Narrative:

During cruise flight I heard a pop sound; flight level 310. I noticed the cabin rate of climb indicating 1;000 ft. [Per] minute climb. I closed off the cargo heat outflow; cabin rate decreased to 500 ft. [Per] minute. Unable to control the cabin rate using the manual cabin pressure control; the captain called ATC to request a descent and called for the appropriate checklist as the cabin altitude reached 7;000 ft. Review of the failure of automatic pneumatic pressurization controller checklist didn't help our situation. We leveled off at flight level 270; cabin kept climbing; at this time we donned our oxygen masks. Captain requested another descent to flight level 230 as the cabin reached 11;000 ft. Another request to descend was made; we leveled off at flight level 210 and the cabin decreased to 10;000 ft. The cabin settled at 10;000 ft. For a short while; but then it started to climb again. We descended to flight level 190 where the cabin finally settled at 10;000 ft. A short time later it was time for us to descend into ZZZ. We had to keep engines 1 and 3 at a higher setting in order to maintain a level cabin from climbing above 10;000 ft. Throughout our situation; ATC was very accommodating. We had an uneventful landing into ZZZ. The mechanic inspection revealed that the water separator coalescer unit clamp had become undone and the unit separated from its ducting allowing the air to leak out.

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Original NASA ASRS Text

Title: Flight Engineer reported a progressive loss of cabin pressure as the result of a water separator unit clamp coming undone.

Narrative: During cruise flight I heard a pop sound; flight level 310. I noticed the cabin rate of climb indicating 1;000 ft. [per] minute climb. I closed off the cargo heat outflow; cabin rate decreased to 500 ft. [per] minute. Unable to control the cabin rate using the manual cabin pressure control; the Captain called ATC to request a descent and called for the appropriate checklist as the cabin altitude reached 7;000 ft. Review of the Failure of Automatic Pneumatic Pressurization Controller checklist didn't help our situation. We leveled off at flight level 270; cabin kept climbing; at this time we donned our oxygen masks. Captain requested another descent to flight level 230 as the cabin reached 11;000 ft. Another request to descend was made; we leveled off at flight level 210 and the cabin decreased to 10;000 ft. The cabin settled at 10;000 ft. for a short while; but then it started to climb again. We descended to flight level 190 where the cabin finally settled at 10;000 ft. A short time later it was time for us to descend into ZZZ. We had to keep engines 1 and 3 at a higher setting in order to maintain a level cabin from climbing above 10;000 ft. Throughout our situation; ATC was very accommodating. We had an uneventful landing into ZZZ. The mechanic inspection revealed that the water separator coalescer unit clamp had become undone and the unit separated from its ducting allowing the air to leak out.

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.