angel (radar meteorology). An echo caused by a physical phenomenon that is not necessarily discernible to the eye. Angels may be observed when abnormally strong temperature and/or moisture gradients exist,
and they are sometimes attributed to insects or birds flying in the radar beam.
angle. The plane figure formed when two lines extend from the same point. A right angle has 90° between the lines.

angle drill. A type of drilling tool in which a twist drill is held in a chuck mounted at an angle to the spindle of the drill motor. Angle drills are often used in aircraft sheet metal work to drill holes in locations where a normal drill cannot be used.
angle of arrival (radio propagation). The angle, measured at the receiving antenna, between the line of propagation of a radio wave and the surface of the earth.
angle of attack (α). The acute angle formed between the chord line of an airfoil and the direction of the air that strikes the airfoil.

angle of attack (gas turbine engine compressor). The acute angle formed between the chord line of the compressor blades and the direction the air strikes the blades. This angle of attack is affected by both the velocity of the air flowing through the compressor and the RPM of the compressor.
If the RPM of the compressor is too high for the velocity of the air flowing through the engine, the angle of attack will be excessively high, and the compressor blades can stall.
angle of attack (propeller). The acute angle between the chord line of a propeller blade and the relative wind.
The angle of attack is affected by both the engine RPM and the forward speed of the aircraft.
angle of attack indicator (aircraft flight instrument). An instrument that measures the angle between the local airflow around the direction detector and the fuselage reference plane.
angle of azimuth. An angle, measured in a horizontal plane, clockwise from north.
angle of bank. The angle formed by the lateral axis of an aircraft and a horizontal plane.
angle of departure (radio propagation). The angle, measured at the transmitting antenna, between the angle of propagation of a radio wave and a horizontal plane.
angle of incidence (airplane specification). 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 incidence (electromagnetic energy). The angle between a beam of electromagnetic radiation and a line perpendicular to the surface the beam strikes.
angle of refraction (light). The angle between a beam of light as it passes through a medium and a line perpendicular to the surface of the medium.
angle of yaw. The acute horizontal angle formed between the plane of symmetry of an aircraft and the direction from which the wind is striking the aircraft.

angstrom. A convenient unit used to measure the wavelength of light. One angstrom, or A, is one
ten-millionth of a micron. This is 0.000 000 01 (10-8) centimeters. The wavelength of visible light extends from about 4,000 to 8,000 angstroms.