Narrative:

While cruising at FL330 northeast of atl, we had an intern pilot in the jump seat of our aircraft. He was asking questions about the various system of the airplane. We were talking about the pressurization system and I was describing how the automatic and manual system worked. I then pushed down on the manual system handle to put the system in manual. The cabin altitude started a small descent. Not a problem. I then pulled the handle back up to put the system back in the 'automatic' mode. The cabin altitude started a 1500 FPM climb. Then after a few seconds the cabin rate of climb started to increase. I then proceeded to put the system back into the manual mode to get positive control of the outflow valves. The way the system is designed, positive control of the outflow valves can always be achieved by using the manual mode. I've flown entire flts with the automatic mode MEL'ed 'inoperative.' well, this was not what happened on this day. The cabin was climbing and I was rotating the cabin outflow valve forward to close the valves, but nothing was happening. I had the outflows completely closed and the cabin was still climbing. We got the cabin altitude warning light and audio annunciation. At that time we donned our oxygen masks and started an emergency descent. The oxygen system in the cabin started and masks were dropped. The captain made a PA to the passenger and I started the checklist. We also called ZTL on the radio and got clearance to 15000 ft. Then requested 14000 ft. Just as we got to 14000 ft, the cabin started to pressurize again. I got positive control of the cabin, used the manual mode to bring the cabin down to 1000 ft. We requested clearance to atl. We turned toward atl in preparation for an overweight landing. On our approach to runway 26R, we overshot the final and got an RA on the TCASII from an aircraft on approach to runway 27L. We reduced rate of descent to resolve the RA. Normal landing was accomplished. No one was hurt or injured in the incident. In retrospect, my overconfidence of the system and putting the system in manual to start with, should have never happened. However, I 'should' have been able to control the cabin manually. I don't know what happened mechanically that I couldn't control the cabin altitude. From now on and only in the event of an emergency, I will only touch the 'shiny switches.' supplemental information from acn 511740: aircraft experienced a loss of cabin pressure at FL330 and procedures were followed to descend aircraft to safe altitude. After emergency was completed and at around 10000 ft, aircraft received clearance to divert to atl. This required a turn to heading 240 degrees to VOR. This also required getting approach plates out, ATIS, etc. At the time of divert we were 40 mi from atl. We received clearance for a visual to runway 26R following a B757. I tuned in the ILS frequency and when localizer needle centered I turned to place the VOR CDI needle at the top of heading indicator, still 240 degrees. We were doing checklist, configuring the aircraft and still a little shook up from the high dive. When we realized we overshot the runway and started to turn back, we got an RA to descend from parallel traffic on runway 27L. During the turn, atl approach also said we needed to turn right to the runway, which by now we saw and reported that we were doing so. We did comply with the RA as far as descending the aircraft and also turned back to runway 26R. No further incident.

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Original NASA ASRS Text

Title: MD80 CREW LOST CTL OF CABIN PRESSURIZATION AND MADE AN EMER DSCNT IN ZTL CLASS A AIRSPACE.

