Abstract: A method for controlling instability in a combustion system comprising a combustion chamber and a passage external of the combustion chamber which supplies a secondary non-combustible fluid to the combustor chamber. The passage external to the combustion chamber is arranged such that the phase of perturbations in the mass flow of the secondary non-combustible fluid in the passage acts to reduce the amplitude of unsteady instability in the combustor chamber. Furthermore, the geometry of the external passage may be actively adjusted to tune the system if the frequency of the unstable instability differs from that for which the combustor was originally designed to attenuate.