Pressure. The amount of force acting on a specific amount of surface area, typically measured in pounds per square inch or psi.

Preventive maintenance. Simple or minor preservation operations and the replacement of small standard parts not involving complex assembly operations.

Product. The result of multiplication.

Progressive inspection. Breaking down the large task of conducting a major inspection into smaller tasks which can be accomplished periodically without taking the aircraft out of service for an extended period of time.

Proportion. A proportion is a statement of equality between two or more ratios. The example of A is to B as C is to D can be represented A:B = C:D or A/B = C/D.

Pythagorean Theorem. An equation used to find the length of a third side of any right triangle when the lengths of two sides are known. The Pythagorean Theorem states that a2 + b2 = c2. The square of the hypotenuse (side opposite the right angle) is equal to the sum of the squares of the other two sides (a and b).

Quenching. The rapid cooling of metal in the heat-treatment process.

Quotient. The result of dividing two numbers.

Radiant energy. Electromagnetic waves of certain frequencies produce heat when they are absorbed by the bodies they strike such as x-rays, light rays, and infrared rays.

Radiation. The continuous emission of energy from the surface of all bodies.

Radical sign. The symbol √, used to indicate the root of a number.

Radiographic inspection. Inspection using radiography to locate defects or flaws in airframe structures or engines with little or no disassembly.

Radius (circle). Equal to one-half the diameter of a circle.

Ratio. The comparison of two numbers or quantities.

Reamers. Tools made of either carbon tool steel or high-speed steel that are used to smooth and enlarge holes to exact size.

Rectifier. A device for converting alternating current to direct current.

Relative humidity. The ratio of the amount of water vapor actually present in the atmosphere to the amount that would be present if the air were saturated at the prevailing temperature and pressure.

Remainder. The leftover number in the process of division.