B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 156
Workload; Human-Machine Interface; Time Pressure; Troubleshooting
Aircraft Equipment Problem Critical
Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Flight Crew Diverted; Flight Crew Landed in Emergency Condition
Aircraft
It was my leg and we were at cruise flight at 40000 feet. We got an autofail light on the pressurization panel. I immediately grabbed the QRH to reference the emergency procedure. We started running the checklist. We had an autofail light with an uncontrollable cabin altitude. We then asked ATC for a lower altitude; but were not allowed to descend. The QRH directed us to go to alternate on the pressurization panel. We then selected alternate (ALTN). At this point the autofail light was still illuminated and the cabin altitude was uncontrollable. The QRH then says to go to manual mode and to manually open and close the valve as needed to control the cabin altitude. The Captain then began controlling the cabin manually with the toggle selector. He pressed the manual switch slightly and I started to see the differential pressure climb into the caution range and moving toward the max differential limit. I then heard the Captain say oxygen masks on and 100 percent. After I donned the mask I looked up and saw that we had exceeded max differential pressure. We established communication and checked that the seatbelt sign was on. The Captain kept running the checklist and I notified ATC that we needed a lower altitude immediately. ATC allowed us to descend to 36000 feet initially and then 32000 feet. I initiated an emergency descent and I continued to ask for lower altitudes and after consulting with the Captain I [notified ATC] for a pressurization problem. I asked for 10;000 feet and asked to divert. Meanwhile the Captain worked through the autofail checklist and had moved to the rapid depressurization and emergency descent checklist. The Captain was also communicating to the flight attendants; passengers and was attempting to contact Dispatch. After getting things stabilized the Captain and I came back together as a team to communicate what had been completed by each of us; so that we both could gain situational awareness of what had transpired. He told me that he had completed the checklist down to the deferred items and I told him that I had the airplane set up to land and was ready to brief him for the landing whenever he was caught up. We then briefed and continued uneventfully. After landing the mechanic verified that the primary and alternate pressurization controller were not working and he was suspicious that the manual mode wasn't working correctly. We did not receive the final diagnosis before leaving the aircraft. The Captain and I debriefed the flight and determined the following. Communication with the masks on made it challenging; but we were able to work through it. We did a good job of dividing duties to make sure that the airplane was being flown; checklists were being completed and situational awareness remained high. I would attribute this to our CRM training program.
Enroute at 40000 feet the First Officer called our attention to an MCP overhead annunciation; caused by a fault in the pressurization system. The PRESS panel had AUTO FAIL and ALTN lights illuminated. The First Officer reached for the QRH in order for us to take an initial look at the accompanying procedure. Initially the First Officer began to go through the steps since he sat closer to the panel. At this point both of us noticed that the cabin was descending and that the DIFF PRESS was increasing into the yellow arc on the indicator. At this point I took over the QRH procedure and he resumed as pilot flying. He also assumed ATC communication responsibilities. I then asked him to request a lower altitude for us to perhaps make it easier to deal with the problem. Center responded with a negative due to traffic. Each step on the QRH procedure was followed in sequence. But at this point in the narrative I will only mention the steps that bear significance; or provide a turning point in the sequence of events. At this point the QRH procedure directed me to (step 5) select the mode to ALTN; which was done. The cabin continued as before with a slow descent and increasing cabin pressure. The AUTO FAIL light remained on and at this point the mode selector was placed in MAN (step 7). I then made one single short/snap toggle for the outflow valve (OV) to open to stop the descent and bring the diff press down. The result was a large (approx) 1000+fpm rise in cabin alt. It was even physiological as I felt the result of the rapid pressure change. Also the quick snap of the toggle switch appeared to produce a disproportionate movement of the outflow valve indicator needle. After a couple seconds I reversed the step and made a quick (single) snap toggle back to close. The cabin stabilized but now the diff pressure was in the red and climbing. Fearing an imminent and uncontrollable triggering of the positive pressure relief valve I called out for the First Officer  to don our masks and I went over to the Cabin depressurization QRC. We [notified] ATC after we began our descent via the QRC procedures; which included selecting PASS OXYGEN on. The First Officer was flying during the descent and did a good job of flying the profile and complying with ATC off vectors as well as adjusting speed for the turbulence and maintaining safe distance from some convective activity we needed to clear. While I struggled to get my glasses back on and readjusted enough to resume the checklist. It also took a few seconds to reestablish communications as his ACP (Audio Control Panel) got bumped out of the intercom position during the flurry of activity. At this point we [notified] ATC. Several attempts were made via service interphone by the cabin crew to contact the cockpit which were answered briefly with a 'stand by' but were apparently not understood by them due to mask noise. Center initially cleared us to 32000 feet; but then re-cleared us to 18000 feet then finally to 10000 feet; where we remained the rest of the flight. We were over water so terrain was no factor. Upon reaching 10000 feet we came off our o2 masks and resumed normal cockpit communications. I next contacted the cabin for an assessment. They stated that the passengers were all fine and that no injuries had been sustained. After a brief explanation of what happened I told them I'd get back to them in a minute. I then took a minute to consolidate; and make sure all the checklists had been complete and that ATC was fully caught up as well. I then attempted to contact Dispatch over ARINC but due to distance and altitude; was unable to establish a VHF connection. I proceeded to ACARS Dispatch to establish communications with them. We then discussed as a crew that we didn't have the fuel to continue to our destination at 10;000 feet. After coordinating with ATC and Dispatch we diverted. At this point the workload was reduce significantly and I made a PA to the passengers; explaining that our pressurization had malfunctioned and that the masks had been deployed as a precaution; and that we would not be able to proceed to our destination. Approach and landing were uneventful. After seeing the passengers off; the First Officer and I debriefed the event and flight as a whole. We both agreed that communication while on the masks made CRM more challenging; but we were able to overcome this to the extent necessary to make sure that we both were completing our tasks and maintaining overall situational awareness. The mechanic who met the aircraft and did the initial tests on the system made the statement that ' both controllers were failed.' He also mentioned that when he moved the outflow valve manually it was moving in a strange way. We noticed this as well.
A Flight Crew reported loss of pressurization in the cockpit requiring an emergency descent and diversion to the nearest suitable airport.