second in command. 14 CFR Part 1: “A pilot who is designated to be second in command of an aircraft during flight time.”
sectional chart. An aeronautical chart drawn to a scale of 1:500,000 (one inch is equal to eight statute miles). Sectional charts are designed for visual navigation of slow- or medium-speed aircraft. Topographic information on these charts feature the portrayal of relief and a judicious selection of visual checkpoints for VFR flight. Aeronautical information includes visual and radio aids to navigation, airports, controlled airspace, restricted areas, obstructions, and related data.
sectional view (aircraft drawing). A detail view in an aircraft drawing that shows the inside of a part. A view of the main drawing is cut with a cutting plane, and the sectional view is drawn to show the shape and construction of the piece along this cutting plane.
sectioning (aircraft drawing). The use of cross-hatching to indicate the type of material used in a sectional view of a part on an aircraft drawing.
sector (part of a circle). A part of a circle bounded by two radii and the arc that lies between them.

sector gear. A piece of a gear wheel that contains the hub and a portion of the rim with the teeth. A sector gear, which is used where only a small amount of angular movement is needed, usually meshes with and drives a small pinion gear. The shaft of the pinion gear can turn several revolutions, while the shaft of the sector gear is turning through only a few degrees.
sector visibility (meteorology). Meteorological visibility within a specified sector of the horizon circle.
sediment. Contaminants that settle to the bottom of a reservoir or container.
seesaw rotor (helicopter rotor). Another name for a semirigid rotor. A semirigid rotor is a helicopter rotor whose blades can rotate about their feathering axis, but there are no separate flapping hinges. The rotor pivots on the mast in much the same way a seesaw pivots on its fulcrum.
segment (part of a circle). The part of a circle formed by a chord (a line passing across the circle and touching the circumference at two points) and the arc between the two points.

segmented circle. A visual indicator around a wind sock or tetrahedron designed to show the traffic pattern for each runway. Segmented circles are used at airports without operating control towers, and which have some runways with right-hand traffic patterns and others with left-hand patterns.

segmented-rotor brake (multiple-disk brake). A heavy-duty, multiple-disk brake used on large, high-speed aircraft. Stators, surfaced with a material that retains its friction characteristics at high temperatures, are keyed to the axle. Rotors, keyed into the wheels, rotate between each pair of stators. The rotor disks are made in segments to aid in dissipation of heat and to prevent their warping.
seize (equipment failure). A type of failure in which parts stick or jam because of friction, pressure, or excessive temperature.