“Say again” (air traffic control). A phrase used by ATC to request a repeat of the last transmission. This usually specifies the transmission or the portion that was not understood or received, such as “Say again all after Abram VOR.”
“Say altitude” (air traffic control). A phrase used by ATC to ascertain an aircraft’s specific altitude or flight level. When the aircraft is climbing or descending, the pilot should state the indicated altitude rounded off to the nearest 100 feet.
Saybolt universal viscosity. A measure of the viscosity (resistance to flow) of a lubricating oil. The number of seconds needed for 60 milliliters of the liquid, at a specified temperature, to flow through a calibrated orifice is measured and is called the Saybolt Seconds Universal (SSU) viscosity of the liquid.
The viscosity number used for commercial aviation engine lubricating oil relates closely to the SSU of the oil at 210°F. Aviation 80 engine oil has an SSU viscosity at 210°F of 79.2, and Aviation 100 oil has an SSU viscosity of 103.0 at the same temperature.
“Say heading” (air traffic control). A phrase used by ATC to request an aircraft heading. The pilot should state the actual heading of the aircraft.
S-band radar. Radar that operates in the frequency band between 1,550 and 5,200 megahertz. In this frequency band, the wavelengths are between 19.35 and 5.77 centimeters.
scale model. A copy of something made in the same proportions as the original, but of a smaller size. When an airplane is designed, a scale model is built and tested in a wind tunnel. The model has the same shape as the actual aircraft, but it is much smaller. It is a scaled-down copy of the real thing.
scalene triangle. A closed, three-sided, plane (flat) figure that has no two sides of the same length, nor are any of the angles the same.

scandium. A silvery-white, very lightweight, rare-earth, metallic chemical element. Scandium’s symbol is Sc, its atomic number is 21, and its atomic weight is 44.956. A radioactive isotope of scandium is used in petroleum exploration.
scarfed patch. A type of patch made to a thin plywood structure. The damaged area is cleaned out and the edges of the hole tapered, with the length of the taper about 12 times the thickness of the plywood. Temporary backing blocks support the area while the patch is glued in place. A scarfed patch is the most difficult to make, but it is the preferred patch for aircraft plywood skins.
scarf joint (woodworking). A joint in a wood structure in which the ends to be joined are cut in a long taper, normally 12:1, and fastened together by gluing. A glued scarf joint makes a strong splice, because the joint is made along the side of the wood fibers rather than along their ends.
scavenge oil system. The subsystem in the lubrication system of a gas turbine engine that collects the oil after it has lubricated the bearings and gears and returns it to the oil tank.