Narrative:

After a normal takeoff and initial climb out; the after takeoff check was called for and accomplished. Climbing through 15000 ft; the 'master caution' light came on with an 'air cond' annunciator. Looked up and saw the 'automatic fail' and the green 'stby' light illuminated. Noticing that the cabin was now over 7000 ft and climbing; signifying that the standby system was not controling the cabin; we called ATC and told them that we were having pressurization problems and requested a descent to 10000 ft and a turn back towards anc and away from high terrain. I then got out the automatic fail checklist and proceeded to accomplish it. The copilot was controling the airplane and getting set up for a return to anc. The highest the cabin climbed to was 10000 ft. Passenger oxygen mask did not deploy. An emergency was not declared. In following the steps to the automatic fail checklist I got to step 7; outflow valve switch -- as required and noticed that I had some control of the cabin; but I was not comfortable that I had full control because the cabin was not descending slower than the airplane was descending. I relayed to dispatch that I would rather return to anc. An uneventful approach and landing was done at anc. There were no complaints or difficulties reported from flight attendants or passenger. After a couple of hours troubleshooting the automatic and standby pressurization systems and finding nothing wrong with either system controllers; the anc mechanics pressurized the airplane on the ground using one engine. The mechanic came to us and reported that when the psid was past 2.0; pressurization was lost through the ground air conditioning ducting -- evidentially a 'flapper' type valve in the ducting opened when the psid reached 2.0. He said; 'the door just shot right out.'

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Original NASA ASRS Text

Title: A B737-400'S OVER PRESSURIZATION RELIEF VALVE OPENED AT 2.0 PSID; CAUSING THE ACFT TO NOT PRESSURIZE PROPERLY. FLT RETURNED TO LAND.

Narrative: AFTER A NORMAL TKOF AND INITIAL CLB OUT; THE AFTER TKOF CHK WAS CALLED FOR AND ACCOMPLISHED. CLBING THROUGH 15000 FT; THE 'MASTER CAUTION' LIGHT CAME ON WITH AN 'AIR COND' ANNUNCIATOR. LOOKED UP AND SAW THE 'AUTO FAIL' AND THE GREEN 'STBY' LIGHT ILLUMINATED. NOTICING THAT THE CABIN WAS NOW OVER 7000 FT AND CLBING; SIGNIFYING THAT THE STANDBY SYSTEM WAS NOT CTLING THE CABIN; WE CALLED ATC AND TOLD THEM THAT WE WERE HAVING PRESSURIZATION PROBLEMS AND REQUESTED A DSCNT TO 10000 FT AND A TURN BACK TOWARDS ANC AND AWAY FROM HIGH TERRAIN. I THEN GOT OUT THE AUTO FAIL CHKLIST AND PROCEEDED TO ACCOMPLISH IT. THE COPLT WAS CTLING THE AIRPLANE AND GETTING SET UP FOR A RETURN TO ANC. THE HIGHEST THE CABIN CLBED TO WAS 10000 FT. PAX OXYGEN MASK DID NOT DEPLOY. AN EMER WAS NOT DECLARED. IN FOLLOWING THE STEPS TO THE AUTO FAIL CHKLIST I GOT TO STEP 7; OUTFLOW VALVE SWITCH -- AS REQUIRED AND NOTICED THAT I HAD SOME CTL OF THE CABIN; BUT I WAS NOT COMFORTABLE THAT I HAD FULL CTL BECAUSE THE CABIN WAS NOT DSNDING SLOWER THAN THE AIRPLANE WAS DSNDING. I RELAYED TO DISPATCH THAT I WOULD RATHER RETURN TO ANC. AN UNEVENTFUL APCH AND LNDG WAS DONE AT ANC. THERE WERE NO COMPLAINTS OR DIFFICULTIES RPTED FROM FLT ATTENDANTS OR PAX. AFTER A COUPLE OF HOURS TROUBLESHOOTING THE AUTO AND STANDBY PRESSURIZATION SYSTEMS AND FINDING NOTHING WRONG WITH EITHER SYSTEM CTLRS; THE ANC MECHANICS PRESSURIZED THE AIRPLANE ON THE GND USING ONE ENG. THE MECHANIC CAME TO US AND RPTED THAT WHEN THE PSID WAS PAST 2.0; PRESSURIZATION WAS LOST THROUGH THE GND AIR CONDITIONING DUCTING -- EVIDENTIALLY A 'FLAPPER' TYPE VALVE IN THE DUCTING OPENED WHEN THE PSID REACHED 2.0. HE SAID; 'THE DOOR JUST SHOT RIGHT OUT.'

Data retrieved from NASA's ASRS site as of January 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.