  The principal component analysis (PCA), a mathematical tool commonly used in
statistics, has recently been employed to interpret the $p_T$-dependent
fluctuations of harmonic flow $v_n$ in terms of leading and subleading flow
modes in heavy ion collisions. Using simulated data from AMPT and HIJING
models, we show that the PCA modes are not fixed, but depend on the choice of
the particle weight and the $p_T$ range. Furthermore, the shape of the leading
mode is affected by the presence of non-flow correlations, and fake subleading
mode may arise from the mixing of non-flow correlations with leading flow mode
with a magnitude that could be larger than the genuine subleading flow mode.
Therefore, the meaning of PCA modes and their relations to physical leading and
subleading flow modes associated initial state eccentricities need to be
further clarified/validated in realistic model simulations.
