  The three-dimensional tilted axis cranking covariant density functional
theory (3D-TAC CDFT) is used to study the chiral modes in $^{135}$Nd. By
modeling the motion of the nucleus in rotating mean field as the interplay
between the single-particle motions of several valence particle(s) and hole(s)
and the collective motion of a core-like part, a classical Routhian is
extracted. This classical Routhian gives qualitative agreement with the 3D-TAC
CDFT result for the critical frequency corresponding to the transition from
planar to aplanar rotation. Based on this investigation a possible
understanding of tilted rotation appearing in a microscopic theory is provided.
