Narrative:

Level at FL430 I heard a hum/wind noise. Checked with the copilot and they heard it too. No abnormal indications. Sound became a hissing noise. Discussed the noise and source and agreed it was coming from the door area. As a crew we discussed possible rain water in the door seal and reviewed the door seal abnormal checklist. Still no changes in pressurization and no indications but we both agreed a descent to a lower alt would be best. ATC cleared us to FL380 and all indications and pressurization remained steady and normal in the descent. While my first officer was trying to coordinate a lower altitude with ATC I saw a small climb on the cabin alt indicator but all other indications remained normal. I voiced my observation to my first officer and we agreed pressurization could become an issue. ATC coordinated a lower altitude and asked why the requested lower altitude. My first officer replied 'pressurization issue' with a thumbs up from me and we continued a normal rate of descent at normal speed. Still no cockpit indications other than a slight increase in cabin altitude. ATC gave us priority. Descending through FL300 a climb on the cabin alt gauge was now noticeable and the hissing sound still present. I elected to divert to ZZZ and tried to communicate this to our passengers but they couldn't hear and returned to reading. Noticing a continued climb in cabin alt I pointed at cabin alt gauge and together we put on our O2 masks and established communication in anticipation of a cabin altitude warning. ATC asked about an alternate and I elected to land at ZZZ 60 miles ahead. Descending through FL180 the cabin altitude waning illuminated indicating cabin altitude was above 10;000. Checked and the passengers was still relaxed and reading. Having already accomplished the emergency descent items we acknowledged the warning and continued to set up for a VFR landing at ZZZ. Normal landing. Emergency vehicles followed us to the FBO and checked on our safety once parked.

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Original NASA ASRS Text

Title: CE525 flight crew reported a failure of the pressurization system resulted in a gradual loss of cabin pressure and a successful divert.

Narrative: Level at FL430 I heard a hum/wind noise. Checked with the copilot and they heard it too. No abnormal indications. Sound became a hissing noise. Discussed the noise and source and agreed it was coming from the door area. As a crew we discussed possible rain water in the door seal and reviewed the Door Seal Abnormal Checklist. Still no changes in pressurization and no indications but we both agreed a descent to a lower alt would be best. ATC cleared us to FL380 and all indications and pressurization remained steady and normal in the descent. While my FO was trying to coordinate a lower altitude with ATC I saw a small climb on the cabin alt indicator but all other indications remained normal. I voiced my observation to my FO and we agreed pressurization could become an issue. ATC coordinated a lower altitude and asked why the requested lower altitude. My FO replied 'pressurization issue' with a thumbs up from me and we continued a normal rate of descent at normal speed. Still no cockpit indications other than a slight increase in cabin altitude. ATC gave us priority. Descending through FL300 a climb on the cabin alt gauge was now noticeable and the hissing sound still present. I elected to divert to ZZZ and tried to communicate this to our passengers but they couldn't hear and returned to reading. Noticing a continued climb in cabin alt I pointed at cabin alt gauge and together we put on our O2 masks and established communication in anticipation of a cabin altitude warning. ATC asked about an alternate and I elected to land at ZZZ 60 miles ahead. Descending through FL180 the cabin altitude waning illuminated indicating cabin altitude was above 10;000. Checked and the passengers was still relaxed and reading. Having already accomplished the emergency descent items we acknowledged the warning and continued to set up for a VFR landing at ZZZ. Normal landing. Emergency vehicles followed us to the FBO and checked on our safety once parked.

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.