Why do we need to weigh an aircraft?

 An aircraft has a maximum authorised weight that it must not exceed. In order to calculate its safe payload the weight in its basic empty configuration must be determined periodically

Why do aircraft get heavier?
This may be caused by several factors:

1. Moisture retention, typically when the aircraft climbs and descends, or as temperature and humidity changes, moisture present in the atmosphere forms water droplets that may be absorbed by the aircraft’s soft furnishings and soundproofing.

2. Retention of dirt in the cabin, under the floor and in some of the aircraft compartments that may be exposed to the elements.

3. Owners’ and manufacturers’ modifications and/or repairs to the airframe.

4. Paint schemes, light coloured paint generally needs to be thicker than dark to cover other dark colours.

Why does the Weight multiplied by the CG not equal the Moment?

A common mistake when looking at the resultant weight and balance of an aircraft is to confuse Lever Arm with C G. The Lever Arm is a fixed dimension relative to the Datum and is multiplied by the Weight of the component, whose centroid is at that position, to give its Moment.  The C G, which is a variable dimension, is calculated by dividing the Total Moment of the aircraft by its Total Weight. .The figure obtained is usually rounded to a manageable number of decimal places; this is what causes a discrepancy in the Moment figure when re-multiplied by the Weight. Only use Lever Arms to calculate individual Moments. Only use Total Moment divided by Total Weight to calculate resultant CofG.

Common Weight and Balance Terms

Datum - A vertical plane or line to which the C G position and all Lever Arm measurements relate.

Centre of Gravity - This is the point about which an aircraft would balance if it were possible to suspend or lift it at that point. It is found by dividing the Total Moment by the Total Weight.

Arm or Lever Arm - This is the distance from the Datum to an installed item or component’s own CG or centroid. The algebraic sign is plus (+) if measured aft of the Datum and minus (-) if measured forward of the Datum. It is a fixed measurement.

Moment - This is the Weight of an item or component multiplied by its Lever Arm. The Total Moment is the sum of all the individual Moments.

Empty Weight - This is normally the Weight of the aircraft including all its basic essential equipment. When the Empty Weight includes all the unusable fuel it is normally termed Basic Weight or Basic Empty Weight.