or speed and lasts for no more than one or two cycles
Shroud    A portion of a component which covers or shields
Sideslip    The sideways movement produced by a component of lift produced when an aircraft rolls
Single-acting Propeller    A propeller that incorporates a pitch change mechanism that selectively uses governor oil pressure on only one side of the pitch change piston. The hydraulic force acting on the piston is opposed by a combination of spring and blade counterweight force. The oil pressure is used to drive the pitch change piston towards fine pitch.
Sink Rate    Loss of altitude with time during a gliding descent for endurance
Sintered Metal    A porous material made up by fusing powdered metal under heat and pressure
Skin Friction    The frictional force produced as a result of the shearing action between the layers of varying speed air flows in the boundary layer over the surface of an aircraft. It is a component of profile drag.
Slat    A leading edge device used to control the boundary layer and prevent leading edge Separation. Increases the stalling angle of attack of an aerofoil.
Slender Delta    A type of delta wing designed to operate with separated flow over the wing designed to produce vortex lift
Slinger Ring    A 'U' section ring mounted on the rear of a propeller hub that enables de-icing fluid to transfer from the engine to the rotating propeller.
Slip    The difference between the geometric pitch and the effective pitch. It is
    often presented as a percentage ratio between the two. Slip occurs
    because the air is not a solid medium. The blade angle of attack varies with
    the value of the slip. If there were no slip, the angle of attack would be
    zero.
Slip Ring    A circular copper track that makes contact with carbon brushes. Slip rings
    are used to transfer electrical power from the engine to a rotating
    propeller.
Slot    A device used to control the boundary layer and prevent flow Separation. May be used as a leading edge device or with trailing edge flaps
Snap Ring    An extractor ring fitted along with a propeller retaining nut to aid in the removal of a propeller from its shaft.
Solidity    The ratio of the total blade area to the propeller disc area. It can be determined by calculating the ratio between the circumference of the propeller disc at a given radius and the sum of the blade chord lengths at that radius. As the solidity is increased in design, the amount of engine power that can be absorbed by the propeller also increases.
Sonic    The condition where all the airflow over a body is travelling at the local speed of sound in air
Software    Software gives the computer instructions on how to function. This is carried by loadable programs from CD-ROMs, floppy disks etc.
Span    The length of a wing taken at right angles to the longitudinal axis to the wing tip. Wingspan is the total straight-line distance across the aircraft from wing tip to opposite wing tip.
Speed of Sound in Air    The speed at which sound waves, travel through air. The speed is proportional to the absolute temperature of the air only.
Speeder Spring    A spring incorporated into the propeller governor head that acts on the governor flyweights. Changes in engine speed in the constant speed range will cause the flyweights to either be overcome by, or overcome the load of the speeder spring. RPM control inputs alter the loading on the spring.
Specific Fuel Consumption    The weight of fuel required to produce a unit of output an hour; expressed as pounds of fuel/brake horse power/hour, or, pounds of fuel/pounds of thrust/hour
Specific Gravity (Relative Density)    The weight of a fluid by comparison to the same volume or water
Spider    A high tensile steel component that is incorporated into a variable pitch propeller hub. It has tapered arms designed to fit into the reverse taper counter bores of the propeller blades, locating them within the hub. The spider also incorporates a hub drive centre that has internal splines that mate with the propeller drive shaft and thus transmits the drive to the blades.
Spigot    An extension which will enter and help to locate one engine part to another, to ensure concentricity
Spin    A spiral dive created when directional and lateral stabilities become unbalanced usually initiated by a wing tip stalling
Spinner    A light alloy, streamlined shell fairing that encloses the thick-sectioned blade roots and the hub and pitch change mechanism. The spinner smoothes the airflow that passes back to the engine.
Spiral Instability    Occurs when directional stability is much higher than lateral stability. A yaw produces a roll that produces a sideslip that produces a further yaw into a spiral dive.
