Aircraft have a tendency to gain weight because of the accumulation of dirt, greases, etc., in areas not readily accessible for washing and cleaning. The weight gained in any given period of time will depend on the function of the aircraft, its hours in flight, atmospheric conditions, and the type landing field from which it is operating. For this reason, periodic aircraft weighings are desirable and, in the case of air carrier and air taxi aircraft, are required by Federal Aviation Regulations.
Privately owned and operated aircraft are not required by regulation to be weighed periodically. They are usually weighed when originally certificated, or after making major alterations that can affect the weight and balance. Even though the aircraft need not be weighed, it must be loaded so that the maximum weight and CG limits are not exceeded during operation.
Airline aircraft (scheduled and nonscheduled) carrying passengers or cargo are subject to certain rules that require owners to show that the aircraft is properly loaded and will not exceed the authorized weight and balance limitations during operation.