vernier coupling. A timing coupling used with base-mounted magnetos. The vernier coupling allows the timing to be adjusted in increments of considerably less than one degree.
vernier micrometer caliper. A precision measuring device calibrated in units of one ten-thousandth of an inch. A vernier scale consisting of 11 lines (10 spaces), is marked on the barrel of the caliper. Only one of these marks lines up with a mark on the caliper thimble. To find the distance the caliper spindle has moved out from its zero position, add the number of ten-thousandths of an inch shown on the vernier scale to the number of thousandths of an inch read on the sleeve and the thimble.
In the illustration, the thimble has moved out more than 0.222 inch, as shown by the lines on the barrel scale and the thimble. The 4 line on the vernier scale lines up with a line on the thimble so the final reading is 0.2224.

vernier micrometer caliper (metric). A precision measuring device calibrated in units of one-thousandth of a millimeter. A vernier scale consisting of 6 lines (5 spaces), is marked on the barrel of the caliper. Only one of these marks lines up with a mark on the caliper thimble. To find the distance the caliper spindle has moved out from its zero position, add the number of thousandths of a millimeter shown on the vernier scale to the number of millimeters read on the sleeve and the thimble.
In the illustration, the thimble has moved out more than 8.50 millimeters, as shown by the lines on the barrel scale, and more than 0.25 millimeters as shown on the thimble. The 6 line on the vernier scale lines up with a line on the thimble so the final reading is 8.756 millimeters.

vertex (air traffic control). The last fix for an arriving aircraft. Normally, it is the outer marker of the runway in use. However, it may be the actual threshold or other suitable common point on the approach path for the particular runway configuration.
vertex (part of a cone). The point of a cone, or the part of the cone the greatest distance from the base.

vertex time of arrival. See VTA.
vertical axis of an aircraft. An imaginary line passing vertically through the center of gravity of an aircraft. The vertical axis is called the z-axis, or the yaw axis.

vertical fin (aircraft component). The fixed vertical surface in the empennage of an airplane. The vertical fin acts as a weather vane to give the airplane directional stability.
vertical-speed indicator (aircraft instrument). An instrument that gives the pilot an indication of the rate at which an aircraft is rising or descending. A vertical-speed indicator is a pressure instrument with a calibrated restriction between the case and a pressure bellows. When the aircraft changes altitude, the pressure inside the case changes immediately. But because of the calibrated restriction, the pressure change inside the bellows lags behind the pressure change in the case.