Splined Shaft    A propeller drive shaft that has external splines that mate with internal splines in the hub drive centre of a propeller. There may be a 'master' spline that ensures the correct positional relationship between the propeller and its shaft.
Splines or Serrations    A series to longitudinal ridges on the outer surface of a shaft, separated by grooves, these fit into a similarly grooved and ridged counterpart
Spoilers    Control surfaces consisting of a hinged flaps on the top wing surface that can be used to differentially spoil lift and support the ailerons in lateral control or collectively to act as speed brakes in the air or lift dumpers on the ground
Stagger Angle    The angle formed between the chord line of a compressor blade and the horizontal centreline of the engine
Stagnation    A region of stationary air usually occurring just below the leading edge of an aerofoil where the airflow divides to pass on either side
Stall    The point where the boundary layer separates from the surface resulting in a sharp loss of lift. Occurs around fifteen degrees angle of attack on an aerofoil.
Stall    An aerodynamic condition in which the smooth flow of air has broken away from the upper surface of an airfoil, and the flow is turbulent, decreasing the amount of life produced
Stalling Angle    The angle of attack of an aerofoil at which stall occurs
Stamping    Shaping and/or cutting by means of dies in a press
Standard Atmosphere    The conditions for a standard atmosphere (ICAO) are:
    Pressure = 1013.25mb (14.7 Ibf/in²)
    Temperature = 15°C
    Density = 1.22250 Kg/m³
Starting Vortex    A horizontal vortex that is formed and left behind the aircraft whenever there is an increase in wing circulation such as on take-off and when pulling out of a dive. Wing bound vortex.
Static    Still, not moving. A condition of rest
Static Balance    Achieved when the centre of gravity of a propeller is aligned with its axis of rotation and no periodic out of balance forces exist.
Static Pressure    Atmospheric pressure measured at a point where there is no external disturbance, and the flow of air over the surface is perfectly smooth
Static Pressure    The ambient atmospheric pressure
Static RPM    The engine RPM obtained when the aircraft is stationary on the ground.
Static Stability    The ability of an aircraft to recover to its original Position following a disturbance around its axes
Stator    To be stationary, fixed compressor blades
Stoichiometric    Chemical combination which completely uses all the products of the reaction. In the case of a stoichiometric mixture, all the oxygen and all the hydrocarbon fuel are used. There is no oxygen or free carbon left
Streamlined shape    A shape where the air flows around it in streamlines without separating and becoming turbulent
Streamlines    The lines traced out in smoke delineating smoothly flowing layers of air in laminar flow
Stress    An applied load. A tensile stress is a force that tends to stretch a body, a shear stress a force that tends to cut through its section, and a compressive stress a force that tends to collapse it
Stroke    The distance that the piston moves from one extreme end of its travel to the other
Subsonic    The condition where all the airflow over a body is travelling below the local speed of sound in air
Suction    The production of a partial vacuum causing a fluid to move or adhere
Superfine    A very low blade angle in the Beta range that is used as the Ground Fine Pitch setting.
Surge    To move up and down or to and fro, in waves
Swirl    The rotary motion given to a fluid
Swirl Vanes    Air circulation vanes which surround fuel nozzles causing a vortex in which fuel vapour is made to recirculate and more completely ignite
Synchronise    To cause two or more events to happen at the same time
Sychronising    The matching of the engine RPMs on a multi-engine aircraft to reduce noise and vibration.
Synchrophasing    A refinement of the engine synchronisation system to further reduce noise and vibration by maintaining blade angular position separation on adjoining propellers to avoid aerodynamic interference occurring as a result of the blade tips passing in close proximity to each other.
   
Tachometer    Engine rev/min gauge may be in % of max rev/min
Tailpipe Inserts    Small sheet metal wedge-shaped tabs that are inserted into the tailpipe of some older engines to reduce the nozzle opening and increase thrust
Tailplane    The structural component that is fitted to give longitudinal stability. Sometimes referred to as the horizontal stabiliser.
