Narrative:

Airplane thought it was still on the ground (air/ground sensing switch). On climb out; initially got 'check position' on FMC; soon followed by numerous issues such as TCAS fail; no autothrottles; no LNAV; no VNAV; etc; including loss of pressurization. Passing FL180; got altitude warning horn with pressure altitude reading (and holding) at 10000 ft. Captain leveled off; and began a slow descent while I coordinated with ATC. We entered present position holding while attempting to run appropriate checklist. Completed loss of cabin pressurization checklist; coordinated with maintenance; ATC; flight attendants; dispatch; etc. Returned to ZZZ with uneventful; though overweight (119000 pounds) landing. Landing was smooth with minimum vertical speed. Thrust reversers deployed normally; as did speed brakes. Long rollout with very minimum braking. Returned to gate. Callback conversation with reporter revealed the following information: the reporter stated that when the cabin altitude warning horn sounded the crew identified the horn immediately and leveled off. They switched the cabin altitude controller to stby but that did not solve the problem. The reporter does not believe the cabin pressurized at all and that the ground/air sense switch failed on takeoff. An emergency was declared and they headed back to the departure airport. The secondary indications did confuse them and were not connected to a ground air sense switch failure until they were on the ground. The check list for loss of pressurization did not address the flight station lights that were on as well as the loss of the automatic throttles and VNAV.

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Original NASA ASRS Text

Title: A B737-300'S CABIN ALT WARNING HORN SOUNDED CLBING THROUGH FL180. WITH OTHER NON RELATED FAILURES THE CREW DECLARED AN EMER AND RETURNED TO LAND.

Narrative: AIRPLANE THOUGHT IT WAS STILL ON THE GND (AIR/GND SENSING SWITCH). ON CLBOUT; INITIALLY GOT 'CHK POS' ON FMC; SOON FOLLOWED BY NUMEROUS ISSUES SUCH AS TCAS FAIL; NO AUTOTHROTTLES; NO LNAV; NO VNAV; ETC; INCLUDING LOSS OF PRESSURIZATION. PASSING FL180; GOT ALT WARNING HORN WITH PRESSURE ALT READING (AND HOLDING) AT 10000 FT. CAPT LEVELED OFF; AND BEGAN A SLOW DSCNT WHILE I COORDINATED WITH ATC. WE ENTERED PRESENT POS HOLDING WHILE ATTEMPTING TO RUN APPROPRIATE CHKLIST. COMPLETED LOSS OF CABIN PRESSURIZATION CHKLIST; COORDINATED WITH MAINT; ATC; FLT ATTENDANTS; DISPATCH; ETC. RETURNED TO ZZZ WITH UNEVENTFUL; THOUGH OVERWT (119000 LBS) LNDG. LNDG WAS SMOOTH WITH MINIMUM VERT SPD. THRUST REVERSERS DEPLOYED NORMALLY; AS DID SPD BRAKES. LONG ROLLOUT WITH VERY MINIMUM BRAKING. RETURNED TO GATE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER STATED THAT WHEN THE CABIN ALT WARNING HORN SOUNDED THE CREW IDENTIFIED THE HORN IMMEDIATELY AND LEVELED OFF. THEY SWITCHED THE CABIN ALT CONTROLLER TO STBY BUT THAT DID NOT SOLVE THE PROBLEM. THE REPORTER DOES NOT BELIEVE THE CABIN PRESSURIZED AT ALL AND THAT THE GND/AIR SENSE SWITCH FAILED ON TKOF. AN EMERGENCY WAS DECLARED AND THEY HEADED BACK TO THE DEPARTURE ARPT. THE SECONDARY INDICATIONS DID CONFUSE THEM AND WERE NOT CONNECTED TO A GROUND AIR SENSE SWITCH FAILURE UNTIL THEY WERE ON THE GROUND. THE CHECK LIST FOR LOSS OF PRESSURIZATION DID NOT ADDRESS THE FLT STATION LIGHTS THAT WERE ON AS WELL AS THE LOSS OF THE AUTO THROTTLES AND VNAV.

Data retrieved from NASA's ASRS site as of May 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.