A DC9-30 in cruise at FL290 had the automatic pressurization system failure causing cabin to over pressurize allowing pressure relief valve to open. Switched to manual control.

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Attributes
ACN	493370
Time
Date	200011
Day	Tue
Local Time Of Day	1801 To 2400
Place
Locale Reference	navaid : vxv.vortac
Altitude	msl single value : 29000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : ztl.artcc
Operator	common carrier : air carrier
Make Model Name	DC-9 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	other vortac
Flight Phase	cruise : level
Route In Use	enroute airway : j46.airway
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : flight engineer pilot : cfi pilot : atp
Experience	flight time last 90 days : 150 flight time total : 21000 flight time type : 2500
ASRS Report	493370
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : multi engine pilot : instrument pilot : commercial pilot : cfi pilot : atp
ASRS Report	493366
Events
Anomaly	aircraft equipment problem : critical other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : cabin alt indicator other flight crewa other flight crewb
Resolutory Action	flight crew : overcame equipment problem flight crew : overrode automation
Consequence	other
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft
Primary Problem	Aircraft

Narrative:

En route from clt to stl at FL290, encountered pressure control problem, unable to control the cabin in the automatic mode we used the manual control to regain the cabin also we started a descent with ATC guidance. The flight was able to continue on at a selected lower altitude. There was a large pressure bump. Callback conversation with reporter revealed the following information: the reporter stated the problem was caused by the automatic pressurization system over pressurizing the cabin allowing the over pressure relief valve to open and close causing the pressure bump. The reporter said switching to manual mode allowed control of the cabin pressure to normal. The reporter said the automatic pressure controller was replaced by maintenance.

Original NASA ASRS Text

Title: A DC9-30 IN CRUISE AT FL290 HAD THE AUTO PRESSURIZATION SYS FAILURE CAUSING CABIN TO OVER PRESSURIZE ALLOWING PRESSURE RELIEF VALVE TO OPEN. SWITCHED TO MANUAL CTL.

Narrative: ENRTE FROM CLT TO STL AT FL290, ENCOUNTERED PRESSURE CTL PROB, UNABLE TO CTL THE CABIN IN THE AUTO MODE WE USED THE MANUAL CTL TO REGAIN THE CABIN ALSO WE STARTED A DSCNT WITH ATC GUIDANCE. THE FLT WAS ABLE TO CONTINUE ON AT A SELECTED LOWER ALT. THERE WAS A LARGE PRESSURE BUMP. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE PROB WAS CAUSED BY THE AUTO PRESSURIZATION SYS OVER PRESSURIZING THE CABIN ALLOWING THE OVER PRESSURE RELIEF VALVE TO OPEN AND CLOSE CAUSING THE PRESSURE BUMP. THE RPTR SAID SWITCHING TO MANUAL MODE ALLOWED CTL OF THE CABIN PRESSURE TO NORMAL. THE RPTR SAID THE AUTO PRESSURE CTLR WAS REPLACED BY MAINT.

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.