As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLT 7989 - IAD - ON ARRIVAL DESCENT AT 210 KNOTS AS F/O TOUCHED TRIM DOWN TO DISCONNECT AP, BOTH PITCH TRIM MAIN AND PITCH TRIM BACKUP WARNING MESSAGES ILLUMINATED. MAINTENANCE CLEANED MAIN AND BACKUP TRIM SWITCHES AND PERFORMED TEST OF ALL SWITCHES RELATED TO PITCH TRIM. NO DEFECTS NOTED.