Narrative:

About one hour 15-minutes into the flight at FL330; the cabin pressurization controller began acting erratically. This was also accompanied with rapid pressure fluctuations that could be felt in the ears. The cabin altitude began climbing at an approximate rate of 1;500 FPM initially and later would increase further. We initiated QRH 21.X -'cabin pressure off schedule or erratic'. We selected the pressurization controller to stby with no change that corrected the problem. The outflow valve control wheel appeared to be sticking or stuck and every time the automatic system would recoil; the outflow valve would stick in a position more 'open' than the last. We selected the system to manual and manually attempted to move (muscle) the control wheel forward to close the outflow valve. We were unsuccessful in being able to position the outflow valve in a 'closed' position sufficient to maintain cabin pressure. While attempting to manually close the outflow valve; we were approaching a cabin altitude of 9;000-10;000 ft prior to reaching step # 3 on QRH 21.X; the 'cabin altitude' master warning illuminated. We elected to stop performing 21.X and performed the memory items required for QRH 21.Y 'cabin altitude/ rapid depressurization' followed by 21.Z 'rapid descent'. The first officer was pilot flying with the autopilot 'on'. We donned our oxygen masks and established communication with ATC. I took control of the aircraft; declared an emergency; and initiated an emergency descent. During the descent; while running QRH 21.Z; we ensured the blue 'cabin O2 on' light was illuminated. We elected to turn towards and divert into ZZZ. When approaching 10;000 ft; we removed our oxygen masks and I made a call to the flight attendants and briefed them followed by a clear public announcement (PA) to the passengers stating what had happened; that we were at a safe altitude to remove oxygen masks; and that we were diverting to ZZZ. Once below 10;000 ft; all operations/communications were normal; as well as a normal landing. We contacted dispatch via commercial radio around 10;000 ft to inform them of our situation. We taxied to the gate and deplaned. Passengers were in good spirits as they deplaned and no injuries were noted. Once on the ground; movement of the outflow valve was still restricted even with no air load on the valve. Two logbook entries were made; one for the loss of cabin pressure and one for three personal oxygen bottles (pobs) that were used by the flight attendants. All aircraft emergency equipment appeared to operate normally. The flight attendants assisted a special needs passenger and an infant with the donning of their oxygen masks. After the flight deplaned; our flight attendants went above and beyond the duty to assist a special needs teen to make accommodations to catch another flight later that night. Prior to our crew deplaning; we had a formal debrief as a crew in its entirety about the event. This aircraft had been recently painted. In august 2014; this aircraft had a similar problem with contract pilots. They returned to [their departure airport] when they were unable to pressurize. Ensure outflow valves (scoop) are painted properly; or not painted over at all.

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Original NASA ASRS Text

Title: A Captain reports he and the First Officer were unable to control the cabin rate of climb in an MD-80 that was approaching 10;000 FT at FL330. Efforts to move the Outflow Valve to the closed position using Manual Control Wheel were unsuccessful. Emergency declared. Rapid Depressurization and Descent performed. Excessive paint around the Outflow Nozzle (Sugar Scoop) was a contributing factor.

Narrative: About one hour 15-minutes into the flight at FL330; the Cabin Pressurization Controller began acting erratically. This was also accompanied with rapid pressure fluctuations that could be felt in the ears. The cabin ALT began climbing at an approximate rate of 1;500 FPM initially and later would increase further. We initiated QRH 21.X -'Cabin Pressure off Schedule or Erratic'. We selected the Pressurization Controller to STBY with no change that corrected the problem. The Outflow Valve Control Wheel appeared to be sticking or stuck and every time the Automatic System would recoil; the outflow valve would stick in a position more 'open' than the last. We selected the System to Manual and manually attempted to move (muscle) the Control Wheel forward to close the outflow valve. We were unsuccessful in being able to position the outflow valve in a 'closed' position sufficient to maintain cabin pressure. While attempting to manually close the outflow valve; we were approaching a cabin ALT of 9;000-10;000 FT prior to reaching Step # 3 on QRH 21.X; the 'Cabin ALT' Master Warning illuminated. We elected to stop performing 21.X and performed the memory items required for QRH 21.Y 'Cabin ALT/ Rapid Depressurization' followed by 21.Z 'Rapid Descent'. The First Officer was pilot flying with the autopilot 'On'. We donned our oxygen masks and established communication with ATC. I took control of the aircraft; declared an emergency; and initiated an Emergency Descent. During the descent; while running QRH 21.Z; we ensured the Blue 'Cabin O2 On' light was illuminated. We elected to turn towards and divert into ZZZ. When approaching 10;000 FT; we removed our oxygen masks and I made a call to the flight attendants and briefed them followed by a clear Public Announcement (PA) to the passengers stating what had happened; that we were at a safe altitude to remove oxygen masks; and that we were diverting to ZZZ. Once below 10;000 FT; all operations/communications were normal; as well as a normal landing. We contacted Dispatch via Commercial Radio around 10;000 FT to inform them of our situation. We taxied to the gate and deplaned. Passengers were in good spirits as they deplaned and no injuries were noted. Once on the ground; movement of the outflow valve was still restricted even with no air load on the valve. Two logbook entries were made; One for the loss of cabin pressure and one for three Personal Oxygen Bottles (POBs) that were used by the flight attendants. All aircraft Emergency Equipment appeared to operate normally. The flight attendants assisted a special needs passenger and an infant with the donning of their oxygen masks. After the flight deplaned; our flight attendants went above and beyond the duty to assist a special needs teen to make accommodations to catch another flight later that night. Prior to our crew deplaning; we had a formal debrief as a crew in its entirety about the event. This aircraft had been recently painted. In August 2014; this aircraft had a similar problem with Contract pilots. They returned to [their departure airport] when they were unable to pressurize. Ensure outflow valves (scoop) are painted properly; or NOT painted over at all.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.