Aircraft communication addressing and reporting system (ACARS).  A two-way communication link between an airliner in flight and the airline’s main ground facilities. Data is collected in the aircraft by digital sensors and is transmitted to the ground facilities. Replies from the ground may be printed out so the appropriate flight crewmember can have a hard copy of the response. 

Airfoil.  Any surface designed to obtain a useful reaction, or lift, from air passing over it. 

Airspeed indicator.  A flight instrument that measures the pressure differential between the pitot, or ram, air pressure, and the static pressure of the air surrounding the aircraft. This differential pressure is shown in units of miles per hour, knots, or kilometers per hour. 

Airworthiness Directive (AD note).  Airworthiness Directives (ADs) are legally enforceable rules issued by the FAA in accordance with 14 CFR part 39 to correct an unsafe condition in a product. 14 CFR part 39 defines a product as an aircraft, aircraft engine, propeller, or appliance. 

Alclad. A registered trade name for clad aluminum alloy. 

Alodine. The registered trade name for a popular conversion coating chemical used to produce a hard, airtight, oxide film on aluminum alloy for corrosion protection. 

Alphanumeric symbols. Symbols made up of all of the letters in our alphabet, numerals, punctuation marks, and certain other special symbols. 

Alternator. An electrical generator that produces alternating current. The popular DC alternator used on light aircraft produces three-phase AC in its stator windings. This AC is changed into DC by a six-diode, solid-state rectifier before it leaves the alternator. 

Altimeter setting.  The barometric pressure at a given location corrected to mean (average) sea level. 

Altitude engine.  A reciprocating engine whose rated sea-level takeoff power can be produced to an established higher altitude. 

Alumel.  An alloy of nickel, aluminum, manganese, and silicon that is the negative element in a thermocouple used to measure exhaust gas temperature. 

Ambient pressure.  The pressure of the air surrounding a person or an object. 

Ambient temperature.  The temperature of the air surrounding a person or an object. 

American wire gauge.  The system of measurement of wire size used in aircraft electrical systems. 

Amphibian.  An airplane with landing gear that allows it to operate from both water and land surfaces. 

Amplifier.  An electronic circuit in which a small change in voltage or current controls a much larger change in voltage or current. 

Analog electronics.  Electronics in which values change in a linear fashion. Output values vary in direct relationship to changes of input values. 

Analog-type indicator.  An electrical meter that indicates values by the amount a pointer moves across a graduated numerical scale. 

Aneroid.  The sensitive component in an altimeter or barometer that measures the absolute pressure of the air. The aneroid is a sealed, flat capsule made of thin corrugated disks of metal soldered together and evacuated by pumping all of the air out of it. Evacuating the aneroid allows it to expand or collapse as the air pressure on the outside changes. 

Angle of attack.  The acute angle formed between the chord line of an airfoil and the direction of the air that strikes the airfoil. 

Angle of attack indicator.  An instrument that measures the angle between the local airflow around the direction detector and the fuselage reference plane. 

Angle of incidence.  The acute angle formed between the chord line of an airfoil and the longitudinal axis of the aircraft on which it is mounted. 

Annual rings.  The rings that appear in the end of a log cut from a tree. The number of annual rings per inch gives an indication of the strength of the wood. The more rings there are and the closer they are together, the stronger the wood. The pattern of alternating light and dark rings is caused by the seasonal variations in the growth rate of the tree. A tree grows quickly in the spring and produces the light-colored, less dense rings. The slower growth during the summer, or latter part of the growing season, produces the dark-colored, denser rings. 

