Rigging Checks Structural Alignment Checking Incidence Checking Dihedral maximum jacking weight (if applicable) specified by the  manufacturer is not exceeded.  With a few exceptions, the dihedral and incidence angles  of conventional modern aircraft cannot be adjusted. Some  manufacturers permit adjusting the wing angle of incidence  to correct for a wing-heavy condition. The dihedral and  incidence angles should be checked after hard landings or  after experiencing abnormal flight loads to ensure that the  components are not distorted and that the angles are within  the specified limits.  All aircraft assembly and rigging must be performed in  accordance with the requirements prescribed by the specific  aircraft and/or aircraft component manufacturer. Correctly  following the procedures provides for proper operation  of the components in regard to their mechanical and  aerodynamic function and ensures the structural integrity  of the aircraft. Rigging procedures are detailed in the  applicable manufacturer’s maintenance or service manuals  and applicable structural repair manuals. Additionally,  aircraft specification or TCDS also provide information  regarding control surface movement and weight and balance  limits.  The purpose of this section is to explain the methods of  checking the relative alignment and adjustment of an aircraft’s  main structural components. It is not intended to imply that the  procedures are exactly as they may be in a particular aircraft.  When rigging an aircraft, always follow the procedures and  methods specified by the aircraft manufacturer.  There are several methods for checking structural alignment  and rigging angles. Special rigging boards that incorporate,  or on which can be placed, a special instrument (spirit level  or inclinometer) for determining the angle are used on some  aircraft. On a number of aircraft, the alignment is checked  using a transit and plumb bobs or a theodolite and sighting  rods. The particular equipment to use is usually specified in  the manufacturer’s maintenance manual.  When checking alignment, a suitable sequence should be  developed and followed to be certain that the checks are  made at all the positions specified. The alignment checks  specified usually include:  The position or angle of the main structural components is  related to a longitudinal datum line parallel to the aircraft  center line and a lateral datum line parallel to a line joining  the wing tips. Before checking the position or angle of the  main components, the aircraft must be jacked and leveled.  Small aircraft usually have fixed pegs or blocks attached to  the fuselage parallel to or coincident with the datum lines.  A spirit level and a straight edge are rested across the pegs  or blocks to check the level of the aircraft. This method of  checking aircraft level also applies to many of the larger types  of aircraft. However, the grid method is sometimes used on  large aircraft. The grid plate is a permanent fixture installed  on the aircraft floor or supporting structure.  [Figure 2-83]  When the aircraft is to be leveled, a plumb bob is suspended  from a predetermined position in the ceiling of the aircraft  over the grid plate. The adjustments to the jacks necessary to  level the aircraft are indicated on the grid scale. The aircraft  is level when the plumb bob is suspended over the center  point of the grid.  The dihedral angle should be checked in the specified  positions using the special boards provided by the aircraft  manufacturer. If no such boards are available, a straight edge  and a inclinometer can be used. The methods for checking  dihedral are shown in  Figure 2-84 .  It is important that the dihedral be checked at the positions  specified by the manufacturer. Certain portions of the wings  or horizontal stabilizer may sometimes be horizontal or, on  rare occasions, anhedral angles may be present.  Certain precautions must be observed in all instances when  jacking an aircraft. Normally, rigging and alignment checks  should be performed in an enclosed hangar. If this cannot  be accomplished, the aircraft should be positioned with the  nose into the wind.  The weight and loading of the aircraft should be exactly as  described in the manufacturer’s manual. In all cases, the  aircraft should not be jacked until it is determined that the  Incidence is usually checked in at least two specified  positions on the surface of the wing to ensure that the wing  is free from twist. A variety of incidence boards are used to  check the incidence angle. Some have stops at the forward  2-48 •  Horizontal stabilizer dihedral •  Horizontal stabilizer incidence •  A symmetry check •  Engine alignment •  Verticality of the fin •  Wing incidence angle •  Wing dihedral angle