Severe turbulence and RVSM
A month ago, a XLA B738 at FL370, suddenly appeared at FL345 (on RDD). It was only after a controller had intervened with request to verify level, that they said they're climbing again to FL370 and that they were unable to maintain level due severe turbulence (RVSM status not mentioned) It was all with the attitude like it was totally normal situation (descending and climbing without clearance), but that's different story.
Almost identical situation happened yesterday. ELG F100 lost also 2500 feet (FL330 to FL305) and informed the controller that they are climbing again to FL330. Again reason was severe turbulence, and again UNABLE RVSM DUE TURBULENCE was not reported.
So, may someone explain to me when aircraft is no longer capable of maintaining RVSM status in severe turbulence?