Date: 201411
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Climb
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pressurization Control System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Second reporter function: Pilot Flying; Captain
Anomaly: Aircraft Equipment Problem Critical
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport; General Declared Emergency
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Upon climbing through approximately 33;000 ft; I noticed a pressure bump with a large pressure change. It was obvious something had occurred to the pressurization system. I looked up and noticed the cabin VSI showing a rapid climb. As I was placing the mask on my face; the cabin altitude alert began to sound and the Captain placed his mask on as well; and complied with the immediate action Items for a loss of cabin pressurization. It was apparent the cabin was not controllable per the QRH; and we initiated a descent. The cabin masks had already deployed by the time I was instructed to deploy them per the QRH. The highest cabin altitude noticed was approximately 17-20;000 ft. It occurred within a very short time frame of likely less than 1 minute from the sound of the intermittent horn to the max cabin altitude. Upon descent there were no unusual sounds and at arrival at 10;000 ft altitude after conferring with the flight attendants; everything appeared normal in the rear of the aircraft as well; other than the cold air that entered the aircraft. MMO/VMO was exceeded slightly as we were in the emergency descent. (As a side note; the pressurization controller in manual controlled the cabin altitude extremely erratically when at a lower altitude. A simple touch of the valve switch would change the cabin altitude VSI by +/-1500-2000 fpm and would not affect a change at all at higher altitudes.) We proceeded to ZZZ as an emergency aircraft and had medics standing by as a precaution to anyone needing medical attention. At the request of the Captain; all the flight crew were all evaluated by the medics and released.


Second narrative: During climb to cruise altitude; we were on a vector for traffic; cleared to FL390. Between FL330 and FL340; the cabin altitude started to climb rapidly. Both pilots donned crew oxygen. The first officer declared an emergency and requested descent. I was the flying pilot; so I initiated the descent. I disconnected the autopilot; pulled the throttles to idle; opened the speed brake to the flight detent; and accelerated to max forward airspeed. The FO read and executed the QRH for Emergency Descent. ARTCC initially cleared us to a lower altitude. We stayed on the assigned vector; until the FO coordinated for 9;000 feet altitude and initial vector direct ZZZ. I pulled up the PA to listen to the Flight Attendants. I heard the Lead Flight Attendant announce the aircraft was experiencing a loss of pressure; was executing commands and instructing the passengers to don their oxygen masks. Satisfied that the Lead FA was doing her job nicely; I delayed making a PA to the passengers until later in the descent. At approximately 12;000 altitude; and heading direct to ZZZ; I looked up at the cabin altitude controller and saw that the cabin altitude was around 2;000 feet; we took our masks off. It was at this time I called the FA on the cabin interphone to assess what was happening in the cabin. The Lead FA said everyone was doing well; and passengers were calm. I asked her about any medical conditions. She said only complaints about ears. I decided at that point that ZZZ was the best location; and pressed forward to ZZZ. The FO concurred with that plan. I briefed the Lead FA on the loss of control of the cabin pressure and told her we would go back to ZZZ. I did not direct her to prepare the cabin for evacuation. I told her the plan would be that we would land; taxi to the gate; and have medical responders meet the aircraft at the gate. She was happy with that plan. I then made a passenger PA that it we were at a safe altitude; and they could stop breathing from their masks. I assured the passengers that there would be a normal landing at ZZZ; with emergency vehicles meeting the aircraft upon arrival for anyone that wanted to get checked out. I later confirmed with the Lead FA; and only one person wanted to have their blood pressure checked; but no other passenger complaints. The FO coordinated for a gate. We landed via visual approach and taxied to the gate. Unfortunately; vehicles and cargo handling equipment were in the way; so ground crews cleared the parking pad; and we taxied to the gate after a short delay; maybe 5 minutes. I wrote several discrepancies in the aircraft logbook: First that the cabin lost pressure; and climbed to and peaked at approximately 20;000 feet. Secondly; passenger oxygen failed to automatically deploy at seats 7ABC; and seats 8DEF. Additionally; O2 deployment at the Flight Attendant stations 1L and 2L were problematic. Door 2L jump seat; the FA O2 canisters fell to the floor during deployment; i.e.; they were not attached to the aircraft and were quite useless; so they had to retrieve other O2 supply from the other door. I also wrote up an overweight landing. Landing gross weight was approximately 130;000 lb. After the event was over; I realized I did not write up an aircraft over speed; so I returned to the aircraft to make that write up; but I could not locate the aircraft logbook or the maintenance technician that met us at the gate; so I called the Duty Manager and reported that during the descent; in clean configuration; with speed brake deployed; I got the clacker and 5 knots over speed; and asked him to report it to Maintenance Control. Additionally; the cabin controller was very sensitive to inputs and erratic in behavior; causing the cabin altitude to fluctuate during our recovery. The FO did an excellent job managing the cabin pressure from fluctuating wildly. Eventually; at about 3;000 feet aircraft altitude; the FO was finally able to open the outflow valve and depressurize the cabin fully;and we landed depressurized. The crew debriefed with EMT's as a precaution prior to departing the gate area.


Synopsis: B737 flight crew experiences a loss of cabin pressurization passing FL330 in the climb. The cabin altitude cannot be controlled and an emergency descent is initiated and a return to the departure airport is accomplished. The crew reported problems in the cabin with both passenger and Flight Attendant oxygen systems.