the Aviation Dictionary

dual-spool compressor. dye penetrant inspection
such a direction that they tend to cancel the torque of
each other.
dual-spool compressor - A turbine engine with two
separate compressors, each with its own stage of
turbine. The low-pressure compressor is NI and the
high-pressure compressor is N2.
duckbill pliers - Flat-nosed pliers used extensively in
safety wiring.
duct - A hollow tube used to transmit and direct the
flow of air through an aircraft.
duct support systems - Methods and apparatus used to
support cabin air supply ducts.
ducted-fan engine - An engine-propeller combination
that has the propeller enclosed in a radial shroud.
Enclosing the propeller improves the efficiency of the
propeller.
ductility - The property that allows metal to be drawn
into thinner sections without breaking.
ductwork - The channels or tubing through which the
cabin air supply is distributed.
due regard - A phase of flight wherein an aircraft
commander of a State-operated aircraft assumes
responsibility to separate his aircraft from all other
aircraft.
dump chute - In an aircraft fuel system, a device
designed to carry dumped fuel away from the aircraft
to prevent it from being ignited by static electricity or
engine exhaust.
dump valve - The valve that allows the fuel in a tank to
be dumped in flight in order to decrease the landing
weight of the aircraft.
duo-servo brakes - Brakes that use the momentum of
the aircraft to wedge the lining against the drum and
assist in braking when the aircraft is rolling either
forward or backward.
duplex bearing - A matched pair of bearings with a
surface ground on each to make contact with the other
matched surface. When three bearings are used they are
called triplex bearings; when four bearings are used
they are called quadpiex bearings, and so on. These are
usually ball bearings.
duplex fuel nozzle - A turbine engine fuel nozzle that
has two different spray patterns: one for low airflow
and one for high airflow. This keeps the flame pattern
centered in the combustor section.
duplex operation - In communications, the capability
of all stations to transmit and receive simultaneously.
Telephone communication is a duplex operation. For
radio communication to be a duplex operation, two
frequencies are required.
duplcxer - A circuit that makes it possible to use the
same antenna for transmitting and receiving without
allowing excessive power to flow to the receiver.
durability - The ability to withstand hard wear.
DuralB - A high strength, low weight alloy consisting
of 95% aluminum, 4% copper, and 1% magnesium.
Duralumin - The original name of the aluminum alloy
now known as 2017. First produced in Germany and
used in its Zeppelin fleet of WWI.
dust - Small soil particles suspended in the atmosphere.
dust devil - An unusual, frequently severe weather
condition characterized by strong winds and dust-filled
air over an extensive area.. A small vigorous
circulation that can pick up dust or other debris near
the surface to form a column hundreds or even
thousands of feet deep. At the ground, winds can be
strong enough to flip an unattended aircraft. Dust
devils mark the location where a thermal is leaving the
ground.
Dutch roll - A combination of rolling and yawing
oscillations that normally occurs when the dihedral
effects of an aircraft are more powerful than the
directional stability.
duty cycle - The comparison of the time a piece of
equipment can operate to how long it must cool before
being operated again. A device that can operate one
minute before having to cool off for four minutes
would have a 25% duty cycle.
duty runway - See runway in use and active runway.
D-value - Departure of true altitude from pressure
altitude; obtained by algebraically subtracting true
altitude from pressure altitude.
DVFR flight plan - A flight plan filed for a VFR
aircraft which intends to operate in airspace within
which the ready identification, location, and control of
aircraft are reauired in the interest of national security.
DVOR (Doppler VOR) - Provides increased accuracy
compared to a conventional VOR. Employed in areas
such as mountainous terrain.
Dwell angle - In ignition systems, the time that the
ignition points remain closed. During this time, the
magnetic field builds in the primary-coil. When the
points open, the magnetic field collapses, creating a
much higher voltage in the secondary coil, which is
connected to the spark plugs.
dye - A material added to a substance to add or change
its color. In aviation, dye is added to aviation fuels to
identify the grade of fuel.
dye penetrant inspection - An inspection method for
surface cracks in which a penetrating dye is allowed to
enter any cracks present and is pulled out of the crack
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