The air-core plays an important role in the operation of a conical hydrocyclone. In the apex, the air-core increases in diameter with the possibility of reaching a size close to the apex diameter. In this case, roping may be induced. There is a range of values for the apex to vortex ratio where roping is possible. Using some simple physical models, this work shows that perturbations propagating through the air-core interface are amplified in the direction of the apex and may be responsible for the fluctuations of the underflow that are characteristic of roping. (c) 2005 Published by Elsevier B.V.