Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
plummet [ˈplʌmɪt] vi
to plummet (down) (bird, aircraft) ру́хнуть (perf) (price, amount) ре́зко па́дать (упа́сть*perf)
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.