The control of empty weight is just as important after a transport airplane has been delivered by the manufacturer to its operator as it was during its design, testing and production stages, because it is obvious that the maximum revenue obtainable during its useful life depends upon the retainment of its highest permissible ratio of useful load to gross weight, all other factors and characteristics being equal.

However, the designer must not originally achieve maximum load carrying capacity at the expense of marginal structures or lack of rigidity, which later on would demand excessive maintenance and heavy reinforcements.

The penalty to the air transport operator caused by excessive empty weight can be best emphasized by stating that on Eastern Air Lines, for instance one hundred pounds of payload is worth approximately $1,650.00 of revenue per year.

Nevertheless, and in spite of close watch of airplane empty weight, a gradual increase almost invariably occurs with any particular type of transport aircraft over a period of years in scheduled operation. Such weight increase can be segregated into three categories which are listed as follows:

(1) Weight increases demanded by safety. These include reinforcements of structural members found to be necessary in service, additions in radio equipment, fire protection, oxygen equipment, and so on

(2) Weight increases to reduce maintenance, such as the use of heavier tires, cactus—proof tubes, removable panels and heavier gauge sheet metal for cowling removed at regular intervals.

(3) Weight increases to improve passenger comfort. This includes changes in cabin appointments, food service, lavatory facilities, heavier carpets, exterior painting, and so on.

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