Narrative:

On descent to FL280, upon passing FL290 from FL310, cabin pressure started fluctuating +/-500 FPM. Checked cabin pressure controller circuit breaker (normal) and started to reference cockpit operating manual. At that point, cabin pressure momentarily spiked down 2000 FPM, and then completely reversed to maximum cabin climb on the indicator. Anticipating a cabin pressure loss, we began to initiate cabin pressure loss/rapid descent when it was noticed that the outflow valve was not fully closed. We manually started the outflow valve to the closed position and then noted that we were regaining control of the cabin altitude. During this time the aircraft descended to FL275 before we regained control of the cabin. At this time, ATC called and asked about our altitude. We told ATC about our pressurization problems, and told ATC we wanted a continued gradual constant descent into bna. ATC responded and we were able to descend the cabin manually with a minimum of throttle changes, which minimized the increased cockpit workload required by a 2-M crew utilizing manual cabin pressurization procedures--especially with IMC conditions at bna. Cooperation by ATC, in this instance, was a great help to the crew. Back in dtw, maintenance found that a faulty cabin pressure control amp had been replaced 2 days prior to our incident. Maintenance replaced the amp again, and replaced the cabin pressure controller. After repairs, system operated normally.

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Original NASA ASRS Text

Title: ACR MLG ALT DEVIATION OVERSHOT DURING DESCENT WHILE FLT CREW WAS CHANGING OVER FROM AUTO PRESSURIZATION CONTROL TO MANUAL CONTROL.

Narrative: ON DSCNT TO FL280, UPON PASSING FL290 FROM FL310, CABIN PRESSURE STARTED FLUCTUATING +/-500 FPM. CHKED CABIN PRESSURE CTLR CB (NORMAL) AND STARTED TO REF COCKPIT OPERATING MANUAL. AT THAT POINT, CABIN PRESSURE MOMENTARILY SPIKED DOWN 2000 FPM, AND THEN COMPLETELY REVERSED TO MAX CABIN CLB ON THE INDICATOR. ANTICIPATING A CABIN PRESSURE LOSS, WE BEGAN TO INITIATE CABIN PRESSURE LOSS/RAPID DSCNT WHEN IT WAS NOTICED THAT THE OUTFLOW VALVE WAS NOT FULLY CLOSED. WE MANUALLY STARTED THE OUTFLOW VALVE TO THE CLOSED POS AND THEN NOTED THAT WE WERE REGAINING CONTROL OF THE CABIN ALT. DURING THIS TIME THE ACFT DSNDED TO FL275 BEFORE WE REGAINED CONTROL OF THE CABIN. AT THIS TIME, ATC CALLED AND ASKED ABOUT OUR ALT. WE TOLD ATC ABOUT OUR PRESSURIZATION PROBS, AND TOLD ATC WE WANTED A CONTINUED GRADUAL CONSTANT DSCNT INTO BNA. ATC RESPONDED AND WE WERE ABLE TO DSND THE CABIN MANUALLY WITH A MINIMUM OF THROTTLE CHANGES, WHICH MINIMIZED THE INCREASED COCKPIT WORKLOAD REQUIRED BY A 2-M CREW UTILIZING MANUAL CABIN PRESSURIZATION PROCS--ESPECIALLY WITH IMC CONDITIONS AT BNA. COOPERATION BY ATC, IN THIS INSTANCE, WAS A GREAT HELP TO THE CREW. BACK IN DTW, MAINT FOUND THAT A FAULTY CABIN PRESSURE CONTROL AMP HAD BEEN REPLACED 2 DAYS PRIOR TO OUR INCIDENT. MAINT REPLACED THE AMP AGAIN, AND REPLACED THE CABIN PRESSURE CTLR. AFTER REPAIRS, SYS OPERATED NORMALLY.

Data retrieved from NASA's ASRS site as of August 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.