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difference, or potential, between a point at ground potential and the ground point in the circuit.
ground power unit. See GPU.
ground resonance (helicopter operation). Destructive vibration that occurs with a helicopter, usually on
landing. If the helicopter touches down rough and unevenly, the shock can throw a load into the lead-lag
hinges of the rotor blades and cause the blades to oscillate (move back and forth) about this hinge.
If the frequency of this oscillation is the same as the resonant frequency of the fuselage, the energy
will cause the helicopter to strike the ground hard with the opposite skid or wheel. This will increase the
energy and cause more severe contact with the ground. If corrective action is not taken immediately, either
lifting the helicopter or placing it firmly on the ground, ground resonance can destroy it.
ground-return circuit (electrical circuit). An electrical circuit using the ground or the structure of a
vehicle as one of its conductors. Almost all automobiles and aircraft use ground-return electrical systems.
One of the terminals of the battery, usually the negative terminal, is connected directly to the framework of
the vehicle.
One wire is used to connect the components to the positive terminal of the battery, and another
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 285
wire connects the component to the structure. The return current from the component flows back to the
battery through the structure.
ground speed (aircraft navigation). The speed at which an aircraft is moving over the ground. When an
aircraft is flying, its speed through the air is its airspeed. The air moves over the ground and carries the
aircraft with it, so the ground speed is the vector sum of the airspeed and the direction of flight of the
aircraft, combined with the speed and direction of the air movement over the ground (the wind speed and
direction).
If the air is moving in the same direction the aircraft is flying, the ground speed is equal to the
airspeed plus the speed of the air movement. If the wind is blowing in the direction opposite that which the
aircraft is flying, the ground speed is equal to the airspeed minus the speed of the wind. When the wind is
blowing at an angle to the flight path, the ground speed is found by adding the airspeed and the wind speed
as vectors.
ground support equipment (aircraft operations). All the equipment needed to service and maintain
aircraft on the ground. Ground support equipment includes such equipment as tugs to move the aircraft and
auxiliary power units to provide electrical power and compressed air when the engines are not running.
ground visibility. 14 CFR Part 1: “Prevailing horizontal visibility near the earth’s surface as reported by
the United States National Weather Surface or an accredited observer.”
ground wave (radio transmission). The component of a transmitted electromagnetic wave that travels
from the transmitting antenna along the surface of the earth.
growler. A piece of electrical test equipment used to check armatures in DC motors and generators for
open or shorted turns in the windings.
The armature to be tested is placed on the growler, and the AC power is turned on. The growler
acts as the primary of a transformer and the armature as the secondary. A voltmeter is used to measure the
voltage induced into the armature windings. A growler gets its name from the growling noise made when
an armature is being tested.
GTAW (gas tungsten arc welding). A method of electric arc welding in which the electrode in the torch is
a fine, nonconsumable tungsten wire. The arc is enveloped in a flow of an inert gas such as argon or helium.
GTAW was formerly called heli-arc welding.
guarded switch (type of electrical switch). A special switch whose operating control is inside a guard that
must be raised before the switch can be actuated. Guarded switches are used for circuits that must not be
accidentally operated.
gudgeon pin (reciprocating engine component). The British name for the wrist pin in a reciprocating
engine. See wrist pin.
GUMP. An acronym used by a pilot for a pretakeoff check to be sure that none of the most important items
are missed:
G — Gasoline (The correct fuel tank should be selected for takeoff.)
U — Undercarriage (The landing gear selector should be in the down position and the indicator
lights show that the gear is locked down.)
M — Mixture (The mixture control should be in the correct position for takeoff.)
P — Propeller (The propeller pitch control should be in the low-pitch, high RPM position for
takeoff.)
guncotton. A highly explosive form of cellulose nitrate made by treating cotton fibers with nitric and
　
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