While cruising at FL240 in instrument meteorological conditions, on a flight from Brisbane to Townsville, the crew of a BAe 146 noticed the amber engine vibration (ENG VIBN) annunciator had illuminated. The engine thrust management system (TMS) was configured to 830 degrees Celsius in turbine gas temperature (TGT) mode, and all engine and airframe anti-ice systems were selected ON at the time.
A check by the crew revealed the number 1 engine vibration indication to be at 1.5 units, a reading that was outside the Manufacturer's prescribed limits. All other engine indications were normal. The crew disconnected the TMS and shut down the number 1 engine. The crew then decided to return to Brisbane.
Shortly after, the ENG VIBN annunciator again illuminated and the number 3 engine vibration indicator showed 1.5 units, with all other engine indications normal. The number 3 engine power lever was then retarded to below 80%. The ENG VIBN annunciator extinguished, and the indicated vibration level dropped to approximately 0.5 units.
During descent, the operation of the number 3 engine was monitored with nothing abnormal noted, and an uneventful one engine inoperative landing was subsequently carried out.
The pilot later reported that the weather conditions in the area were conducive to intake icing, with the possibility that the aircraft was flying through moderate freezing rain at the time of the incident.
A maintenance investigation carried out by the operator following the incident, discovered water ingress into the number 3 engine vibration transducer, in the lower fan cowling area. This has been known to result in erroneous vibration instrument indications in the past. No other mechanical defect was discovered that would have resulted in the vibration in either engine. Following the completion of operational tests, the aircraft was returned to service. The vibration has not recurred.
The ATSB investigation concluded that the indicated engine vibrations were consistent with a build up of ice on the fan areas of engines 1 and 3 while the aircraft was being flown in icing conditions. This conclusion is reinforced by the report from the pilot in command that the engine vibrations subsided as engine RPM was reduced, and did not return when the number 3 engine RPM was increased when clear of the icing conditions. The vibration indications on the number 3 engine may have been further affected by the water ingress into the area of the vibration transducer.