Narrative:

Climbing through about 28;000 feet; we experienced excessive cabin altitude. The cabin altitude indicated 9000 feet and was continuing to climb. I stopped climbing the aircraft; and the first officer and I both donned oxygen masks. I then asked her to advise ATC of the problem and request a lower altitude; which she did. We were then cleared to FL200 with a subsequent clearance to 15;000. I then initiated a rapid descent towards that altitude. In the meantime; the cabin altitude had climbed to a maximum of 10;300; but as we descended it turned around and began descending back below 10;000 again. The passenger oxygen masks did not deploy. I asked the first officer to run the cabin altitude checklist. Structural integrity was not in doubt as there was no vibration. We then requested an air return back to ZZZ and [advised ATC of] emergency and were given a clearance back towards ZZZ VOR then ZZZ. As we approached 15;000 the cabin altitude was now approximately 4500 and under control; but we requested to continue descent to 10;000 where the cabin altitude descended to about 3;500 feet. Once at this altitude; we were cleared to hold at present position about 80 miles southwest of ZZZ to dump fuel and advised ATC and recorded the aircraft position. It was also during this hold I finally had time to call dispatch and advise them of our situation. At the completion of the dump; recorded position again and later gave this to dispatch. It was also in this phase I coordinated with the flight attendants; gave them the test and advised the passengers of our situation. The dump time was about 23 minutes; at the completion of which; we requested a clearance towards ZZZ VOR and then flew the ILS approach to runway xy. The approach and landing were all normal.

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Original NASA ASRS Text

Title: B787 flight crew reported the loss of cabin pressurization control; resulting in dumping fuel and diverting to the nearest airport.

Narrative: Climbing through about 28;000 feet; we experienced excessive cabin altitude. The cabin altitude indicated 9000 feet and was continuing to climb. I stopped climbing the aircraft; and the First Officer and I both donned oxygen masks. I then asked her to advise ATC of the problem and request a lower altitude; which she did. We were then cleared to FL200 with a subsequent clearance to 15;000. I then initiated a rapid descent towards that altitude. In the meantime; the cabin altitude had climbed to a maximum of 10;300; but as we descended it turned around and began descending back below 10;000 again. The passenger oxygen masks did not deploy. I asked the FO to run the Cabin Altitude checklist. Structural integrity was not in doubt as there was no vibration. We then requested an air return back to ZZZ and [advised ATC of] emergency and were given a clearance back towards ZZZ VOR then ZZZ. As we approached 15;000 the cabin altitude was now approximately 4500 and under control; but we requested to continue descent to 10;000 where the cabin altitude descended to about 3;500 feet. Once at this altitude; we were cleared to hold at present position about 80 miles southwest of ZZZ to dump fuel and advised ATC and recorded the aircraft position. It was also during this hold I finally had time to call dispatch and advise them of our situation. At the completion of the dump; recorded position again and later gave this to dispatch. It was also in this phase I coordinated with the flight attendants; gave them the TEST and advised the passengers of our situation. The dump time was about 23 minutes; at the completion of which; we requested a clearance towards ZZZ VOR and then flew the ILS approach to runway XY. The approach and landing were all normal.

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.