As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DESCENDING FROM FL 410, LOAD POP HEARD FROM REAR OF AIRPLANE AND SMOKE SMELL FILLED THE COCKPIT. PER CHECKLIST ISOLATED THE COCKPIT PAC AND THE SMOKE DISSIPATED. TROUBLESHOT TO COCKPIT ACM TURBINE (COMING APART). REMOVED & REPLACED COCKPIT ACM IAW CE 750 MM 21-51-00. OPERATIONAL CHECKS GOOD.