## Datum.

Datum:

In weight and balance calculations it is not only the total weight onboard the aircraft that is important but also where the weight is situated.
The relative location of all loaded weight is expressed using a datum line.
This datum line, or in simpler terms the datum, is an imaginary vertical line that can be found in or outside of an aircraft, it is always vertical.

There is no fixid rule for the location of the datum line, except that it must be a location that will not change during the life of the aircraft. For example, it would be not be a good idea to have the datum be the tip of the propellor spinner or the front edge of the pilot seat, because changing to a new design of the spinner or moving the seat of the the pilot would change the the datum.
The datum is mostly to be found at a recognisable position in the aircraft, the nose, leading edge or any other bulkhead.

The manufacturer has the choise of locating the datum where it is most convenient for measurement , equipment location and weight & balance computation

The figure below shows an aircraft with the leading edge of the wing being the datum.

The datum line through the nose of the aircraft.

The chosen datum line for weight and balance calculation purposes is through the nose of the aircraft, which makes the centre of gravity calculations easier to calculate. We have already seen that all clockwise moments/forces are positive and all anti-clockwise moments/forces are negative. If the datum line runs through the nose of the aircraft then all other moments to the datum line are therefore positive.

The figure below shows an aircraft with the datum line through the nose of the aircraft.