This paper discusses a few simple ideas which may be supposed to form, consciously or unconsciously, a schematic basis for the servicing of given aircraft in given circumstances. It is not concerned with engineering technique but rather with the assumptions behind it.

The purpose of an aircraft's upkeep, by which is meant its servicing and maintenance in the widest sense, is to enable the machine to furnish the maximum output of flying work of the required kind and intensity. “Intensity” is used to mean an aircraft's rate of output of flying work in a calendar period.

For commercial aircraft the average intensity required is usually near the maximum that can be sustained at reasonable upkeep expense, because this tends to maximise net profit (or to minimise net loss if there is no profit) by lowering the overheads related to the capital cost of aircraft.