Tapered Shaft    A propeller drive shaft that has a tapered seating that mates with a similar reverse taper in the propeller hub. Tapered shafts incorporate a keyway and key to transmit the drive and ensure the correct propeller to shaft position. Tapered shafts offer a greater seating contact area and centralise the propeller.
Tapper Ratio    The ratio of the chord lengths of the wing root and the wing tip. Taper ratio zero refers to a pointed tip.
Temperature Ratio    A ratio of two engine temperatures, used in certain performance calculations
Tertiary    Third
Test    To make certain, by using the necessary test equipment that a component functions correctly
Thermal Efficiency    The ratio of the amount of heat energy converted into useful work, to the amount of heat energy in the fuel used
Thermal Fatigue    A condition in turbine metals caused by the heating and cooling (compression and tension forces) each time a power setting is changed
Third Oil Line    A line that transfers engine oil under pressure from the oil transfer housing, through the propeller drive shaft and the FFPS withdrawal solenoid valve to the propeller flight fine pitch lock piston. The oil is used to withdraw the FFPS.
Thrust    The forward acting propelling force that opposes drag
Thrust    The forward acting component of the resultant force that is at right angles to the plane of a propeller's rotation.
Thrust Bearing    A shaft bearing designated to take an axial load
Tip    End of blade furthest from hub
Tip Clearance    The minimum specified clearances that should exist between a propeller blade tip and the ground, water and aircraft structure.
Tip Loss    The loss of propeller efficiency experienced at a blade tip due to the induced drag from tip vortices and from compressibility effects.
Tip Vortex    A vortex that trails from a blade tip and is created by high-pressure air constantly spilling from the pressure side over the tip and attempting to enter the low pressure region of the camber side. The vortex increases in intensity in proportion to thrust and creates induced drag.
Torque    A resistance to turning or twisting
Torque    The name given to the force that resists the rotation of a propeller through the air. The force acts against the direction of rotation in the plane of rotation and thus opposes the engine power output.
Torquemeter    An indicator used to indicate the amount of torque the engine is producing
Total Reaction    The resultant of the lift and drag forces acting on an aircraft
Tracking Check    A check that is carried out to compare the "track' of all the blades on a propeller with each other. The total measured difference should be within a specified minimum value.
Tractor    The term used to describe a propeller that is mounted on the front of an engine and produces thrust that pulls the aircraft forwards.
Trailing Edge    The rear edge of an aerofoil
Trailing Vortex    The vortex formed at a wing tip resulting from the airflow under the wing trying to curl over into the Iow-pressure region above the wing. This vortex is the primary cause of induced drag. Its intensity is inversely proportional to airspeed and proportional to aircraft weight.
Transient Conditions    Conditions which may occur briefly while accelerating or decelerating, or while passing through a specific range of engine operation
Transient Vibration    When an object is struck and vibrates at its resonant frequency the vibration will slowly die away. This is known as transient vibration.
Transition point    The point on the surface of an aerofoil where laminar airflow becomes turbulent. The point moves forward as airspeed increases.
Transition Range    A region of flight deck power lever dead movement between the ground and flight ranges.
Transonic    The condition where the airflow velocity over a body is part sub-sonic and part sonic or supersonic.
Trim    Adjustment to fuel flow, e.g. top temperature trimming
Trim Drag    The profile drag created by the tail plane.
Turbine    That part of an engine which is rotated by the medium of gas flow
Turbulent Flow    The situation where streamlines cannot maintain their separation and intermingle creating vortices and re-circulatory flow
Two-pitch Propeller    A propeller that incorporates only two blade angle settings. A low or fine setting for take-off and climbing and a high or coarse pitch setting for cruise.
   
Un-feathering Accumulator    A pneumatically charged accumulator that stores engine oil under pressure to assist in the un-feathering of a single acting counterweight propeller.
