Narrative:

The only explanation I can think of is that we experienced some icing on the flow sensor in the heat exchanger ram air duct since we were in and out of some cirrus clouds at FL310 but with an OAT of -40 degrees. There was no indication of a pack shutdown (normal pack pressures) unless this was one of those shutdowns that still shows normal pressure. Even so; this only shuts down 1 pack and it was obvious that the remaining pack was not going to maintain anywhere near the cabin altitude it should have. Southwest of rochester; mn; at cruising altitude of FL310; we asked and received clearance to FL290 to get out of some turbulence. Shortly after leveling at FL290; we noticed the 'flow light' on the pressurization panel was on and the cabin rate of climb indicator showed 500 FPM climb. Cabin altitude indicated 7000 ft; which was considerably higher than schedule for FL290; in fact it should have been descending. Turned on airfoil anti-ice and consulted QRH for flow light and then erratic cabin altitude and asked for a lower altitude. Donned oxygen masks and established communications; selected manual cabin altitude control and discovered the outflow valve was already fully closed or indicated as such. Nothing left to do but expedite the descent with ATC before the masks in the cabin deployed. At 14500 ft the cabin rate of climb spiked down (probably because the outflow valve was still closed) and I selected automatic pressurization. The cabin rate of descent returned to a normal 500 FPM descent. We asked for 15000 ft; took our oxygen off; informed the cabin crew what was going on and continued to mke for a normal descent and landing. The cabin altitude never got above about 9800 ft and the descent was not that abrupt so I did not tell the passenger anything out of the ordinary. I probably should have turned on the airfoil anti-ice sooner but I'm not entirely sure that is what happened. The air-conditioning system and pressurization on all the MD80's seem to be weak and inefficient. There needs to be more emphasis on ducts being connected and leaks corrected. Sometimes I think most of our pressurization is provided to the tail cone. We never find out how weak the air from a bad air cycle machine is until the other one fails.

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Original NASA ASRS Text

Title: AN MD80 EXPERIENCED A PRESSURIZATION ABNORMALITY THAT REQUIRED DONNING OXYGEN MASKS AND DESCENDING BEFORE REGAINING CONTROL OF THE SYSTEM.

Narrative: THE ONLY EXPLANATION I CAN THINK OF IS THAT WE EXPERIENCED SOME ICING ON THE FLOW SENSOR IN THE HEAT EXCHANGER RAM AIR DUCT SINCE WE WERE IN AND OUT OF SOME CIRRUS CLOUDS AT FL310 BUT WITH AN OAT OF -40 DEGS. THERE WAS NO INDICATION OF A PACK SHUTDOWN (NORMAL PACK PRESSURES) UNLESS THIS WAS ONE OF THOSE SHUTDOWNS THAT STILL SHOWS NORMAL PRESSURE. EVEN SO; THIS ONLY SHUTS DOWN 1 PACK AND IT WAS OBVIOUS THAT THE REMAINING PACK WAS NOT GOING TO MAINTAIN ANYWHERE NEAR THE CABIN ALT IT SHOULD HAVE. SW OF ROCHESTER; MN; AT CRUISING ALT OF FL310; WE ASKED AND RECEIVED CLRNC TO FL290 TO GET OUT OF SOME TURB. SHORTLY AFTER LEVELING AT FL290; WE NOTICED THE 'FLOW LIGHT' ON THE PRESSURIZATION PANEL WAS ON AND THE CABIN RATE OF CLB INDICATOR SHOWED 500 FPM CLB. CABIN ALT INDICATED 7000 FT; WHICH WAS CONSIDERABLY HIGHER THAN SCHEDULE FOR FL290; IN FACT IT SHOULD HAVE BEEN DSNDING. TURNED ON AIRFOIL ANTI-ICE AND CONSULTED QRH FOR FLOW LIGHT AND THEN ERRATIC CABIN ALT AND ASKED FOR A LOWER ALT. DONNED OXYGEN MASKS AND ESTABLISHED COMS; SELECTED MANUAL CABIN ALT CTL AND DISCOVERED THE OUTFLOW VALVE WAS ALREADY FULLY CLOSED OR INDICATED AS SUCH. NOTHING LEFT TO DO BUT EXPEDITE THE DSCNT WITH ATC BEFORE THE MASKS IN THE CABIN DEPLOYED. AT 14500 FT THE CABIN RATE OF CLB SPIKED DOWN (PROBABLY BECAUSE THE OUTFLOW VALVE WAS STILL CLOSED) AND I SELECTED AUTO PRESSURIZATION. THE CABIN RATE OF DSCNT RETURNED TO A NORMAL 500 FPM DSCNT. WE ASKED FOR 15000 FT; TOOK OUR OXYGEN OFF; INFORMED THE CABIN CREW WHAT WAS GOING ON AND CONTINUED TO MKE FOR A NORMAL DSCNT AND LNDG. THE CABIN ALT NEVER GOT ABOVE ABOUT 9800 FT AND THE DSCNT WAS NOT THAT ABRUPT SO I DID NOT TELL THE PAX ANYTHING OUT OF THE ORDINARY. I PROBABLY SHOULD HAVE TURNED ON THE AIRFOIL ANTI-ICE SOONER BUT I'M NOT ENTIRELY SURE THAT IS WHAT HAPPENED. THE AIR-CONDITIONING SYS AND PRESSURIZATION ON ALL THE MD80'S SEEM TO BE WEAK AND INEFFICIENT. THERE NEEDS TO BE MORE EMPHASIS ON DUCTS BEING CONNECTED AND LEAKS CORRECTED. SOMETIMES I THINK MOST OF OUR PRESSURIZATION IS PROVIDED TO THE TAIL CONE. WE NEVER FIND OUT HOW WEAK THE AIR FROM A BAD AIR CYCLE MACHINE IS UNTIL THE OTHER ONE FAILS.

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.