stepping resonance/vibration

[dipsplash]

I have recently analysed the 'cyclic and loud' resonance emanating form a Rolls royse gas turbine engine. I eventually diagnosed this as 'stepping'; a well known but rarely found characteristic which occurs at high turbine spool speed. Can anyone tell the precise cause and give me a 'text book' definition of this phenomena. For info, it occurs as a result of high speed output line imbalance and is correctable by in situ balance techniques.

Dip