What was found
CLT - FLT 61 - CREW WAS UNABLE TO PRESSURIZE AIRCRAFT UPON CLIMB-OUT FROM CLT. FLIGHT RETURNED TO FIELD AT CLT AND LANDED WITHOUT FURTHER INCIDENT. NO EMERGENCY WAS DECLARED. MAINTENANCE FOUND AN INSULATION BLANKET STUCK IN THE UPPER PRESSURIZATION SAFETY RELIEF VALVE. REMOVED INSULATION AND SECURED. ACCOMPLISHED PRESSURIZATION CONTROL SYSTEM TEST. PRESSURIZED AIRCRAFT ON GROUND TO 6.5 PSI WITH APU AND ONE PACK. ACCOMPLISHED HULL LEAK DOWN CHECK AND FOUND TO BE WITHIN LIMITS PER MM CHAPTER 21. AIRCRAFT OK FOR SERVICE. (M)