For commercial aircraft when it is weighed to certify its basic weight it is done in accordance with the manufacturers weighing process, this usually entails defueling and sump draining the tanks.

In certain cases the manufacturer has a process to weigh the aircraft with fuel this may be followed to weigh and calculate the basic empty weight for certification.

For light aircraft with much smaller capacity tanks and usually more symmetrical it is permissible to weigh the aircraft with full tanks or with multiple tanks mains full & aux empty, the need for full and not part full is that it would not be possible to accurately determine quantity of fuel in the tanks, full tanks can be verified by visual inspection, the full tank quantity of usable fuel and corresponding lever arm is stated by the manufacturer and can then be calculated off the aircraft.

Over time the basic empty weight and balance of an aircraft will change through modification, repair and ingress of dirt and moisture. An aircraft has a set of weight and centre of gravity limits which it must not exceed during its operation. In order to prevent the aircraft being operated outside those limits we must determine the empty weight and balance periodically by physical weighing. On aircraft operated under an aircraft operator certificate it is a requirement to carry out a physical re-weigh every 4 years.

It is possible to calculate changes that occur to the aircraft this is a normal part of tracking the weight and balance changes between physical weighing calendar requirements. Typically changes that may be calculated are service bulletin, some repairs, avionic changes, cabin configuration changes and paint scheme changes.

Inside the mandated weighing interval unless stated in the modification documents it is acceptable to re-determine the aircraft basic weight by calculation

For aircraft moisture retention is a problem typically when the aircraft descends and the cabin depressurises water is formed and is absorbed by the aircraft’s soft furnishings.

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

Repairs to airframe, operators and manufacturers modifications such as blended winglets and paint schemes.

An airframe may get lighter occasionally, if it has had several layers of paint removed prior to re-painting or if an aircraft is on a long maintenance program in a hangar the items that retain moisture will have a chance to dry out, other reasons may be from a configuration change to lighter seats.

A common mistake when looking at the weight and balance of an aircraft is to confuse Lever Arm with Centre of Gravity.

The Lever Arm is a fixed dimension and is multiplied by the weight to give the Moments.

The Centre of Gravity which is a variable dimension, is calculated by dividing the Total Moments by the Total weight. The figure obtained is usually rounded to manageable number of decimal places, using the rounded CG figure to multiply the weight will show a discrepancy in the moment figure as shown on a weighing report.

Only use lever arms to calculate individual Moments.

Only use Total Moments divided by Total Weight to calculate resultant Centre of Gravity