normal (geometry). Normal, in geometry, means perpendicular. A line is normal to another line if it is perpendicular to it, and it is normal to a curve if it is perpendicular to a line tangent to the curve at the point the line intersects it.
normal (meteorology). The value of an element averaged for a given location over a period of years and recognized as a standard.
normal category aircraft. An aircraft certificated under 14 CFR Part 23 that is not certificated in the utility, acrobatic, or commuter category.
normal heptane. A hydrocarbon, C7H16, that has a very low critical pressure and temperature. Normal heptane is used as the low reference in measuring the anti-detonation characteristics of aviation gasoline.
normalize (steel heat treatment). A process of strain-relieving steel that has been machined, welded, or worked in any way that leaves it in a strained condition. The steel is heated to a specified temperature, usually red-hot, and then allowed to cool in still air to room temperature.
normally aspirated engine (reciprocating engine). See naturally aspirated engine.
normally closed (NC) relay. A relay whose contacts are held closed by a spring. The contacts are opened by the magnetic pull of an electromagnet.
normally open (NO) relay. A relay whose contacts are held open by a spring. The contacts are closed by the magnetic pull of an electromagnet.
normal operating speed (aircraft performance specification). The level-flight speed of an aircraft at its design altitude, with the engine operating at no more than 75% of its rated horsepower.

normal operating zone. See NOZ.
normal refraction (radar technology). Refraction of a radar beam under normal atmospheric conditions. The normal radius of curvature of the beam is about four times the radius of the curvature of the earth.
normal shock wave (aerodynamics). A shock wave that forms ahead of a blunt object moving through the air at the speed of sound. The shock wave is perpendicular (normal) to the air approaching the object. Air passing through a normal shock wave is slowed to a subsonic speed, and its static pressure is increased. See shock wave.
northerly turning error (magnetic compass error). An error inherent in an aircraft magnetic compass. The float on which the compass magnets are mounted is unbalanced to compensate for the vertical component of the earth’s magnetic field, and it is this unbalanced condition that causes northerly turning error.
When a turn is made from either an easterly or westerly heading toward the north, the heavy end of the float causes the compass to overshoot the heading, and when turning toward the south, to lag behind the turn. Northerly turning error is also called dip error.
north pole of a magnet. The pole of a magnet from which the lines of magnetic flux are considered to leave. The north pole is actually the north-seeking pole. When a magnet is suspended so it is free to rotate, it will turn until its north pole points to the magnetic north pole of the earth.