Sorry to be so acerbic/ caustic in my reply there Triple F, guess I also had a similarly poor day
Whirl Mode The BE-1900 had a problem with broken or cracked engine mounts before Service Bulletin 2255 mandated stronger trusses and increased inspection period. Seems that whirl mode flutter may have been the cause of the cracks. The accident offshore Block Island, (29 December, 1991) may have been due to engine separation as the result of wmf. That seems to be ALPAs position, the NTSB maintains it was pilot error, loss of aircraft control. Though the finding was pilot error, the manufacturer issued SB 2255 shortly after the accident.
Whirl mode is present in most rotating bodies, such as a ceiling fan that not only rotates, but if not rigid will wobble or precess.