servo-type carburetor. A type of carburetor or continuous-flow fuel injection system that uses the pressure drop across a servo bleed orifice (a metering jet) to control the amount of metered fuel allowed to flow to the cylinders. The servo system adjusts for changes in inlet fuel pressure so the pressure of the fuel sent to the cylinders is proportional only to the amount of air being drawn into the cylinders.
sesquiplane (type of airplane). A biplane in which one wing, usually the lower wing, has an area less than one half the area of the other wing.
setback (sheet metal layout). A measurement used in sheet metal layout. Setback is the distance the jaws of a brake must be set back from the mold line to form a bend. Setback for a 90° bend is equal to the inside radius of the bend, plus the thickness of the metal being bent. For a bend of other than 90°, a factor called a K-factor must be used. See K-factor.

setscrew. A headless screw used to prevent relative movement between a wheel and a shaft. A hole is drilled through the hub boss of the wheel, and it is tapped so a setscrew can be screwed through it and tightened against the shaft. The set screw prevents the wheel turning on the shaft.
settling with power (helicopter operation). An operation of a helicopter in which the main rotor is operating in its own downwash. The flow of air through the center portion of the rotor disk is upward, and the flow through the outer portion of the disk is downward, as it should be. Settling with power is more correctly called “operating in the vortex ring.”
severe weather avoidance plan. See SWAP.
sewed-in panel repair (aircraft fabric repair). A repair to a fabric-covered airplane wing in which a section of fabric extending from the leading edge to the trailing edge between two or more ribs is replaced by sewing the new fabric to the old. All the seams are reinforced with surface tape.
sextant (navigation instrument). An optical instrument used in celestial navigation to measure the angle between the horizon and a line to one of the navigational stars. By finding this angle and referring to the appropriate tables in a nautical almanac for the exact time the observation was made, the observer can determine his latitude and longitude.
Seyboth fabric tester (aircraft fabric tester). A type of tester used to determine the condition of the fabric installed on an aircraft. A specially shaped, spring-loaded, sharp-pointed plunger is pressed into the fabric until a shoulder around the point contacts the fabric. The amount of force required to press the shoulder to the fabric is measured on a colored scale in the handle of the tester. Weak fabric requires very little force to press the shoulder against the fabric, and it shows up as a red band. Airworthy fabric requires enough force to compress the spring that the indicator moves up to one of the green bands.
A Seyboth test does not indicate the strength of the fabric in pounds per inch, but it is sufficiently accurate to distinguish between fabric that is either definitely good or definitely bad.