Unlike the motor-car engine, which is generally designed, built and tested in association with the vehicle which it eventually propels, the aero engine is often designed and built with little or no knowledge of the types of aircraft in which it will be employed. The remainder of the power plant, fuel, water and oil systems of the car are also designed at the same time and to suit the chassis, but the aero engine, in the various types of aircraft, is likely to be coupled to many different forms of these systems, and consequently numerous problems arise between the completion of the acceptance tests of an engine and the final approval of an aircraft power plant for flight tests.