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.

Before the Second World War there were few airfields in Great Britain with paved runways.

The grassed flight strips of the pre-war years could carry the wheel loads of the aircraft then in use with little maintenance, other than grass cutting and rolling clinker or stone into local soft patches. From 1939 onwards, the increasing weight of aircraft, and more intensive flying, made it necessary to construct airfields with paved runways to permit operation in all weathers.

From 1939 to 1945, 444 airfields were constructed with paved runways for the Royal Air Force at a cost of £200,000,000, excluding the cost of building construction. Since 1945 the emphasis has been upon airfields for civil use, and the great airfields at Idlewild, Boston, Johannesburg and London Airport have been built.

In this paper it is shown how, in an angular contact ball bearing, the motion of a ball depends on the dimensions, loading and speed of the bearing. In general, ball motion in an angular contact bearing differs from the simple motion usually visualised by including oscillatory occurrences, shown here analytically, graphically and visually. These oscillatory occurrences are shown to lead to premature fatigue of the bearing metal, as a function of the speed of rotation, thereby imposing the present limitations on maximum speed and “ life.”

It is further shown that only when the dimensions of an angular contact ball bearing are related in a certain way, a motion is produced in which the oscillatory phenomena are eliminated. These relations are given as functions of loading and speed in a system of equations and limiting conditions (hereafter referred to as “ Relations of the Dimensions ”).