slotted flap (aircraft secondary control). A type of trailing-edge flap that forms a slot between its leading edge and the inside of the flap well in the wing. This slot directs fast-moving air over the lowered flap and prevents it separating from the flap upper surface. The air flowing over the top of the flap increases the lift produced by the wing when the flap is lowered.

slotted-head screw. A screw having a single, straight slot cut across its head for the blade of a screwdriver
to fit.
slotted nut. A regular hexagon nut with slots cut across its top. A cotter pin is installed through a hole in the shank of the bolt and through the slots in the nut to safety the nut to the bolt. Slotted nuts differ from castle nuts in that the sides of slotted nuts are straight, rather than domed.
slow-blow fuse. A special type of fuse that allows a large flow of current for a short period of time, but opens the circuit if current in excess of its rating flows for a longer period. Slow-blow fuses are used in electric motor circuits and circuits with incandescent lamps. Both of these circuits have a large inrush of current when the switch is first closed, but when the motor begins to turn, or when the lamp filaments heat up, the current drops to a much lower value.
slow taxi (float plane operation). To taxi a float plane at low power or low RPM.
sludge. A heavy contaminant that forms in an aircraft engine lubricating oil because of oxidation and chemical decomposition of the oil.
sludge plugs (aircraft reciprocating engine component). Spool-shaped, sheet-metal plugs installed inside hollow crankshaft throws. Sludge carried in the lubricating oil passing through the crankshaft is thrown by centrifugal force to the outside of the chamber formed by the sludge plugs, where it is trapped and held until the engine is disassembled at overhaul.
slug (electrical relay component). A heavy ring of copper or other metal placed over a relay coil. The slug acts as a short-circuited winding, and the magnetic field produced by current induced in the slug slows the operation of the relay.