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VFR (visual flight rules). Flight rules adopted by the Federal Aviation Administration to govern aircraft
flight when the pilot has visual reference to the ground at all times. VFR operations specify the amount of
ceiling (the distance between the surface of the earth and the base of the clouds) and the visibility (the
horizontal distance the pilot can see) that the pilot must have in order to operate according to these rules.
When the weather conditions are not such that the pilot can operate according to VFR, he or she
must use another set of rules, instrument flight rules (IFR).
VFR over-the-top. 14 CFR Part 1: “The operation of an aircraft over-the-top under VFR when it is not
being operated on an IFR flight plan.”
VFR-On-Top (flight conditions). A flight condition in which a pilot flies an aircraft according to visual
flight rules (VFR) above a solid layer of clouds. While flying in the clear above the clouds, the pilot can
operate under visual flight rules, but in order to reach VFR-On-Top conditions, the aircraft must pass
through the clouds. When flying through the clouds, the aircraft must be operated according to instrument
flight rules (IFR).
VFR terminal area chart. An aeronautical chart, drawn to a scale of 1:250,000 (one inch is equal to four
statute miles). This chart depicts terminal control area (TCA) airspace which provides for the control or
segregation of all aircraft within the TCA.
The topographic and aeronautical information depicted on terminal area charts includes visual and
radio aids to navigation, airports, controlled airspace, restricted areas, obstructions, and related data.
VHF (very-high frequency) (radio frequencies). The range of radio frequencies between 30 megahertz
(30,000,000 cycles per second) and 300 megahertz (300,000,000 cycles per second) that are used for
aviation communications and navigation. VHF radio waves operate much like light waves, in that they
travel in straight lines and are usable only for a line-of-sight distance. VHF radio waves do not follow the
curvature of the earth.
VI (viscosity index). A measure of the change in the viscosity of a fluid with a change in its temperature.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 618
The higher the viscosity index, the less the viscosity changes with the temperature.
vibrating-reed frequency meter. A type of panel-mounted instrument used to show the frequency of the
alternating current being supplied to certain electrical equipment. Metal reeds of different lengths are
vibrated by an electromagnet powered by the AC whose frequency is being measured. The reed whose
resonant frequency is the same as the frequency of the AC will vibrate with a much greater amplitude than
the other reeds.
The ends of all of the reeds are visible on the face of the instrument, and each reed is identified by
the frequency at which it is resonant. The number beside the reed seen as a wide blur is the frequency of the
AC.
vibration. A periodically reversing motion in which a body moves from one side to the other of its at-rest
position. Every physical body has a natural resonant frequency at which it vibrates with the greatest
amplitude for a given amount of excitation.
vibration isolator. A device installed between a component and a structure to prevent any vibrations
present in the structure from being transmitted into the component. Instruments and electronic components
are mounted in aircraft with vibration isolators to keep the shocks and vibrations from damaging them.
Vibration isolators are often called shock mounts.
vibration loop (fluid lines). A loop formed in a rigid fluid line to prevent vibration from concentrating
stresses that could cause the line to break.
vibrator (electrical component). An electromagnetic relay used to change steady direct current into
pulsating DC. Current flowing through a relay coil produces an electromagnetic pull that opens a set of
contacts that are normally held closed by a spring. The coil and contacts are connected in series so the
current flowing in the coil stops as soon as the contacts open.
When the contacts open, the coil loses its electromagnetic pull, and the spring closes the contacts,
restoring the current to the coil. The frequency of the pulsating DC is determined by the magnetic
characteristics of the relay.
vibrator-type voltage regulator (DC generator control). A voltage control used with some of the simpler
direct current generators. The voltage sensor is an electromagnetic relay whose coil is connected across the
generator output. The relay contacts are in the generator field circuit.
As soon as the generator output voltage reaches the value for which the voltage regulator is set,
　
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