A Cobham Aviation Avro RJ-100, registration VH-NJY performing flight NC-219 from Prominent Hill, SA to Port Augusta, SA (Australia), was en route at FL250 when the crew received an engine vibration warning concerning engine #4 (LF507, right outboard), the engine was vibrating at 1.8 units (limit 1.2 units), at the same time the crew noticed ice build up at the windscreen. The crew worked the related checklists before the checklist was completed the vibrations ceased. Shortly afterwards another engine vibration occurred, this time engine #2 (left inboard) showed 2.1 units of vibrations. Considering the weather conditions and icing conditions the crew decided to descend the aircraft to FL190 into warmer air, the vibrations ceased. In consultation with dispatch, the crew decided to divert to Adelaide, SA (Australia), where the aircraft landed without further incident.
The Australian TSB reported they had opened an investigation but now discontinued the investigation as there were no indications of a systemic safety issue. The ATSB reported significant weather with severe turbulence and icing was forecast along the route. The flight crew was aware of the SIGMET, the actual conditions were matching the SIGMET. The crew had activated engine anti-ice prior to the occurrence, this and fuselage anti-ice systems were working properly.
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