  It is well-known that 7-dimensional 3-Sasakian manifolds carry a
one-parametric family of compatible G_2 structures and that they do not admit a
characteristic connection. In this note, we show that there is nevertheless a
distinguished cocalibrated G_2 structure in this family whose characteristic
connection along with its parallel spinor field can be used for a thorough
investigation of the geometric properties of 7-dimensional 3-Sasakian
manifolds. Many known and some new properties can be easily derived from the
properties of this connection and the spinor field, yielding thus an
appropriate substitute for the missing characteristic connection.
