3-5 Configuration of the Aircraf Jacking the Aircraft Safety Considerations Determining the CG Leveling the Aircraft Figure 3-5.  Locating the CG of an airplane relative to the datum. conditions of both longitudinal and lateral level. The actual adjustments to level the aircraft using load cells  are made with the jacks. When weighing from the wheels,  leveling is normally done by adjusting the air pressure in the  nosewheel shock strut. Consult the aircraft service manual regarding position of  the landing gear shock struts and the control surfaces weighing. When weighing a helicopter, the main rotor must  be in its correct position. Aircraft are often weighed by rolling them onto ramps in  which load cells are embedded. This eliminates the problems  associated with jacking the aircraft off the ground. However,  many aircraft are weighed by jacking the aircraft up and then  lowering them onto scales or load cells. Special precautions must be taken when raising an aircraft  on jacks. Extra care must be used when raising an aircraft on jacks for  weighing. If the aircraft has spring steel landing gear and it  is jacked at the wheel, the landing gear will slide inward as  the weight is taken off of the tire, and care must be taken to  prevent the jack from tipping over. For some aircraft, stress panels or plates mustbeinstalled before  the aircraft is raised with wing jacks to distribute the weight  over the jack pad. Be sure to follow the recommendations  of the aircraft manufacturer in detail anytime an aircraft is  jacked. When using two wing jacks, take special care to raise  them simultaneously, keeping the aircraft so it does not slip  off the jacks. As the jacks are raised, keep the safety collars  screwed down against the jack cylinder to prevent the aircraft  from tilting if one of the jacks should lose hydraulic pressure. When the aircraft is in its level flight attitude, drop a plumb  line from the datum and make a mark on the hangar floor below the tip of the bob. Draw a chalk line through this point  parallel to the longitudinal axis of the aircraft. When an aircraft is weighed, it must be in its level fligh attitude so that all of the components are at their correct  distance from the datum. This attitude is determined by  information in the TCDS. Some aircraft require a plumb line  to be dropped from a specified location so that the point of the  weight (the bob) hangs directly above an identifiable point.  Others specify that a spirit level be placed across two leveling  lugs, often special screws on the outside of the fuselage. Other  aircraft call for a spirit level to be placed on the upper door sill. Then, draw lateral lines between the actual points for the main wheels, and make a mark along longitudinal line at the weighing point for or the tailwheel. These lines and marks on the floor allow  accurate measurements between the datum and the weighting  points to determine their arms. Determine the CG by adding the weight and moment of  each weighing point to determine the total weight and total  moment. Then, divide the total moment by the total weight  to determine the CG relative to the datum. As an example of  locating the CG with respect to the datum, which in this case  is the firewall, consider the tricycle landing gear airplane as  detailed in the  Figure 3-5  table and illustrated in  Figure 3-6. Lateral level is not specified for all light aircraft, but  provisions are normally made on helicopters for determining  both longitudinal and lateral level. This may be done by  built-in leveling indicators, or by a plumb bob that shows the  When the airplane is on the scales with the parking brakes  Weighing point Net weight (lb) TARE weight (lb) Scale reading (lb) Arm (in) Moment (lb-in) CG 46.0 46.0 −32.0 32.8 Right side Left side Nose Total 16 16 8 830 836 340 2,006 38,180 38,456 −10,880 65,756 846 852 348 4.  All jacks must be raised simultaneously and the safety  devices are against the jack cylinder to prevent the  aircraft tipping if any jack should lose pressure. Not  all jacks have screw down collars, some use drop pins  or friction locks. 3.  The jacks must be straight under the jack pads before  beginning to raise the aircraft. 2.  If anyone is required to be in the aircraft while it is  being jacked, there must be no movement. 1.  Stress plates must be installed under the jack pads if  the manufacturer specifies them