In the early days of aero-engines the test equipment was limited to little more than a brake or dynamometer, a fuel flow meter and a few thermometers, and the development of these engines in the direction of better performance and reliability, to a large extent, was an art, built up of experience aided by instinct, of deducing the happenings inside an engine from the few meagre measurements obtainable.

Nevertheless, the aero-engine made healthy progress, became more complicated in the process, and introduced many more problems.