N2 Vibration at engine start
The GE90 engine family experiences "bowed rotor starts". The phenomenon was first reported early in the GE90-76B program, and occurs to varying degrees on a regular basis. A bowed rotor start can be characterized as an indicated N2 vibration during the engine start sequence, frequently associated with felt vibration on the flight deck. It then dissipates approximately 15 to 30 seconds after the engine reaches idle. A bowed rotor start will not result in any significant engine distress nor is it an indication of an impeding engine operational event.