Date: 201605
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: Mixed
Weather elements / visibility: 6
Light: Daylight
Aircraft: B737 Undifferentiated or Other Model
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 Control System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Departure Airport; Flight Crew Landed As Precaution; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Climbing thru FL230 we heard the Cabin Altitude Warning horn and observed the red Cabin Altitude Warning lights. Cabin pressure was climbing through 10;000 ft on the overhead panel gauge. We immediately donned O2 masks and established Crew communication with each other. I notified ARTCC that we were initiating a descent. We were cleared to 12;000 ft. I observed that both Engine Bleed switches were ON and both PACK switches were in AUTO. I selected MAN AC on the analog pressurization controller; and held the Outflow Valve switch to CLOSE. The FLT/GRD switch was in FLT. The cabin altitude was now 11;000 ft and climbing at about 1500 fpm. I manually selected the PASS OXYGEN switch to ON. Simultaneously; my F/O had begun a descent toward 12;000 ft. During descent; we accomplished the QRC Procedure for Rapid Depressurization; followed by the QRH Procedure for Unscheduled Pressurization Change. We never had an AUTO FAIL light on the pressurization panel. Going thru the QRH; the STANDBY mode did not seem to function properly; and allowed cabin pressure to rise. Cabin altitude was decreased to about 2000 ft by manual control. Maximum cabin altitude experienced was about 11;000 ft. The cabin altitude rate of change was smooth except for the bump we caused by closing the Outflow Valve. During descent; I made a PA to notify the F/As and passengers that we had lost pressurization. Shortly thereafter; I made another PA to notify everyone that cabin [altitude] was then below 10;000 ft; and O2 masks were no longer necessary; and that we would return to [departure airport]; expecting a normal arrival and landing. Because of fuel weight; we held for about 20 minutes using flight spoilers; and lowered landing gear early on arrival. (Landing weight was 114;700 pounds; max was 115;000 pounds for this aircraft.) During this flight; a number of other system malfunctions occurred. On initial climbout; about 15;000 ft; the HGS control panel FAULT light illuminated and we lost HUD symbology. During holding and arrival we experienced single channel MACH TRIM FAIL; single channel SPEED TRIM FAIL; the Captain's airspeed indicator flag appeared; and the MCP Speed Window would not control the Captain's Speed bug (although the actual airspeed indicated by the needle matched the F/O's indicator); the Captain side CMPTR Flag appeared; and lost the Captain side FD; and both side FMA A/P and A/T warning lights began to flicker on and off. (The F/O's autopilot stayed on and functioned normally.) During holding; we attempted to use AUTO and STBY pressurization modes with the Cabin Altitude set to 200 ft (field elevation minus 200 ft.) In both modes the cabin pressure slowly began to rise above what we expected to see. The remainder of the flight was flown in MAN DC for smoother control of the outflow valve. We depressurized to zero below 2000 ft on approach. The approach; landing and taxi to gate were uneventful. Passengers and F/As were kept informed often. No one seemed stressed during deplaning. Contract Maintenance said the E&E compartments smelled strongly of lav fluid; but they observed no actual leaks; nor fluid. Each system malfunction was recorded individually in the aircraft logbook.




Synopsis: B737 Captain reported returning to the departure airport following loss of cabin pressure. Several other system anomalies were noted; possibly related to lavatory fluid leaking into the Electrical and Engineering Compartment.