There are two parts of this paper, the first dealing with Servicing and the second with Reliability. Although they are connected in many ways, and indeed servicing has been taken to mean checking parts which might be unreliable, it is convenient to separate them.

A practical factor of increasing importance is the burial of the accessory machines in modern aeroplanes.

Compared with, say, a plough, the parts of Blériot's Anzani engine might be thought to be dangerously concealed; dangerous, that is to say, if its working depended on any condition of the internal parts which could be judged when looked at. But the Anzani engine was simplicity itself, compared with any modern machine, even an accessory machine, and these machines are usually covered by, or rather entangled in, the structure.