Aviation Maintenance Terms beginning with D
Dacron
The registered trade name for a cloth woven from polyester fibers.
Damped oscillation
Oscillation whose amplitude decreases with time.
Data bus
A wire or group of wires that are used to move data within a computer system.
Database
A body of information that is available on any particular subject.
Debooster valve
A valve in a power brake system between the power brake control valve and the wheel cylinder. This valve lowers the pressure of the fluid going to the brake and increases its volume. A debooster valve increases the smoothness of brake application and aids in rapid release of the brakes.
Decay
The breakdown of the structure of wood fibers. Wood that shows any indication of decay must be rejected for use in aircraft structure.
Deciduous
A type of tree that sheds its foliage at the end of the growing season. Hardwoods come from deciduous trees.
Decomposition
The breakdown of the structure of wood fibers. Wood that shows any indication of decay must be rejected for use in aircraft structure.
Dedicated computer
A small digital computer, often built into an instrument or control device that contains a built-in program that causes it to perform a specific function.
Deep-vacuum pump
A vacuum pump capable of removing almost all of the air from a refrigeration system. A deep- vacuum pump can reduce the pressure inside the system to a few microns of pressure.
Deflator cap
A cap for a tire, strut, or accumulator air valve that, when screwed onto the valve, depresses the valve stem and allows the air to escape safely through a hole in the side of the cap.
Deicer system
A system that removes ice after it has formed on an aircraft.
Delamination
The separation of the layers of a laminated material.
Delivery air duct check valve
An isolation valve at the discharge side of the air turbine that prevents the loss of pressurization through a disengaged cabin air compressor.
Delta airplane
An airplane with a triangular-shaped wing. This wing has an extreme amount of sweepback on its leading edge, and a trailing edge that is almost perpendicular to the longitudinal axis of the airplane.
Delta connection (electrical connection)
A method of connecting three electrical coils into a ring or, as they are drawn on a schematic diagram as a triangle, a delta (D).
Denier
A measure of the fineness of the yarns in a fabric.
Density altitude
The altitude in standard air at which the density is the same as that of the existing air.
Density ratio (σ)
The ratio of the density of the air at a given altitude to the density of the air at sea level under standard conditions.
Derated (electrical specification)
Reduction in the rated voltage or current of an electrical component. Derating is done to extend the life or reliability of the device.
Desiccant (air conditioning component)
A drying agent used in an air conditioning system to remove water from the refrigerant. A desiccant is made of silica-gel or some similar material.
Detent
A spring-loaded pin or tab that enters a hole or groove when the device to which it is attached is in a certain position. Detents are used on a fuel valve to provide a positive means of identifying the fully on and fully off position of the valve.
Detonation
An explosion, or uncontrolled burning of the fuel-air mixture inside the cylinder of a reciprocating engine. Detonation occurs when the pressure and the temperature inside the cylinder become higher than the critical pressure and temperature of the fuel. Detonation is often confused with preignition.
Deviation error
An error in a magnetic compass caused by localized magnetic fields in the aircraft. Deviation error, which is different on each heading, is compensated by the technician “swinging” the compass. A compass must be compensated so the deviation error on any heading is no greater than 10 degrees.
Dewar bottle
A vessel designed to hold liquefied gases. It has double walls with the space between being evacuated to prevent the transfer of heat. The surfaces in the vacuum area are made heat-reflective.
Differential aileron travel
Aileron movement in which the upward-moving aileron deflects a greater distance than the one moving downward. The up aileron produces parasite drag to counteract the induced drag caused by the down aileron. Differential aileron travel is used to counteract adverse yaw.
Differential pressure
The difference between two pressures.
Differential-voltage reverse-current cutout
A type of reverse-current cutout switch used with heavy-duty electrical systems. This switch connects the generator to the electrical bus when the generator voltage is a specific amount higher than the battery voltage.
Digital multimeter
An electrical test instrument that can be used to measure voltage, current, and resistance. The indication is in the form of a liquid crystal display in discrete numbers.
Dihedral
The positive angle formed between the lateral axis of an airplane and a line that passes through the center of the wing or horizontal stabilizer. Dihedral increases the lateral stability of an airplane.
Diluter-demand oxygen system
A popular type of oxygen system in which the oxygen is metered to the mask, where it is diluted with cabin air by an airflow-metering aneroid assembly which regulates the amount of air allowed to dilute the oxygen on the basis of cabin altitude. The mixture of oxygen and air flows only when the wearer of the mask inhales. The percentage of oxygen in the air delivered to the mask is regulated, on the basis of altitude, by the regulator. A diluter-demand regulator has an emergency position which allows 100 percent oxygen to flow to the mask, by-passing the regulating mechanism.
