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produced by the retreating blade decreases, it flaps downward, increasing both its angle of attack and the
lift it produces.
distance available (ALD). Available landing distance is that portion of a runway available for landing and
roll-out for aircraft cleared for land-and-hold-short operation (LAHSO). This distance is measured from the
landing threshold to the hold-short point.
distance measuring equipment. See DME.
distillate. A liquid obtained by condensing the vapors boiled off a material when it is heated in the process
of distillation.
distillation. A process by which components of a liquid mixture are separated. Crude oil, for example, is a
source of many hydrocarbon fuels and lubricants which are obtained by the process of distillation.
The crude oil is heated to a specific temperature and some vapors are driven off, and these vapors
are cooled until they condense into a liquid. The temperature of the oil is increased and more vapors are
driven off and condensed.
Butane, gasoline, kerosine, diesel fuel, furnace oil, lubricating oil, and tar all have different boiling
points and are all produced by distilling crude oil.
distilled water. Water that has been purified by distillation. The water is boiled to convert it into water
vapor, and the vapor is condensed into liquid water. Minerals and other contaminants in the liquid water do
not stay with the water when it is changed into a vapor. They are removed by the process of distillation.
distress (air traffic control). A condition of being threatened by serious and/or imminent danger, and of
requiring immediate assistance.
distress frequencies (radio communications). Frequencies for radio transmission recognized by most
nations as universal frequencies for distress, or emergency, purposes.
In the low-frequency band, 500 kilohertz is the distress frequency. In the VHF (very high
frequency) band, it is 121.5 megahertz, and in the UHF (ultra high frequency) band, it is 243.0 megahertz.
These distress (or emergency) frequencies are continuously monitored by many military and commercial
radio operators.
distributed pole motor. A type of electric motor whose stator windings are wound in a series of slots in
the motor frame. A distributed pole motor is different from a salient pole motor whose field windings are
wound around separate pole shoes that project inward from the frame toward the rotor.
distributor (magneto ignition system component). A high-voltage rotary selector switch used in an
aircraft engine magneto to direct high-voltage electrical energy from the magneto coil secondary winding
to the spark plugs at the time they should fire. The rotor of the distributor is gear-driven from the
crankshaft.
distributor block (magneto ignition system component). The insulated block in a high-voltage magneto
distributor to which the ignition leads attach. The distributor rotor passes near fixed contacts inside the
distributor block, and the high-voltage current jumps from the rotor to the contact, then out the correct
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 195
ignition lead to the spark plug.
Distributor blocks are made of a plastic material having an extremely high dielectric strength.

　
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