johan jnb - 21000 feet per minute... who writes this stuff!?
Pitch attitude x mach = rate of descent(approximately)
Reports says pitch attitude was as low as -23, and the a/c maximum speed was exceeded. Using the rule of thumb above a mach of .91 would be approximately 21,000' FPM so it's seems to be a reasonable number.
Personally I didn't enjoy descent rates above 20,000' - 25,000' FPM but everyone's tolerances are different.