splice knot (aircraft covering material). A special knot used to join two pieces of waxed rib-stitching cord. A splice knot is superior to a square knot because it will not slip.
splined propeller shaft (reciprocating engine component). A propeller shaft with splines along its periphery that fit into mating splines in the steel propeller hub. A splined propeller shaft is indexed to the hub by a master spline which allows the propeller to be installed with only one orientation to the engine crankshaft.

splines. A method of locking a shaft into a wheel or disk. A number of equally spaced parallel slots are cut lengthwise into the surface of a shaft. Matching slots are cut in the inside surface of the hole into which the shaft fits, and the splines prevent the shaft rotating in the hole.
split (wood defect). A longitudinal crack in a piece of wood caused by externally induced stress.
split bus. A type of electrical bus that allows all of the voltage-sensitive avionic equipment to be isolated from the rest of the aircraft electrical system when the engine is being started or when the ground-power unit is connected.
split-field motor. A reversible DC electric motor that has two separate field windings; one for clockwise rotation and the other for counterclockwise rotation.
split flaps (aircraft controls). A type of wing flaps in which a part of the under side of the wing or the trailing edge of the wing splits and folds down to increase the curvature (camber) of the bottom of the wing. The increased camber increases the drag and also the lift produced by the wing under low-speed flight conditions.

split lock washer. A heavy spring-steel washer split at an angle across its face and twisted. When a nut is tightened down over a split lock washer, the twist is flattened out, and the spring action of the washer pushes against the nut and holds its threads tight against the threads of the bolt. This pressure between the threads prevents vibration from loosening the nut.
split needles (helicopter instrument indication). The indication on a helicopter tachometer that shows the rotor clutch is not fully engaged. A helicopter tachometer has two needles; one shows the speed of the engine and the other, the speed of the main rotor. When the clutch is fully engaged and the rotor is coupled to the engine, the needles are “married”—one is directly on top of the other.
When the engine is started and the rotor is coming up to speed, the engine RPM needle is ahead of the rotor RPM needle. When the engine is throttled back, and the rotor freewheels, the rotor needle is ahead of the engine needle. In either case, the needles are said to be split.
split-phase induction motor. A type of alternating current motor that has a main and an auxiliary winding connected in parallel. The phase of the current flowing in the auxiliary, or start, winding is shifted from that flowing in the main winding, either by a capacitor or by a large difference in the resistance in the two windings. The phase difference between the current flowing in the two field windings produces a rotating magnetic field which starts the rotor spinning.