Narrative: WHILE CRUISING AT FL330 NE OF ATL, WE HAD AN INTERN PLT IN THE JUMP SEAT OF OUR ACFT. HE WAS ASKING QUESTIONS ABOUT THE VARIOUS SYS OF THE AIRPLANE. WE WERE TALKING ABOUT THE PRESSURIZATION SYS AND I WAS DESCRIBING HOW THE AUTOMATIC AND MANUAL SYS WORKED. I THEN PUSHED DOWN ON THE MANUAL SYS HANDLE TO PUT THE SYS IN MANUAL. THE CABIN ALT STARTED A SMALL DSCNT. NOT A PROB. I THEN PULLED THE HANDLE BACK UP TO PUT THE SYS BACK IN THE 'AUTO' MODE. THE CABIN ALT STARTED A 1500 FPM CLB. THEN AFTER A FEW SECONDS THE CABIN RATE OF CLB STARTED TO INCREASE. I THEN PROCEEDED TO PUT THE SYS BACK INTO THE MANUAL MODE TO GET POSITIVE CTL OF THE OUTFLOW VALVES. THE WAY THE SYS IS DESIGNED, POSITIVE CTL OF THE OUTFLOW VALVES CAN ALWAYS BE ACHIEVED BY USING THE MANUAL MODE. I'VE FLOWN ENTIRE FLTS WITH THE AUTO MODE MEL'ED 'INOP.' WELL, THIS WAS NOT WHAT HAPPENED ON THIS DAY. THE CABIN WAS CLBING AND I WAS ROTATING THE CABIN OUTFLOW VALVE FORWARD TO CLOSE THE VALVES, BUT NOTHING WAS HAPPENING. I HAD THE OUTFLOWS COMPLETELY CLOSED AND THE CABIN WAS STILL CLBING. WE GOT THE CABIN ALT WARNING LIGHT AND AUDIO ANNUNCIATION. AT THAT TIME WE DONNED OUR OXYGEN MASKS AND STARTED AN EMER DSCNT. THE OXYGEN SYS IN THE CABIN STARTED AND MASKS WERE DROPPED. THE CAPT MADE A PA TO THE PAX AND I STARTED THE CHKLIST. WE ALSO CALLED ZTL ON THE RADIO AND GOT CLRNC TO 15000 FT. THEN REQUESTED 14000 FT. JUST AS WE GOT TO 14000 FT, THE CABIN STARTED TO PRESSURIZE AGAIN. I GOT POSITIVE CTL OF THE CABIN, USED THE MANUAL MODE TO BRING THE CABIN DOWN TO 1000 FT. WE REQUESTED CLRNC TO ATL. WE TURNED TOWARD ATL IN PREPARATION FOR AN OVERWT LNDG. ON OUR APCH TO RWY 26R, WE OVERSHOT THE FINAL AND GOT AN RA ON THE TCASII FROM AN ACFT ON APCH TO RWY 27L. WE REDUCED RATE OF DSCNT TO RESOLVE THE RA. NORMAL LNDG WAS ACCOMPLISHED. NO ONE WAS HURT OR INJURED IN THE INCIDENT. IN RETROSPECT, MY OVERCONFIDENCE OF THE SYS AND PUTTING THE SYS IN MANUAL TO START WITH, SHOULD HAVE NEVER HAPPENED. HOWEVER, I 'SHOULD' HAVE BEEN ABLE TO CTL THE CABIN MANUALLY. I DON'T KNOW WHAT HAPPENED MECHANICALLY THAT I COULDN'T CTL THE CABIN ALT. FROM NOW ON AND ONLY IN THE EVENT OF AN EMER, I WILL ONLY TOUCH THE 'SHINY SWITCHES.' SUPPLEMENTAL INFO FROM ACN 511740: ACFT EXPERIENCED A LOSS OF CABIN PRESSURE AT FL330 AND PROCS WERE FOLLOWED TO DSND ACFT TO SAFE ALT. AFTER EMER WAS COMPLETED AND AT AROUND 10000 FT, ACFT RECEIVED CLRNC TO DIVERT TO ATL. THIS REQUIRED A TURN TO HDG 240 DEGS TO VOR. THIS ALSO REQUIRED GETTING APCH PLATES OUT, ATIS, ETC. AT THE TIME OF DIVERT WE WERE 40 MI FROM ATL. WE RECEIVED CLRNC FOR A VISUAL TO RWY 26R FOLLOWING A B757. I TUNED IN THE ILS FREQ AND WHEN LOC NEEDLE CTRED I TURNED TO PLACE THE VOR CDI NEEDLE AT THE TOP OF HDG INDICATOR, STILL 240 DEGS. WE WERE DOING CHKLIST, CONFIGURING THE ACFT AND STILL A LITTLE SHOOK UP FROM THE HIGH DIVE. WHEN WE REALIZED WE OVERSHOT THE RWY AND STARTED TO TURN BACK, WE GOT AN RA TO DSND FROM PARALLEL TFC ON RWY 27L. DURING THE TURN, ATL APCH ALSO SAID WE NEEDED TO TURN R TO THE RWY, WHICH BY NOW WE SAW AND RPTED THAT WE WERE DOING SO. WE DID COMPLY WITH THE RA AS FAR AS DSNDING THE ACFT AND ALSO TURNED BACK TO RWY 26R. NO FURTHER INCIDENT.

Data retrieved from NASA's ASRS site as of July 2007 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.