Repair. The restoration of an aircraft component to its previous state.

Repair station. A maintenance facility certificated under 14 CFR part 145 to perform maintenance functions.

Resistance. The opposition a device or material offers to the flow or current.

Resonance. The increase in amplitude of vibrations of an electric or mechanical system exposed to a periodic force whose frequency is equal or very close to the natural frequency of the system.

Retentivity. The ability of a material to hold its magnetism after the magnetizing field has been removed.

Rheostat. A variable resistor used to vary the amount of current flowing in a circuit.

Root. A number that when multiplied by itself a specified number of times will produce a given number. The two most commonly used roots are the square root and the cube root.

Routine inspection. A visual examination or check of an item in which no disassembly is required.

Schematic diagram. A diagram that locates components with respect to each other within a system.

Scientific notation. Used as a type of shorthand to express very large or very small numbers. For example, to express 1,250,000,000,000 in scientific notation is 1.25 × 1012.

Sea level pressure. The atmospheric pressure at sea level. Average sea level pressure is 29.92 inches of mercury, or 1013.25 millibars.

Sectional view. A view obtained by cutting away part of an object to show the shape and construction at the cutting plane.

Semiconductor. Any device based on either preferred conduction through a solid in one direction, as in rectifiers, or on a variation in conduction characteristics through a partially conductive material, as in a transistor.

Series circuit. The most basic electrical circuit in which there is only one possible path for current to flow. Current must pass through the circuit components, the battery and the resistor, one after the other, or “in series.”

Series-parallel DC circuits. A grouping of parallel resistors connected in series with other resistors.

Signed numbers. A signed number can be either a positive or negative number. A positive number is a number that is greater than zero. A negative number is a number that is less than zero.

Sine. A trigonometric function comparing two sides of a right triangle as follows: opposite side Sine = hypotenuse

Sine wave. A continuous waveform with a constant frequency and amplitude.

Sketch. A simple rough drawing that is made rapidly and without much detail.