shielded ignition cable (reciprocating engine ignition system component). A type of ignition cable used to carry high voltage from the magneto distributor to the spark plug. Shielded ignition cable is used in aircraft installations where the electromagnetic radiation from the high-voltage spark plug leads could cause radio interference.
The spark plug leads are enclosed in a metal braid. One end of the braid is connected to a terminal in a shielded spark plug, and the other end is connected to the magneto distributor. Any radio-frequency energy that radiates from the wire when the spark is jumping the gap in the spark plug is picked up by the braid and carried to ground, rather than radiating into space and causing radio interference.
shielded spark plug (reciprocating engine ignition system component). A spark plug entirely enclosed in a steel housing. A shielded ignition lead terminates with an insulated connector that fits into an insulated cavity inside the spark plug. The nut on the end of the ignition lead screws onto the spark plug barrel.
Electromagnetic energy released when the spark jumps the gap in the spark plug is picked up by the metal in the spark plug and the shielded ignition lead, and is carried to ground preventing it from causing radio interference.
shielded wire. Electrical wire enclosed in a braided metal jacket. Electromagnetic energy radiated from the wire is trapped by the braid and carried to ground.
shielding. A metal braid that encloses wires which carry high-frequency alternating current or high-voltage DC that has radio-frequency energy superimposed on it. The shielding intercepts any electromagnetic radiation and carries it to the engine structure so it will not interfere with any installed electronic equipment.
shim. A thin piece of material used to fill a space between two objects or to level an object. Thin metal shims are used in some bevel gear systems to adjust the gear preload, and behind some bearing inserts to adjust the clearance between the bearing and the shaft.
shimmy. Abnormal, and often violent, vibration of the nose wheel of an airplane. Airplane nose wheel installations must be loose enough to allow them to be steered, but if they are too loose, they will shimmy. Shimmying is prevented by a shimmy damper. See shimmy damper.
shimmy damper (aircraft landing gear component). A small hydraulic shock absorber installed between the nose wheel fork and the nose wheel cylinder which is attached to the aircraft structure. The piston inside the shimmy damper cylinder is free to move back and forth when it is moved slowly enough to allow fluid to transfer from one side of the piston to the other through a restrictor.
Slow movement of the piston allows the nose wheel to pivot so the aircraft can be steered on the ground. But, the restrictor slows the flow of fluid inside the cylinder and prevents the piston from moving back and forth as fast as it would if the nose wheel were shimmying.
shock absorber (aircraft landing gear component). A device in an aircraft landing gear that absorbs the shock which occurs when an aircraft touches down on landing. The most commonly used shock absorber is the oleo (oil and air) shock strut. The initial landing impact is taken up by oil transferring from one chamber inside the shock absorber to another through a metering orifice. The much smaller shocks that occur when the aircraft is taxiing are taken up by a cushion of compressed air.