For aviation to become commercially usable and reliable, in addition to the development of aerodynamics, of reliable structures and reliable engines, there had to be the development of instruments, communications, and navigational aids. Suppose, for example, that today one could not fly through cloud, or at night, then nothing remotely resembling a regular service would be possible. Flying would still be a sport.

To illustrate what I mean I shall quote the example of another innovation with quite different properties in this respect. The first railway was immediately commercial and usable without any aids. It can be seen at once that the train running along rails is constrained compared with aircraft. It is obvious that the navigation problem here is very simple, and that there is not really a problem of control or stability—the first railway trains did not even have brakes. The mechanical skill of operating and maintaining the new device had to be learned; but very little else.