Date: 201811
Light: Daylight
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Cruise
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Overcame Equipment Problem; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Took Evasive Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Previous flight was uneventful. Preflight; briefing; and checklist were accomplished per guidance. Departure and climb were normal with a brief level off at 10;000 feet for traffic. Climb Checklist was accomplished and both of us looked at all items; including the First Officer (Pilot Monitoring) touching each item as we completed the checklist all items normal. Step climb to our filed altitude of FL370; then requested and received clearance to FL390 as the rides were reported better. After cruising level at FL390 for perhaps 10-15 minutes; the Cabin Altitude Warning horn sounded; accompanied with illumination of the Cabin Altitude light. I called for 'MASKS' and we donned our O2 masks immediately. I glanced at the cabin altitude indicator and it showed 10;000 feet. I verified the packs and bleeds were on. Packs were in AUTO and engine bleeds were ON. I was able to don my mask and establish COMM quickly and while the First Officer was working to get his COMM working I reached up and placed the pressurization mode selector to MAN and depressed the CLOSE position. Since the First Officer was having difficulty with communication/audio; I communicated to ATC that we were beginning an immediate descent due to a pressurization issue. I began a precautionary expedited descent to a lower altitude and was cleared to FL210. We ran the QRH for Cabin Altitude Warning. Immediately as the outflow valve closed; there was a large pressure bump as the cabin altitude decreased quickly. Indicated cabin altitude quickly descended below 10;000 feet and when leveled at FL210 per the QRH. We removed our masks and established normal COMM. I communicated with the flight attendants and advised the situation and told them I would call them back. I made a brief PA to advise of the pressurization issue and that the situation was under control. I sent ACARS to Dispatch to advise of our pressurization issue. Initially discussed possible divert to [an alternate] which was 80 NM but after discussion with Dispatch we all agreed that divert was not directed nor warranted. Flight plan fuel burn for continuance of the flight at FL210 to [destination] was confirmed and we continued to our planned destination. After a query from ATC when we advised we had the situation under control; I advised that we were discontinuing our '[priority] aircraft' status. I checked on flight attendants and made PA's every 15-20 minutes. Flight attendants reported customers were in good spirits and no one expressing significant anxiety. We requested and received early; gradual descent clearances to aid in cabin altitude management. QRH items were completed on final approach. Approach; landing; and taxi-in were uneventful. I greeted customers as they deplaned and no one expressed any significant concerns; mostly thanks.




Synopsis: B737NG Captain reported loss of cabin pressure at FL390. During descent; cabin altitude control was regained with use of manual backup system.