  We perform a linear stability analysis of dynamical, quadratic gravity in the
high-frequency, geometric optics approximation. This analysis is based on a
study of gravitational and scalar modes propagating on spherically-symmetric
and axially-symmetric, vacuum solutions of the theory. We find dispersion
relations that do no lead to exponential growth of the propagating modes,
suggesting the theory is linearly stable on these backgrounds. The modes are
found to propagate at subluminal and superluminal speeds, depending on the
propagating modes' direction relative to the background geometry, just as in
dynamical Chern-Simons gravity.
