Not all Engine Malfunctions are as clear as  ENG FAIL,  FIRE ENG or even Engine Failure Analysis. Sometimes engine malfunctions are as simple as anomalous indications of Oil Temperature or Pressure – or high engine vibrations on the Secondary Engine Display.
Engine VIB-rations (like other secondary engine indications that do not have actual flight manual limitations – such as Oil Quantity) do have a highlight feature when the indications are to be considered unusual. Above a certain value (don’t tell anyone – it’s 4.0) – the display inverts, black text on white background. If not already displayed this will automatically pop up the secondary engine indications on the lower MFD / compacted EICAS display.
There have been times in the past when crew have (and have been trained to) action a precautionary engine shutdown in response to single indications such as high vibration or low oil quantity.
The QRH is however quite clear. Such indications – including the automatic display of secondary engine indications – are not enough on their own to require shutting down the engine. You need some other kind of corroboration of an engine problem – a limit exceedance of one of the other parameters puts the situation beyond doubt – to go down the somewhat radical path of shutting down half your available engines.
Note that even mild accompanying airframe vibration (such as would be felt with a high N1 VIB indication – but not necessarily with N2/N3) might not be enough to require you to secure the engine. There’s clearly something wrong – but you are not required to do anything about it other than perhaps consider a reduction of thrust to see if that contains the problem.
Most airlines will specify a value of continued EICAS VIB indication that should be reported in a Technical Log. Again – don’t confuse this administrative requirement with a need to take action against an engine that is otherwise delivering thrust.
Finally, airline and manufacturer documentation often refer to vibration as an indication of engine damage or otherwise unsuitability for continue operation. Read carefully – in my experience these references are usually to airframe vibration rather than EICAS Secondary VIB indications.
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