Date: 201306
Local time of day: 0601-1200
Location: ZZZ.ARTCC, US
Flight conditions: Mixed
Weather elements / visibility: 8
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pressurization System
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural MEL / CDL
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport; General Maintenance Action
Contributing factors: Human Factors; Procedure; Aircraft
Primary problem: Ambiguous

Narrative: We operated [a] flight under MEL 36-5-01 (Engine Bleed Shutoff Valves Inoperative) for the left engine.  Per the MEL; we operated the right pack utilizing the right engine bleed and the left pack with bleed air from the APU up to 17;000 feet.  At 17;000 feet; we turned off the APU bleed and right pack thereby operating the left pack using the right engine bleed.  We climbed to our cruising altitude of 22;000 feet and leveled off.  We notice the cabin was climbing and the duct pressure was registering approximately 10 PSI.  I requested an immediate descent from Center because of a pressurization issue.  They initially cleared us to 16;000 feet.  We began the descent and I told them we needed to descend down to 10;000 feet.  They cleared us to ten.  At approximately 20;000 feet; the Altitude Warning Horn sounded.  We accomplished the Immediate Action Items; donned our oxygen masks; and established communications.  I accomplished the Altitude Warning Horn/Abnormal Pressurization and Emergency Descent Checklists while the First Officer continued the descent.  Passing approximately 15;000 feet; the Pass Oxy On light illuminated and the oxygen masks dropped in the passenger compartment.  Per the Emergency Descent QRH procedures; I made a PA informing the Passengers of the descent and the deployment of the oxygen masks.  I told them to use the masks as instructed; but emphasized the situation was under control and we would have the cabin at an altitude where the masks were not required momentarily. After the checklists were completed and approaching 10;000 feet; I realized the APU was still available for pressurization.  I reconfigured the air conditioning panel as instructed in MEL 36-5-01 for operations below 17;000 feet.  I elected not to declare an emergency. We had already been cleared to descend to 10;000 feet before the Altitude Warning Horn sounded.  Reaching 10;000 feet; the situation was stabilized.  I requested a return from ATC.  They cleared us on the arrival.  We then referenced the items for diversion on the Normal Checklist card.  We informed Dispatch of the depressurization; mask deployment; and return.  We coordinated with Operations for a gate and Customer Service requirements.  I made several more PAs advising the passengers of our return assuring them that the situation was under control; oxygen masks were no longer required; and the odor caused by the oxygen generators was perfectly normal.  The return was uneventful.  The flight attendants did an excellent job of taking care of the passengers and maintaining calm in the cabin in a very stressful situation.  I said goodbye to the passengers as they exited the aircraft and they were all very complimentary of the calm professionalism of the flight attendants.  I used the IPad EFB during this incident and it was extremely easy to set up the return and have access the appropriate charts.   As an airline; we seem much more willing to MEL items than we were in the past.  The altitude restriction incorporated in the MEL made this a manageable depressurization event.  Dispatch explained there was no airplane available to put on this flight yet; when we returned; there was an airplane at the next gate within approximately 30 minutes.  The MEL is a valuable tool; but it must always be considered preferable to fix airplanes than to MEL them and send them on their way.


Second narrative: Flight dispatched with Engine Bleed closed. MEL called for using APU Air until 17;000 feet then operating opposite pack with isol open. We failed to notice the problem until after reconfiguring the air panel (15;000 feet to 17;000 feet). Level at FL 220; we saw that the cabin was going to climb through 10;000 feet. We got clearance to 10;000 feet and began a descent and; soon after; got the Cabin Altitude Warning; donned masks; established communications; ran QRH; and requested to return. The Captain handled the ATC communications and communication with the crew while I continued to fly the plane. The passenger O2 masks deployed as advertised as the cabin caught up with altitude. Returning to the departure airport; we discovered Maintenance had performed MEL procedures on the wrong (good) engine. There were some clues that could have prevented the issue at several points along the way. Paying more attention to where Maintenance was (we both had commented at the fact that Maintenance had not been out; raised the cowling; etc.) and leveling off when first recognizing something wasn't quite right.


Synopsis: B737-700 flight crew reports loss of cabin pressurization at FL220 after being dispatched with the left engine bleed inoperative and the APU powering the left pack to 17;000 feet.  When the APU bleed air is turned off the problems begin.  Crew returns to the departure airport and discovers that Maintenance performed the MEL procedure on the wrong engine.