Investigation has shown that the N2 generated vibration does not harm the N2 rotating or static components. During engine re-start, shortly after engine shut-down (also known as a “quick turn”), an N2 generated vibration may occasionally be heard or felt by the flight crew. This phenomenon usually decreases quickly after an initial peak, however, in some instances, it may result in a L or R Engine EICAS (Engine Vibration), and Amber Vibe Icon message display in the cockpit.
Normally, the ECU software prevents this issue by suppressing the cockpit indication during engine start, however, if the level rises above .78 IPS after idle N2 is achieved, the aforementioned CAS messages will display. While the phenomenon is rare on the AS907 (HTF7000) engine, it is typically observed in association with quick-turn engine starts in colder weather or if, during the quick turn, significant wind has been blowing into the tailpipe. The source of the vibration is the result of a somewhat rapid, yet uneven cooling of the engine high pressure (N2) rotating and stationary components shortly after engine shut down.
Investigation has shown that the N2 generated vibration does not harm the N2 rotating or static components. With instances in which the EICAS (Engine Vibration), and Amber Vibe Icon messages have displayed, it is typical to conduct a download of the engine ECUs in order to clear the cockpit messages and to return the aircraft to service. This results in extended aircraft downtime while a maintenance crew can be dispatched, download equipment installed, ECUs downloaded, etc.
The Service Information Letter (SIL) will be issued in March 2013.