Date: 201206
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: Mixed
Weather elements / visibility: 10
Light: Daylight
Aircraft: B737-300
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Climb
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: FMS/FMC
Component problem: Improperly Operated; Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: First Officer
Anomaly: Aircraft Equipment Problem Critical
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Departure Airport; General Flight Cancelled / Delayed; General Maintenance Action; General Declared Emergency
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: Just after takeoff; got an IRS/POS Disagree light. At 400 FT; called for LNAV and FD bars disappeared. LNAV info looked incorrect so we notified ATC we were unable to utilize the RNAV departure. ATC gave us vectors toward TRM. Out of 10;000 FT I called for the Climb Checklist and noted nothing abnormal. As we reached FL230; the intermittent Cabin Altitude Warning horn sounded. Preformed memory items; O2 mask on 100%; and communication established. Called for checklist; notified ATC; and declared an emergency. Continued to run the checklist; and not seeing an immediate cabin change; we asked for and initiated an emergency descent to 7;000 FT. We called the Flight Attendants; had them secure the cabin and asked ATC for a clearance to return to our departure airport. We then got the cabin pressure under control and informed the Passengers we were returning. ATC headed us back to LAX; and we then asked for holding or vector to slow things down [while we checked with Dispatch and Maintenance on overweight landing data due to being 7000 to 8000 pounds over maximum structural landing weight. We landed uneventfully; filled out logbook; and contacted Dispatch; Maintenance; and the Chief Pilot on duty.With the things that were happening; it was an early indicator that the aircraft was not sure it was in the air. Closer and more frequent checks of the cabin pressure could have been done.


Second narrative: It was a high workload environment due to ATC clearances; i.e. headings; speeds; altitudes. We completed the Climb Checklist quickly where neither one of us noticed anything unusual. I believe the cabin was climbing at 500 FPM. The Captain then noticed that the TCAS had failed; and I remembered that it had tested on the ground. Then I noticed that the GPWS had failed also. Now we were trying to figure out why all this stuff was failing. The cabin was at 10;000 FT when we started the descent and never got above 12;000 FT. No pressurization switches were in an abnormal position and we were able to control the cabin in standby DC. At some point while this coordination was going on; the TCAS/GPWS starting working and normal FMC functions returned ([it had] initially shown us 540 miles away from the FAF).


Synopsis: When confronted with multiple system anomalies--including an FMS position error and the failure of the pressurization system--shortly after takeoff; the flight crew of a B737-300 declared an emergency; initiated an emergency descent from FL230; and returned to their departure airport.