Flight crew of DC9-30 somehow missets pressurization control causing the cabin altitude to continue climbing. Oxygen masks donned and checklist run to correct the problem.

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Attributes
ACN	398017
Time
Date	199803
Day	Sat
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : pbf
State Reference	AR
Altitude	msl bound lower : 28000 msl bound upper : 28000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zme
Operator	common carrier : air carrier
Make Model Name	DC-9 30
Operating Under FAR Part	Part 121
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : flight engineer pilot : atp pilot : instrument
Experience	flight time last 90 days : 120 flight time total : 13800 flight time type : 5000
ASRS Report	398017
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	aircraft equipment problem : less severe
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course flight crew : overcame equipment problem
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Cabin altitude warning light was noticed while in cruise flight at FL280. Cabin altitude was observed at 9500 ft and still slowly climbing. Oxygen masks were donned and the aircraft descended at a normal rate, 2000 FPM, while problem was taken care of. While oxygen masks were on going back and forth between interphone and communication radio communication there was confusion as to what radio transmission ZME received and did not. During the checklist the pressurization problem was caused by the pressurization control being set improperly. The cabin altitude was reset and normal pressurization established. I have flown this airplane for 10 yrs and never set this wrong. I am unsure how it happened except that possibly I was interrupted during my preflight check and set it improperly. Corrective action would be to eliminate interruptions and distrs and to also be more cautious about doing something that has become rote!

Original NASA ASRS Text

Title: FLC OF DC9-30 SOMEHOW MISSETS PRESSURIZATION CTL CAUSING THE CABIN ALT TO CONTINUE CLBING. OXYGEN MASKS DONNED AND CHKLIST RUN TO CORRECT THE PROB.

Narrative: CABIN ALT WARNING LIGHT WAS NOTICED WHILE IN CRUISE FLT AT FL280. CABIN ALT WAS OBSERVED AT 9500 FT AND STILL SLOWLY CLBING. OXYGEN MASKS WERE DONNED AND THE ACFT DSNDED AT A NORMAL RATE, 2000 FPM, WHILE PROB WAS TAKEN CARE OF. WHILE OXYGEN MASKS WERE ON GOING BACK AND FORTH BTWN INTERPHONE AND COM RADIO COM THERE WAS CONFUSION AS TO WHAT RADIO XMISSION ZME RECEIVED AND DID NOT. DURING THE CHKLIST THE PRESSURIZATION PROB WAS CAUSED BY THE PRESSURIZATION CTL BEING SET IMPROPERLY. THE CABIN ALT WAS RESET AND NORMAL PRESSURIZATION ESTABLISHED. I HAVE FLOWN THIS AIRPLANE FOR 10 YRS AND NEVER SET THIS WRONG. I AM UNSURE HOW IT HAPPENED EXCEPT THAT POSSIBLY I WAS INTERRUPTED DURING MY PREFLT CHK AND SET IT IMPROPERLY. CORRECTIVE ACTION WOULD BE TO ELIMINATE INTERRUPTIONS AND DISTRS AND TO ALSO BE MORE CAUTIOUS ABOUT DOING SOMETHING THAT HAS BECOME ROTE!

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.