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second, or meters per second. Speed, unlike velocity, does not take into consideration the direction of
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 544
travel.
speed brakes (aircraft control). A type of secondary control on an airplane, a control which does not
rotate the aircraft about any of its three axes. Speed brakes produce drag without affecting lift, or causing
the aircraft to pitch. The drag they produce allows a highly streamlined airplane to descend at a steep angle
without picking up excessive speed.
speed of light. The speed at which light travels in a vacuum. The speed of light is 299,792.5 kilometers per
second, or 186,282 miles per second.
speed of sound. The speed at which the mechanical vibrations of sound travel in an elastic medium. Sound
travels in air, under standard atmospheric conditions, at a rate of 760 miles per hour, 340 meters per second,
or 1,116 feet per second. The speed of sound is affected by the density of the medium through which it
travels. In the air this is determined by the air temperature.
speed-rated engine (gas turbine engine specification). A gas turbine engine whose rated thrust is
produced at a specified RPM.
speed segments (air traffic control). Portions of the arrival route between the transition point and the
vertex along the optimum flight path for which speeds and altitudes are specified. There is one set of arrival
speed segments adapted from each transition point to each vertex, and each set may contain up to six
segments.
sphere. A solid with a curved surface, like a ball. All points on the surface of the sphere are the same
distance from a point within the sphere called its center.
spider (propeller component). The high-strength steel component in an airplane propeller that attaches to
the propeller shaft of the engine and supports the propeller blades. The spider and the roots of the blades
are enclosed in the high-strength propeller hub.
spike (electrical disturbance). A transient condition in an electrical circuit in which a pulse of electrical
energy with a high voltage and short duration appears in the circuit. Spikes are produced when an inductive
load in the circuit is interrupted, and by induction from an outside source, such as a flash of lightning.
Special spike-protection circuits are installed in critical electronic equipment to prevent damage from
spikes.
spike knot (wood defect). A knot that runs through the depth of a beam, perpendicular to the annual rings.
Spike knots appear most frequently in quartersawed wood.
spin (aircraft maneuver). A maneuver of an airplane in which one wing is stalled while the other wing
continues to produce lift. An airplane is put into a spin by getting it very nearly into a stall and then quickly
rotating it about its vertical axis by pushing on one of the rudder pedals.
As the airplane rotates about its vertical axis, one wing moves backward and stalls. The other wing
moves forward and picks up a bit of extra lift. The nose drops, and the airplane descends slowly, with the
wing producing lift pulling it around in a spiral path. To recover from a spin, the nose is forced down so air
flows over the stalled wing. With both wings producing lift, the airplane recovers from the spin.
spinner (aircraft propeller component). A streamlined housing over the hub of the propeller. A spinner
smooths the airflow as it enters the cooling openings in the engine cowling. It also gives the airplane a
streamlined shape.
spiral (aircraft maneuver). A maneuver in which an airplane descends in a steeply banked, turning flight
path. A spiral is sometimes confused with a spin, but the two maneuvers are totally different. In a spin, one
wing is stalled and the other is producing lift, while in a spiral, both wings produce lift. The airspeed does
not increase as an airplane spins, but in a spiral, the speed can increase to a dangerous level if the spiral is
allowed to continue.
spirit level. An instrument used to determine whether or not something is level, or perpendicular to a line
that points directly toward the center of the earth. A spirit level is a curved glass tube partially filled with
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 545
liquid. Enough liquid is left out of the tube so there is a bubble in it. The tube is mounted in a long, straight
metal or wood bar in such a way that when the bar is level, the bubble in the liquid is in the center of the
tube. Spirit levels are also called bubble levels.
spirit varnish. A finishing material for wood made of a resin dissolved in a volatile solvent such as alcohol.
The varnish is brushed or sprayed on a surface, and it forms a hard, resin film when the solvent evaporates
　
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