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. 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. 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. 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 attack.  The acute angle formed between the chord  line of an airfoil and the direction of the air that strikes the  airfoil. 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. Analog-type indicator.  An electrical meter that indicates  values by the amount a pointer moves across a graduated  numerical scale. Analog electronics.  Electronics in which values change in  a linear fashion. Output values vary in direct relationship to  changes of input values. Amplifier.  An electronic circuit in which a small change in  voltage or current controls a much larger change in voltage  or current. Amphibian.  An airplane with landing gear that allows it to  operate from both water and land surfaces.  American wire gauge.  The system of measurement of wire  size used in aircraft electrical systems. Ambient temperature.  The temperature of the air  surrounding a person or an object.  G-2