Narrative:

During descent passing through FL180 and cleared to 12000 ft; the descent checklist was initiated. During the preparation for the checklist; one action is to turn on the auxiliary hydraulic pump. When the auxiliary hydraulic pump switch was placed on; there appeared to be an electrical surge or spike; which is fully explained below. The checklist was continued; but we noticed an increase in internal ear pressure. When the pressurization panel was checked it was noted that the cabin altitude was descending at a very rapid rate; in excess of 1500 FPM. As the captain; I was the pilot monitoring. I told my first officer that he had the flight controls and also to take over radio communications. I then referenced the QRH to resolve our pressurization problem. Referencing section 2 of the QRH revealed that there was no procedure for an over-pressurization of the cabin. At this point; we were cleared to 8000 ft. The first officer requested; and received; delaying vectors and maintained 8000 ft. The maximum cabin differential pressure reached approximately 7.9 psi; at 8000 ft. When I was unable to find a procedure that applied to our situation; no lights illuminated on the pressurization panel or oap messages. I relied on my system knowledge of the pressurization system. I tried to test the pressurization panel by pressing the flow light; with no response from the test. There was no illumination of any lights on the pressurization panel. There was no indication of automatic-fail; automatic 1 inoperative; or automatic 2 inoperative. It appeared that the electrical input into the pressurization panel had completely failed. When I was unable to find a QRH procedure that applied to our specific situation or able to test the pressurization panel; I made a decision that I would depressurize the cabin in manual mode by using the automatic pressurization inoperative/manual pressurization checklist in the QRH. While the first officer flew the aircraft on delaying vectors; I slowly manipulated the cabin pressure outflow valve to allow the aircraft to depressurize for landing. After landing; we taxied to the gate where external power was connected to the aircraft. After checking the volts and frequency of the external power; external power was switched on. After completing the shutdown checklist; and while writing the discrepancy in the logbook; I again pressed the flow light on the pressurization panel at which time it tested normal. It is my opinion that the electrical spike; when the auxiliary hydraulic pump was placed on; caused a complete interruption of electrical power in the pressurization controller. Callback conversation with reporter revealed the following information: reporter stated that when the auxiliary hydraulic pump was turned on in flight his pfd also showed a narrowing with momentary distortions typical of an electrical disturbance to electronic equipment. He stated that maintenance performed a pressurization system bite test that revealed no faults. Additionally; the auxiliary hydraulic pump was activated several times and all systems operated normally.

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Original NASA ASRS Text

Title: AN MD88 CAPT REPORTS AN ELECTRICAL SPIKE WHILE TURNING ON THE AUX HYD PUMP DURING DSCNT CAUSED A PRESSURE CONTROLLER MALFUNCTION. NO OVER-PRESSURIZATION CHECKLIST WAS FOUND.

Narrative: DURING DSCNT PASSING THROUGH FL180 AND CLRED TO 12000 FT; THE DSCNT CHKLIST WAS INITIATED. DURING THE PREPARATION FOR THE CHKLIST; ONE ACTION IS TO TURN ON THE AUX HYD PUMP. WHEN THE AUX HYD PUMP SWITCH WAS PLACED ON; THERE APPEARED TO BE AN ELECTRICAL SURGE OR SPIKE; WHICH IS FULLY EXPLAINED BELOW. THE CHKLIST WAS CONTINUED; BUT WE NOTICED AN INCREASE IN INTERNAL EAR PRESSURE. WHEN THE PRESSURIZATION PANEL WAS CHKED IT WAS NOTED THAT THE CABIN ALT WAS DSNDING AT A VERY RAPID RATE; IN EXCESS OF 1500 FPM. AS THE CAPT; I WAS THE PLT MONITORING. I TOLD MY FO THAT HE HAD THE FLT CTLS AND ALSO TO TAKE OVER RADIO COMS. I THEN REFED THE QRH TO RESOLVE OUR PRESSURIZATION PROB. REFING SECTION 2 OF THE QRH REVEALED THAT THERE WAS NO PROC FOR AN OVER-PRESSURIZATION OF THE CABIN. AT THIS POINT; WE WERE CLRED TO 8000 FT. THE FO REQUESTED; AND RECEIVED; DELAYING VECTORS AND MAINTAINED 8000 FT. THE MAX CABIN DIFFERENTIAL PRESSURE REACHED APPROX 7.9 PSI; AT 8000 FT. WHEN I WAS UNABLE TO FIND A PROC THAT APPLIED TO OUR SITUATION; NO LIGHTS ILLUMINATED ON THE PRESSURIZATION PANEL OR OAP MESSAGES. I RELIED ON MY SYS KNOWLEDGE OF THE PRESSURIZATION SYS. I TRIED TO TEST THE PRESSURIZATION PANEL BY PRESSING THE FLOW LIGHT; WITH NO RESPONSE FROM THE TEST. THERE WAS NO ILLUMINATION OF ANY LIGHTS ON THE PRESSURIZATION PANEL. THERE WAS NO INDICATION OF AUTO-FAIL; AUTO 1 INOP; OR AUTO 2 INOP. IT APPEARED THAT THE ELECTRICAL INPUT INTO THE PRESSURIZATION PANEL HAD COMPLETELY FAILED. WHEN I WAS UNABLE TO FIND A QRH PROC THAT APPLIED TO OUR SPECIFIC SITUATION OR ABLE TO TEST THE PRESSURIZATION PANEL; I MADE A DECISION THAT I WOULD DEPRESSURIZE THE CABIN IN MANUAL MODE BY USING THE AUTO PRESSURIZATION INOP/MANUAL PRESSURIZATION CHKLIST IN THE QRH. WHILE THE FO FLEW THE ACFT ON DELAYING VECTORS; I SLOWLY MANIPULATED THE CABIN PRESSURE OUTFLOW VALVE TO ALLOW THE ACFT TO DEPRESSURIZE FOR LNDG. AFTER LNDG; WE TAXIED TO THE GATE WHERE EXTERNAL PWR WAS CONNECTED TO THE ACFT. AFTER CHKING THE VOLTS AND FREQUENCY OF THE EXTERNAL PWR; EXTERNAL PWR WAS SWITCHED ON. AFTER COMPLETING THE SHUTDOWN CHKLIST; AND WHILE WRITING THE DISCREPANCY IN THE LOGBOOK; I AGAIN PRESSED THE FLOW LIGHT ON THE PRESSURIZATION PANEL AT WHICH TIME IT TESTED NORMAL. IT IS MY OPINION THAT THE ELECTRICAL SPIKE; WHEN THE AUX HYD PUMP WAS PLACED ON; CAUSED A COMPLETE INTERRUPTION OF ELECTRICAL PWR IN THE PRESSURIZATION CONTROLLER. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THAT WHEN THE AUX HYDRAULIC PUMP WAS TURNED ON IN FLT HIS PFD ALSO SHOWED A NARROWING WITH MOMENTARY DISTORTIONS TYPICAL OF AN ELECTRICAL DISTURBANCE TO ELECTRONIC EQUIPMENT. HE STATED THAT MAINT PERFORMED A PRESSURIZATION SYSTEM BITE TEST THAT REVEALED NO FAULTS. ADDITIONALLY; THE AUX HYDRAULIC PUMP WAS ACTIVATED SEVERAL TIMES AND ALL SYSTEMS OPERATED NORMALLY.

Data retrieved from NASA's ASRS site as of May 2009 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.