AIR transport operators desire primarily high reliability at lower fuel consumptions than are obtained at present. They look critically at the total economy picture, of which fuel economizing is but a part. If satisfactory schedule completion is possible at lower power at some optimum trip altitude, this method of economizing is highly desirable.
Some decrease in specific consumption is possible with present grades of fuel, given improved mixture indicators or controls. Large improvements in specific consumption must be weighed economically against decreased reliability, increased maintenance, and increased fuel costs.
Test-stand operation cannot be used directly to predict airline economy. Service experience is the only acceptable final test. The road to improved fuel economy is a long one with no safe short-cuts in sight.