Technical Talk

Why do we need to weigh an aircraft
1. 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; on a commercial aircraft that’s normally a requirement every 4 years under EASA/JAR OPS.
2. The aircraft must be loaded so that its Centre of Gravity (CG) falls within the stated forward and aft limits in order to retain full control surface authority e.g. aileron movement in either direction.
Why do aircraft get heavier?
This may be caused by several factors:
1. Moisture retention; typically when the aircraft descends and the cabin depressurises the moisture present in the atmosphere forms water droplets that are 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. Operators’ and manufacturers’ modifications and/or repairs to the airframe.
4. Paint schemes. Remember that light coloured paint generally needs to be thicker than dark to cover other dark colours.
Do aircraft get lighter?
An airframe may get lighter occasionally. For example, if it has had several layers of paint removed prior to re-painting or it is on a long maintenance programme in a warm dry hangar and the items that retain moisture have had a chance to dry out.
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 C G
Why does my aircraft empty CG not fall within the given limits?
The CG limits given in the Pilot’s Handbook or Flight Manual are flight limits so you will have to add in the operational items, such as Pilot and Fuel, then re-calculate the Weight and CG.
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.