axial load (bearing load). The load on a bearing parallel to the shaft on which the bearing is mounted. Axial loads are usually carried by ball bearings or tapered roller bearings.
The thrust load produced by a propeller is an axial load, and it is carried into the engine crankcase through the thrust bearing.

axial loading (gas turbine engine compressor force). An aerodynamic force that tries to move the
compressor forward in its case. Axial loading is supported in a gas turbine engine in ball bearings.
axial turbine (gas turbine engine component). A turbine turned by a fluid flowing through it in a
direction approximately parallel to the shaft on which the turbine wheel is mounted.

axis. A straight line about which a body can rotate.

axis of rotation (helicopter rotor). The line through the rotor head of a helicopter at right angles to the
plane of rotation. The blades rotate around this axis.
axis of rotation (propeller). The center line about which a propeller rotates.
axis of symmetry. An imaginary center line about which a body or object is symmetrical, or has the same

shape on either side.
axle. The shaft on which a wheel is mounted and about which the wheel is free to rotate.
axonometric projection (aircraft drawings). A projection used in mechanical drawing that shows a solid,

rectangular object inclined in such a way that three of its faces are visible. An isometric projection is a form of axonometric projection.

azimuth. Angular measurement made in a horizontal plane and in a clockwise direction from a fixed reference direction to an object.
B
babbitt. A soft, silvery alloy of tin, lead, copper, and antimony used for main bearing inserts in some aircraft reciprocating engines.
Babbitt has a very low coefficient of friction, and its dimensions change very little with changes in its temperature.
back (propeller nomenclature). The curved surface of a propeller blade. The back of a propeller blade corresponds to the upper surface of an airplane wing.
back course (instrument flight). The reciprocal of the localizer course for an ILS (Instrument Landing System).
When flying a back-course approach, an aircraft approaches the instrument runway from the end on which the localizer antennas are installed.
back current (semiconductor device). Current that flows in a semiconductor when the junction is reverse-biased. Back current is also called reverse current.
backfire (reciprocating engine). A loud noise, or explosive sound, made by a reciprocating engine when the fuel-air mixture in the induction system is ignited by gases which are still burning inside the cylinder when the intake valve opens.