What was found
AT FL410 WE ENCOUNTERED SEVERE TURBULENCE. ACFT OVERSPEED TWICE WITH 10KTS IAS ABOVE VMO. SPEED BRAKES DEPLOYED WITH IDLE THRUST TO ATTEMPT TO COUNTERACT CONDITION. OVERSPEED LASTED APPROX 3 SECS EACH WITH AT LEAST 2 AURAL WARNINGS OF HIGH SPEED EACH TIME. AT FL410 ENCOUNTERED SEVERE TURBULENCE. GOT +/-300 FT DEVIATIONS & ACFT HAD SUBSTANTIAL PITCH & ROLL CHANGES. INITIATED AN EMERGENCY DESCENT DUE TO LACK OF CONTROL, & HAD AP, YD, AHRS 1/2 & SWPS FAIL CASS PRESENT. ABLE TO GET TO FL260 & THEN THE CASS MESSAGES WENT AWAY, WITH ALL SYS INDICATING NORMAL. NO ABNORMALITIES NOTICED AFTER TURBULENCE EVENT. POWERED ACFT ON EXTERNAL POWER. TECHS PERFORMED AHRS OPS TEST IAW MM 34-21-00, NO DEFECTS NOTED. DEPOWERED ACFT. POWERED ACFT ON GPU POWER & AGAIN PERFORMED THE OPS TEST OF AHRS IAW MM 34-21-00, NO DEFECTS NOTED. DEPOWERED ACFT. SINCE NO DEFECTSNOTED, ACFT IS TO BE RELEASED & FAULT MONITORED. CONTACTED MCC DISCUSSED FINDINGS. MCC AGREED WITH CONTACT CENTER & ADVISED THAT NO FURTHER ACTION IS REQUIRED.