Absolute humidity. The actual amount of the water vapor in a mixture of air and water.

Absolute pressure. Equal to gauge pressure plus atmospheric pressure. Also known as psia.

Absolute temperature. Temperature measured relative to absolute zero. Absolute temperature scales include Kelvin and Rankine.

Acceleration due to gravity. The acceleration of an object caused by gravity. On earth, it is measured as 32.2 feet per second per second (32.2 fps/s).

Addition. The process in which the value of one number is added to the value of another.

Advisory Circulars (AC). Issued to inform the aviation public in a systematic way of nonregulatory material. An AC is issued to provide guidance and information in a designated subject area or to show a method acceptable to the Administrator for complying with a related 14 CFR part.

Aircraft Specifications. FAA recordkeeping documents issued for both type-certificated and non-typecertificated products which have been found eligible for U.S. airworthiness certification.

Airfoil. Any device that creates a force, or lift, based on Bernoulli’s principles or Newton’s laws, when air is caused to flow over the surface of the device.

Airworthiness certificate. A document required to be onboard an aircraft that indicates the aircraft conforms to type design and is in condition for safe operation.

Airworthiness directive (AD). Issued by the FAA in response to deficiencies and/or unsafe conditions found in aircraft, engines, propellers, or other aircraft parts. Compliance with an AD is mandatory.

Alclad aluminum. Used to designate sheets that consist of an aluminum alloy core coated with a layer of pure aluminum to a depth of approximately 51⁄2 percent on each side.

Algebra. The branch of mathematics that uses letters or symbols to represent numbers in formulas and equations.

Allowance. The difference of the upper and lower variation of a part.

Alodizing. A simple chemical treatment for all aluminum alloys to increase their corrosion resistance and to improve their paint bonding qualities.

Alteration. A change or modification to an aircraft from its previous state

Alternating current. An electric current that reverses direction in a circuit at regular intervals.

Ammeter. An instrument for measuring electric current in amperes.

Ampere. A unit of measure of the rate of electron flow or current in an electrical conductor. One ampere of current represents one coulomb of electrical charge (6.24 × 1018 charge carriers) moving past a specific point in one second.