Originally Posted by
FlightDetent
I was told (CFM-56 at that time) the LP spool will give you airframe shake, unlike the HP where indication without perceived vibration needs to be taken into account straight away.
tdracer, does that resemble any of your experience?
With the previously mentioned of the RB211, I don't believe Boeing has ever published a "high vibe" limit or action - high vibrations are to be used in combination with other engine parameters and pilot judgement. Worst case, leaving an engine with high vibrations can cause economic damage - it's not a safety of flight issue*. Furthermore, vibe indications are not exactly the most robust engine signal - a high vibe indication with no other anomalies is quite possibly nuisance. While high N1 vibes can generally be felt through the airframe, if the HP vibrations are excessive they can usually be felt was well (on the 747-8 we had issues with 'bowed rotor start' which is a HP rotor vibration, and it could clearly be felt in the flight deck - and remember the 747 flight deck is well removed from the engines).
*The RB211 was the exception because it was potentially safety of flight. Three spool engines are a bearing design nightmare, and Rolls has had it's fair share of bearing issues. On the RB211-524, there were several instances where one of the fan shaft bearings would overheat - leading to an oil fire that quite literally cut the fan shaft in half. First time it happened, center engine on an L1011 - the fan departed the rest of the engine and tried to saw the aircraft in half. They landed safely thanks to some good piloting and good luck, but it was a near thing. Rolls instituted a 'fan catcher' (sort of like a disc brake) to prevent the fan from departing the engine if it happened again - which it did on a 747. Fan catcher worked as advertised, but the unloaded turbine rev-ed to well over redline and disintegrated - peppering the wing with shrapnel (fortunately without causing catastrophic damage). Anyway, a vibe limit was instituted on the RB211 - vibes over some limit mandated an immediate shutdown (I think it was fan vibe only but wouldn't swear to it).