B737-800 failed to pressurize after takeoff. Flight returned to departure airport for maintenance.

Attributes
ACN	771136
Time
Date	200801
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : cvg.airport
State Reference	KY
Altitude	msl single value : 15000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zid.artcc tracon : cvg.tracon
Operator	common carrier : air carrier
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Flight Phase	climbout : initial climbout : takeoff
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : multi engine pilot : atp
Experience	flight time last 90 days : 173 flight time type : 311
ASRS Report	771136
Events
Anomaly	aircraft equipment problem : less severe
Independent Detector	other flight crewa
Resolutory Action	flight crew : diverted to another airport flight crew : landed as precaution flight crew : overcame equipment problem
Consequence	other
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Aircraft

Narrative:

While performing the after takeoff checklist; the air conditioning panel and pressurization were set and I saw the cabin altitude climbing; some differential pressure with a cabin altitude lower than our current altitude. During our departure; approximately 12000 ft; the cabin altitude warning horn began to sound. A quick scan of the overhead panel showed a cabin altitude about 11000 ft. I noticed that the bleed pressure was low; and I cycled the bleed switches. This still gave us a low duct pressure. Next I looked at the pack switches and touched them to make sure they were in the automatic position. The captain confirmed that the bleed switches were on and the pack switches in the automatic position. Next I moved over to the differential pressure and cabin altitude gauge and saw that the cabin altitude was still rising. I asked the captain if he wanted to start the APU or descend to a lower altitude. We did not get pressurization and we noticed that the outflow valve was open. During this we applied the memory items for the cabin altitude warning or rapid depressurization. The captain decided not to start the APU but he agreed to descend to a lower altitude. We coordination with ATC and started our descent. Next; I picked up the QRH and looked up the cabin altitude warning and rapid depressurization checklist. I used the communication panel with the speaker on so the captain could hear what I was doing. I followed the checklist after selecting the pressurization mode selector to manual and closing the outflow valve. The cabin altitude started to descend and the cabin was pressurized. The manual pressurization caused some pressurization change which apparently discomforted some passenger; and the captain decided to return to cvg. After the return to cvg; the captain asked the passenger to remain seated to have them checked out by the paramedics. After debriefing maintenance and the chief pilot; we waited until the aircraft was going to be fixed. A couple of hours later; I spoke with one of the maintenance employees who told me they found a faulty stall management computer which gave inputs to the FMS and the altitude controllers.

Original NASA ASRS Text

Title: B737-800 FAILED TO PRESSURIZE AFTER TKOF. FLT RETURNED TO DEP ARPT FOR MAINT.

Narrative: WHILE PERFORMING THE AFTER TKOF CHKLIST; THE AIR CONDITIONING PANEL AND PRESSURIZATION WERE SET AND I SAW THE CABIN ALT CLBING; SOME DIFFERENTIAL PRESSURE WITH A CABIN ALT LOWER THAN OUR CURRENT ALT. DURING OUR DEP; APPROX 12000 FT; THE CABIN ALT WARNING HORN BEGAN TO SOUND. A QUICK SCAN OF THE OVERHEAD PANEL SHOWED A CABIN ALT ABOUT 11000 FT. I NOTICED THAT THE BLEED PRESSURE WAS LOW; AND I CYCLED THE BLEED SWITCHES. THIS STILL GAVE US A LOW DUCT PRESSURE. NEXT I LOOKED AT THE PACK SWITCHES AND TOUCHED THEM TO MAKE SURE THEY WERE IN THE AUTO POS. THE CAPT CONFIRMED THAT THE BLEED SWITCHES WERE ON AND THE PACK SWITCHES IN THE AUTO POS. NEXT I MOVED OVER TO THE DIFFERENTIAL PRESSURE AND CABIN ALT GAUGE AND SAW THAT THE CABIN ALT WAS STILL RISING. I ASKED THE CAPT IF HE WANTED TO START THE APU OR DSND TO A LOWER ALT. WE DID NOT GET PRESSURIZATION AND WE NOTICED THAT THE OUTFLOW VALVE WAS OPEN. DURING THIS WE APPLIED THE MEMORY ITEMS FOR THE CABIN ALT WARNING OR RAPID DEPRESSURIZATION. THE CAPT DECIDED NOT TO START THE APU BUT HE AGREED TO DSND TO A LOWER ALT. WE COORD WITH ATC AND STARTED OUR DSCNT. NEXT; I PICKED UP THE QRH AND LOOKED UP THE CABIN ALT WARNING AND RAPID DEPRESSURIZATION CHKLIST. I USED THE COM PANEL WITH THE SPEAKER ON SO THE CAPT COULD HEAR WHAT I WAS DOING. I FOLLOWED THE CHKLIST AFTER SELECTING THE PRESSURIZATION MODE SELECTOR TO MANUAL AND CLOSING THE OUTFLOW VALVE. THE CABIN ALT STARTED TO DSND AND THE CABIN WAS PRESSURIZED. THE MANUAL PRESSURIZATION CAUSED SOME PRESSURIZATION CHANGE WHICH APPARENTLY DISCOMFORTED SOME PAX; AND THE CAPT DECIDED TO RETURN TO CVG. AFTER THE RETURN TO CVG; THE CAPT ASKED THE PAX TO REMAIN SEATED TO HAVE THEM CHKED OUT BY THE PARAMEDICS. AFTER DEBRIEFING MAINT AND THE CHIEF PLT; WE WAITED UNTIL THE ACFT WAS GOING TO BE FIXED. A COUPLE OF HRS LATER; I SPOKE WITH ONE OF THE MAINT EMPLOYEES WHO TOLD ME THEY FOUND A FAULTY STALL MGMNT COMPUTER WHICH GAVE INPUTS TO THE FMS AND THE ALT CONTROLLERS.

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.