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.