Dipole antenna
A half wavelength, center-fed radio antenna. The length of each of the two arms is approximately one fourth of the wavelength of the center frequency for which the antenna is designed.
Dirigible
A large, cigar-shaped, rigid, lighter-than-air flying machine. Dirigibles are made of a rigid truss structure covered with fabric. Gas bags inside the structure contain the lifting gas, which is either helium or hydrogen.
Disc area (helicopter specification)
The total area swept by the blades of a helicopter main rotor.
Divergent oscillation
Oscillation whose amplitude increases with time.
Diverging duct
A duct, or passage, whose cross-sectional area increases in the direction of fluid flow.
Dope proofing
The treatment of a structure to be covered with fabric to keep the solvents in the dope from softening the protective coating on the structure.
Dope roping
A condition of aircraft dope brushed onto a surface in such a way that it forms a stringy, uneven surface rather than flowing out smoothly.
Double-acting actuator (hydraulic system component)
A linear actuator moved in both directions by fluid power.
Double-acting hand pump (hydraulic system component)
A hand-operated fluid pump that moves fluid during both strokes of the pump handle.
Doubler
A piece of sheet metal used to strengthen and stiffen a repair in a sheet metal structure.
Downtime
Any time during which an aircraft is out of commission and unable to be operated.
Downwash
Air forced down by aerodynamic action below and behind the wing of an airplane or the rotor of a helicopter. Aerodynamic lift is produced when the air is deflected downward. The upward force on the aircraft is the same as the downward force on the air.
Drag (helicopter rotor blade movement)
Fore-and-aft movement of the tip of a helicopter rotor blade in its plane of rotation.
Drag wire
A structural wire inside a Pratt truss airplane wing between the spars. Drag wires run from the front spar inboard, to the rear spar at the next bay outboard. Drag wires oppose the forces that try to drag the wing backward.
Dragging brakes
Brakes that do not fully release when the brake pedal is released. The brakes are partially applied all the time, which causes excessive lining wear and heat.
Drill motor
An electric or pneumatic motor that drives a chuck that holds a twist drill. The best drill motors produce high torque, and their speed can be controlled.
Drip stick
A fuel quantity indicator used to measure the fuel level in the tank when the aircraft is on the ground. The drip stick is pulled down from the bottom of the tank until fuel drips from its opened end. This indicates that the top of the gauge inside the tank is at the level of the fuel. Note the number of inches read on the outside of the gauge at the point it contacts the bottom of the tank, and use a drip stick table to convert this measurement into gallons of fuel in the tank.
Dry air pump
An engine-driven air pump which used carbon vanes. Dry pumps do not use any lubrication, and the vanes are extremely susceptible to damage from the solid airborne particles. These pumps must be operated with filters in their inlet so they will take in only filtered air.
Dry ice
Solidified carbon dioxide. Dry ice sublimates, or changes from a solid directly into a gas, at a temperature of –110 °F (–78.5 °C).
Dry rot
Decomposition of wood fibers caused by fungi. Dry rot destroys all strength in the wood.
Ductility
The property of a material that allows it to be drawn into a thin section without breaking.
Dummy load (electrical load)
A noninductive, high-power, 50-ohm resistor that can be connected to a transmission line in place of the antenna. The transmitter can be operated into the dummy load without transmitting any signal.
Duralumin
The name for the original alloy of aluminum, magnesium, manganese, and copper. Duralumin is the same as the modern 2017 aluminum alloy.
Dutch roll
An undesirable, low-amplitude coupled oscillation about both the yaw and roll axes that affects many swept wing airplanes. Dutch roll is minimized by the use of a yaw damper.
Dutchman shears
A common name for compound-action sheet metal shears.
Dynamic pressure (q)
The pressure a moving fluid would have if it were stopped. Dynamic pressure is measured in pounds per square foot.
Dynamic stability
The stability that causes an aircraft to return to a condition of straight and level flight after it has been disturbed from this condition. When an aircraft is disturbed from the straight and level flight, its static stability starts it back in the correct direction; but it overshoots, and the corrective forces are applied in the opposite direction. The aircraft oscillates back and forth on both sides of the correct condition, with each oscillation smaller than the one before it. Dynamic stability is the decreasing of these restorative oscillations.
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