Underspeed    The condition in which the actual engine speed is lower than the desired engine speed as set by pilot
Under Speed Condition    A propeller governor condition where the speeder spring force is greater than the centrifugal force acting through the governor flyweights. The propeller blades move towards fine pitch to increase the RPM.
Universal Propeller Protractor    The measuring instrument that is used to measure a propeller's blade angle in relation to the normal and longitudinal datum lines of a propeller.
Upwash    The upward deflection of the airflow at the leading edge of an aerofoil
   
Vacuum    A region in which the gas pressure is considerably lower than atmospheric pressure. A perfect vacuum is practically unobtainable
Valve Lift Solenoid    See 'Increase pitch solenoid valve'.
Vane    Term generally used for stationary airfoils in an engine
Vaporisation    The conversion of fluids or solids into a gas
Variable Pitch Propeller    A propeller that has a pitch setting that varies automatically to maintain a pre-selected constant rotational speed.
Vector Quality    That which has both magnitude and direction
Velocity    The velocity of a vibrating body reaches zero at each displacement peak. It reaches a maximum value as it passes through its natural rest position.
Velocity    Distance divided by time or rate of change of distance
Velocity Ratio (of a machine)    Efforts distance moved 4- Loads distance moved
Vent    A small escape pipe which carries off excess pressures or vapours
Venturi    A reduction in the bore of a duct, with convergent upstream and divergent downstream walls, that increases the speed of the fluid flow
Vibration    Vibration is described as the physical and alternating displacement of a body about its rest position. It is caused by disturbing forces. The Parameters of vibration are its displacement, velocity, acceleration and frequency.
Vibration    Oscillation, rapid motion to and fro, of a liquid or solid whose equilibrium has been disturbed
Viscosity    A fluid 's resistance to flow
Viscosity    The reluctance of a fluid to flow, or to change shape easily
Volatile    Easily vaporised
Voltage Proof Test    A test earned out after repairs to the electrical overshoe. The leads from all the elements are connected together and a high voltage is applied between the leads and the blade for a period of at least one minute to ensure there is no breakdown in insulation resistance. The voltage is gradually increased and then gradually decreased.
VOR    VHF Omni-Range
VORTAC    Collocated VOR and Tacan Stations
Vortex    A spinning column of air with a Iow-pressure core
   
Wash-in    An increase in the angle of incidence of a wing from its root to its tip
Wash-out    A decrease in the angle of incidence of a wing from its root to its tip
Weak Mixture    One which has an excess of air
Weight    The force that acts vertically downwards through the centre of gravity that is the product of the mass and the acceleration due to gravity.
Windmill    A condition where a negative angle of attack exists as a result of low blade angle and high forward speed. The condition produces negative thrust and torque resulting in very high drag. The propeller continues to rotate in the same direction of rotation but it is driving the engine. Windmill can only occur when there is an axial airflow component. The condition usually occurs immediately following an engine failure and can be removed by feathering the propeller.
Windmill Brake    A condition that is deliberately selected after landing to improve braking. The condition is induced by the selection of ground fine pitch during the landing run. The condition produces negative thrust and torque with a negative angle of attack.
Windmilling    The act of being turned by the air (motion) stream
Wing Sweep Angle    The angle formed between the longitudinal axis and a line drawn along the wing at one quarter chord length.
Winglet    Aerofoil shaped device fitted at each wing tip to modify and raise the wing tip trailing vortices to reduce their influence on induced drag
Wipe Contact    Where contact is made between a fixed and a moving object, e.g. carbon brushes in a magneto
Yaw dampers    The movement of the longitudinal axis around the normal axis. Stabilised by the fin and controlled by the rudder.
Yaw dampers    An automatic System used to damp un-commanded yawing motions that would otherwise lead to Dutch Roll. Used as an alternative to reducing lateral stability on some swept wing aircraft.