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barrel scale and the thimble. The 4 line on the vernier scale lines up with a line on the thimble so the final
reading is 0.2224.
vernier micrometer caliper (metric). A precision measuring device calibrated in units of one-thousandth
of a millimeter. A vernier scale consisting of 6 lines (5 spaces), is marked on the barrel of the caliper. Only
one of these marks lines up with a mark on the caliper thimble. To find the distance the caliper spindle has
moved out from its zero position, add the number of thousandths of a millimeter shown on the vernier scale
to the number of millimeters read on the sleeve and the thimble.
In the illustration, the thimble has moved out more than 8.50 millimeters, as shown by the lines on
the barrel scale, and more than 0.25 millimeters as shown on the thimble. The 6 line on the vernier scale
lines up with a line on the thimble so the final reading is 8.756 millimeters.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 616
vertex (air traffic control). The last fix for an arriving aircraft. Normally, it is the outer marker of the
runway in use. However, it may be the actual threshold or other suitable common point on the approach
path for the particular runway configuration.
vertex (part of a cone). The point of a cone, or the part of the cone the greatest distance from the base.
vertex time of arrival. See VTA.
vertical axis of an aircraft. An imaginary line passing vertically through the center of gravity of an
aircraft. The vertical axis is called the z-axis, or the yaw axis.
vertical fin (aircraft component). The fixed vertical surface in the empennage of an airplane. The vertical
fin acts as a weather vane to give the airplane directional stability.
vertical-speed indicator (aircraft instrument). An instrument that gives the pilot an indication of the rate
at which an aircraft is rising or descending. A vertical-speed indicator is a pressure instrument with a
calibrated restriction between the case and a pressure bellows. When the aircraft changes altitude, the
pressure inside the case changes immediately. But because of the calibrated restriction, the pressure change
inside the bellows lags behind the pressure change in the case.
The difference between the pressure inside and outside of the bellows causes the bellows to
expand or collapse, and this change in its dimensions causes a pointer to move over its dial showing the
pilot the rate of climb or descent. Vertical-speed indicators are often called rate-of-climb indicators.
vertical stabilizer. See vertical fin.
vertical tape instrument. A tall rectangular instrument that displays the quantity of the parameter being
measured by a movable strip of colored tape. The presentation resembles a vertical bar graph.
vertical vibration (helicopter operation). Up-and-down vibration of a helicopter caused by an
out-of-track main rotor.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 617
vertical visibility (meteorology). The distance one can see upward into a surface-based obscuration. It is
also the maximum height from which a pilot in flight can recognize the ground through a surface-based
obscuration.
vertigo. A type of spatial disorientation caused by the physical senses sending conflicting signals to the
brain. Vertigo is specially hazardous when flying under conditions of poor visibility. The eyes send a signal
to the brain telling it the airplane is in one condition, while the deep muscles, “the seat of the pants,” tell the
brain the airplane is in another condition. The inner ear tells still another story, and the result is confusion
— vertigo.
Vertigo may cause pilot incapacitation, but may be minimized by confidence in the indication of
the flight instruments.
very high frequency. See VHF.
very-high-frequency omnirange. See VOR.
very low frequency. See VLF.
Very signal. A type of emergency light signal used by ships and aircraft. A special pistol, a Very pistol,
fires a cartridge that looks much like a shotgun cartridge. The cartridge sends up a ball of white or colored
fire that can be seen for several miles during the day or night.
VFO (variable-frequency oscillator). An oscillator inside a communications radio receiver that produces
radio-frequency alternating current. The frequency produced by the VFO can be changed until it is almost
the same as the radio frequency of the carrier being received. When the output of the VFO is mixed with a
continuous wave (CW) carrier, an audible signal is produced.
VFOs are used to make Morse code signals audible when they are transmitted by continuous wave
(CW) transmission. When a VFO is used in this way, it is called a beat-frequency oscillator, a BFO.
　
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