This communication deals with the graphical representation of power flows in hydromechanical drives, for use in systems- engineering activities. It points out the need for standardized symbols in mechanics to complement the well-established standards in electrics and hydraulics. Once identified and listed, the mechanical functions are arranged by classes and subclasses, which define the main families of symbols to be designed. A functional view is favoured. Proposals are made for symbolisation in order to improve the readability of the diagrams dealing with multi-physical systems. This is illustrated with the example of the Airbus 320 yaw control, which combines mechanical, electrical and hydraulic devices in a complex architectural arrangement. This work contributes to make foundations for the development of a modern standard of mechanical symbols.