As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ACFT WAS PERFORMING A NON-ROUTINE FLIGHT OPERATION AFTER A HEAVY MX VISIT, ACFT PRESSURIZATION BECAME UNCONTROLLABLE. ACFT RETURN TO DEPARTURE AIRPORT. MX FOUND A FLEX DUCT BETWEEN CONDENSER & ACM ON RT PACK DUCT THAT FAILED AND WAS LEAKING (REF AIPC 21-51-18-06-310). FLEX DUCT WAS REPLACED IAW AMM 21-51-11. ACFT WAS PRESSURIZED AND LEAK CHECKED SYS GOOD IAW AMM. ALL PARAMETERS WERE MET.