We tried to use genuine engine sounds from the various planes as we moved through the ages, and capture the change in character of the engines, so as you move forward in time the sounds got less staccato and more legato, as you would get with the introduction of the jet engine. The general ambiances were filled out too, to convey the rising popularity of air travel. During the second half and in order to build tension, the engines gradually rise in pitch together creating a chorus and climax.