Datum – An aircraft datum line is an imaginary vertical plane or line from which all horizontal measurements of arm are taken, generally for balance purposes. Once the datum has been selected, all moment arms and the location of CG range are measured from this point and the weight and balance may be calculated.
The datum line appears on drawings as a vertical line, which is perpendicular (90°) to the aircraft horizontal axis. The datum is established by the manufacturer and is referenced in the type certificate data sheet as noted in the attached illustration. There is no fixed rule for the position or location of the datum, except that must be positioned in a location that will not change during the life of the aircraft. For example, if the datum were positioned at the tip of the propeller spinner or the front edge of the a seat it is possible that during the life of the aircraft a new spinner or seat design may be installed thus changing the reference position. So, if the datum position was desired by the manufacturer to be located in front of the spinner, manufacturer would reference a fixed design spot on the aircraft such as the leading-edge of a wing and specify that the datum position is a set number of inches forward of that point. To avoid confusion. The datum is positioned in such a manner that all reference dimensions will be a positive number rather than a negative number, which would occur forward of that datum position.