Bubble level Incidence board Straight edge and adjustable level Stop Chord line Figure 2-85.  A typical incidence board. edge, which must be placed in contact with the leading edge  of the wing. Others are equipped with location pegs which  fit into some specified part of the structure. The purpose in  either case is to ensure that the board is fitted in exactly the  position intended. In most instances, the boards are kept clear  of the wing contour by short extensions attached to the board.  A typical incidence board is shown in  Figure 2-85 .  When used, the board is placed at the specified locations on  the surface being checked. If the incidence angle is correct,  a inclinometer on top of the board reads zero, or within a  specified tolerance of zero. Modifications to the areas where  incidence boards are located can affect the reading. For  example, if leading edge deicer boots have been installed,  the position of a board having a leading edge stop is affected. After the rigging of the horizontal stabilizer has been checked,  the verticality of the vertical stabilizer relative to the lateral  datum can be checked. The measurements are taken from a  Engines are usually mounted with the thrust line parallel to  the horizontal longitudinal plane of symmetry. However, this  is not always true when the engines are mounted on the wings.  Checking to ensure that the position of the engines, including  any degree of offset is correct, depends largely on the type of  mounting. Generally, the check entails a measurement from  the center line of the mounting to the longitudinal center  line of the fuselage at the point specified in the applicable  manual.  [Figure 2-87]  given point on either side of the top of the fin to a given point  on the left and right horizontal stabilizers.  [Figure 2-86]  The  measurements should be similar within prescribed limits.  When it is necessary to check the alignment of the rudder  hinges, remove the rudder and pass a plumb bob line through  the rudder hinge attachment holes. The line should pass  centrally through all the holes. It should be noted that some  aircraft have the leading edge of the vertical fin offset to the  longitudinal center line to counteract engine torque.  Checking Engine Alignment Checking Fin Verticality String or tape measure lateral datum Figure 2-86.  Checking fin verticality. 2-50