A note on symmetry and stability for fluid flows

In this note it is shown that Andrews' theorem, which states that Arnol'd stability implies symmetry, is not special to fluid mechanics, but holds in the general context of classical mechanics with symmetry. More importantly, we emphasise that in Andrews' theorem one should consider stability modulo the symmetry group of solution class. We do this by using the energy-Casimir method to prove the stability of two-dimensional ABC flows, even though they are not symmetric.