The datum is an imaginary vertical plane from which all horizontal measurements are taken for balance purposes, with the aircraft in level flight attitude. If the datum was viewed on a drawing or photograph of an aircraft, it would appear as a vertical line which is perpendicular (90 degrees) to the aircraft’s horizontal axis. For each aircraft make and model, the location of all items is identified in reference to the datum. For example, the fuel in a tank might be 60 inches (60″) behind the datum, and a radio on the flight deck might be 90″ forward of the datum.
There is no fixed rule for the location of the datum, except that it must be a location that will not change during the life of the aircraft. For example, it would not be a good idea to have the datum be the tip of the propeller spinner or the front edge of a seat, because changing to a new design of spinner or moving the seat would cause the datum to change. It might be located at or near the nose of the aircraft, a specific number of inches forward of the nose, at the engine firewall, at the center of the main rotor shaft of a helicopter, or any place that can be imagined. The manufacturer has the choice of locating the datum where it is most convenient for measurement, equipment location, and weight and balance computation. Figure 4-1 shows an aircraft with the leading edge of the wing being the datum.
The location of the datum is identified in the Aircraft Specifications or Type Certificate Data Sheet. Aircraft certified prior to 1958 fell under the Civil Aeronautics Administration, and had their weight and balance information contained in a document known as Aircraft Specifications. Aircraft certified since 1958 fall under the FAA and have their weight and balance information contained in a document known as a Type Certificate Data Sheet. The Aircraft Specifications typically included the aircraft equipment list. For aircraft with a Type Certificate Data Sheet, the equipment list is a separate document.