lomapaseo
"Vibratory modes are predictable and calibrated via experience including the hundreds of hours in development testing before the engine is certified. So obviously no mis-designed or undertested severe failure conditions are expected in service before any recommended maintainence cycles. assuming of course no mis-operation or mis-manufacturerd parts."
Problem. That is not ill-informed, but it is wrong. It is the casual expectation of results that gets engineers into trouble. Most vibratory modes are known, especially when building a "replica" of a known model (TRENT). The 900 is quite different from the others, in RPM, discrepant RPM, and bearings. I predict it is exactly what you casually discard that is the problem, just as you have predicted that all vibratory modes are predictable.