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Prior to the introduction of the BAC 1–11 aircraft, constant speed generator drives were primarily the hydro-mechanical type; i.e., the engine speed was trimmed by the use of hydraulic devices. The BAC 1–11 has introduced the Plessey CSDS, a pneumatically trimmed unit which assumes the dual role of a CSD and an engine starter. This unit is basically an epicyclic gear train which takes inputs from both the engine and a reversible roots type air motor/blower to drive an A-C generator at a constant speed. This paper will discuss the design, operation, and performance of this unique unit.