Longitudinal axis Wheel paths Wheels aligned are parallel to the  longitudinal axis of the aircraft Tow-in: wheel paths cross forward of the aircraft Tow-out: wheel paths diverge forward of the aircraft Grease Carpenter’s square Straight edge Aluminum plates Positive camber Negative camber Vertical plain Strut Bubble protractor 13-16 Figure 13-29.  Finding tow-in and tow-out on a light aircraft with  spring steel struts.  Figure 13-30.  Camber of a wheel is the amount the wheel is tilted out  of the vertical plain. It can be measured with a bubble protractor. Figure 13-28.  Wheel alignment on an aircraft.  Support Camber is the alignment of a main wheel in the vertical plain.  It can be checked with a bubble protractor held against the  wheel assembly. The wheel camber is said to be positive if  the top of the wheel tilts outward from vertical. Camber is  negative if the top of the wheel tilts inward.  [Figure 13-30]  Adjustments can be made to correct small amounts of wheel  misalignment. On aircraft with spring steel gear, tapered  shims can be added or removed between the bolt-on wheel  axle and the axle mounting flange on the strut. Aircraft  equipped with air/oil struts typically use shims between the  two arms of the torque links as a means of aligning tow-in  and tow-out.  [Figure 13-31]  Follow all manufacturer’s  instructions. Aircraft landing gear are attached to the wing spars or other  structural members, many of which are designed for the  specific purpose of supporting the landing gear. Retractable  gear must be engineered in such a way as to provide strong  attachment to the aircraft and still be able to move into a  recess or well when stowed. A trunnion arrangement is  typical. The trunnion is a fixed structural extension of the  upper strut cylinder with bearing surfaces that allow the entire  gear assembly to move. It is attached to aircraft structure in  such a way that the gear can pivot from the vertical position  required for landing and taxi to the stowed position used  during flight.  [Figure 13-32] 