Pre-flight review of the route forecast and SIGWX for turbulence is a requirement for RVSM ops. Likewise reporting "unable" is a requirement if you cannot fly ±200FT in RVSM airspace.
If it was a severe turbulence encounter, then they could possibly have been busy dealing with it!
So, may someone explain to me when aircraft is no longer capable of maintaining RVSM status in severe turbulence?
"Aircraft handling difficulties" is one of the textbook definitions of severe turb. At FL370 is it not easy to hand fly within ±200FT when it is smooth! Add severe turb and the autopilot wont be doing it either, if it hasn't disconnected.
Copied from some groundschool notes:
• Very low – below 0.05g – Light oscillations
• Low – 0.05 to 0.2g – Choppy; slight, rapid, rhythmic bumps or cobblestoning
• Moderate – 0.2 to 0.5g – Strong intermittent jolts
• Severe – 0.5 to 1.5g – Aircraft handling made difficult
• Very severe – above 1.5g – Increasing handling difficulty, structural damage possible.
Manufacturers often recommend pilots abandon hard targets or accuracy during flight in severe turbulence. From one Boeing FCOM under Severe Turbulence:
"Use autopilot in turbulence. Closely monitor autopilot
operation and be prepared to disconnect autopilot only if
airplane does not maintain an acceptable attitude. If autopilot
disconnects, pilot should smoothly take control and stabilize
pitch attitude. Fly attitude as the primary pitch reference.
Sacrifice altitude to maintain attitude. Disregard flight director
pitch bar. Do not trim manually. After recovery, autopilot
should be reengaged if available."
You can forget about ±200FT, better off calling for a block clearance in severe turbulence!