Loss of cabin pressure resulting in an emergency descent.

Attributes
ACN	296153
Time
Date	199502
Day	Sun
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : pit
State Reference	PA
Altitude	msl bound lower : 18000 msl bound upper : 35000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zob tower : mia
Operator	common carrier : air carrier
Make Model Name	B737-100
Operating Under FAR Part	Part 121
Flight Phase	cruise other other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : cfi pilot : commercial pilot : instrument
Experience	flight time last 90 days : 235 flight time total : 15000 flight time type : 6500
ASRS Report	296153
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	aircraft equipment problem : critical other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : overcame equipment problem flight crew : declared emergency other
Consequence	Other
Supplementary
Primary Problem	Aircraft

Narrative:

While cruising at 35000 ft the cabin altitude warning horn sounded. I (the PF) noticed the cabin altitude at 9800 ft and climbing at 1200 FPM. The first officer immediately checked the outflow valve full closed and bleeds and packs on. The first officer declared an emergency with ATC as I started a rapid descent. Upon passing approximately 22000 ft the cabin altitude started to stabilize and return to normal pressure. At 18000 ft the cabin stabilized at 1500 ft and 6 ft psi. Company dispatch, maintenance, and operations were notified of the situation. The emergency was terminated. I recommended landing at pittsburgh international. All passenger were ok and the masks did not deploy. All company parties involved recommended proceeding to dayton, ohio (scheduled city). I concurred since all flight operations were back to normal. Remainder of flight was uneventful. The cabin ascent was very insidious. Neither pilot noticed anything abnormal until the cabin altitude warning horn went off.

Original NASA ASRS Text

Title: LOSS OF CABIN PRESSURE RESULTING IN AN EMER DSCNT.

Narrative: WHILE CRUISING AT 35000 FT THE CABIN ALT WARNING HORN SOUNDED. I (THE PF) NOTICED THE CABIN ALT AT 9800 FT AND CLBING AT 1200 FPM. THE FO IMMEDIATELY CHKED THE OUTFLOW VALVE FULL CLOSED AND BLEEDS AND PACKS ON. THE FO DECLARED AN EMER WITH ATC AS I STARTED A RAPID DSCNT. UPON PASSING APPROX 22000 FT THE CABIN ALT STARTED TO STABILIZE AND RETURN TO NORMAL PRESSURE. AT 18000 FT THE CABIN STABILIZED AT 1500 FT AND 6 FT PSI. COMPANY DISPATCH, MAINT, AND OPS WERE NOTIFIED OF THE SIT. THE EMER WAS TERMINATED. I RECOMMENDED LNDG AT PITTSBURGH INTL. ALL PAX WERE OK AND THE MASKS DID NOT DEPLOY. ALL COMPANY PARTIES INVOLVED RECOMMENDED PROCEEDING TO DAYTON, OHIO (SCHEDULED CITY). I CONCURRED SINCE ALL FLT OPS WERE BACK TO NORMAL. REMAINDER OF FLT WAS UNEVENTFUL. THE CABIN ASCENT WAS VERY INSIDIOUS. NEITHER PLT NOTICED ANYTHING ABNORMAL UNTIL THE CABIN ALT WARNING HORN WENT OFF.

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.