Narrative:

While reviewing the maintenance log during my preflight inspection I noticed that the previous day a crew wrote up the main hydraulic accumulator for pressure fluctuations. Maintenance replaced the flap valve and signed it off. We leveled off at 16000 ft. I noticed that the main hydraulic accumulator started cycling on and off every time the pressure would drop down to 2000 psi. It would go up to 3000 psi; bleed down to 2000 psi and then cycle again. I began to time the cycle given the history on the aircraft and that it was happening often. The main accumulator was having pressure fluctuations and cycling every 35 seconds. The emergency; outboard; and inboard accumulators were steady at 3000 psi. The hydraulic quantity was fluctuating between 80-120 cubic inches. We checked our circuit breakers; none were tripped. We then consulted the non normal hydraulic checklist and there was no procedure for the problem we were having. I contacted dispatch and maintenance and explained the problem. We all agreed that we should return to ZZZ. Dispatch gave us our amended release information and I then contacted ATC and told them we needed to return to ZZZ. I explained to them that we were having a hydraulic problem. They asked me if I was declaring an emergency and I said not at this time. The reason I waited a few mins was because I wanted to brief my first officer; flight attendant; and the passenger and alert them of the situation. I did not feel that we were in eminent danger as we had hydraulic fluid and the emergency accumulator had 3000 psi. The first officer and I agreed that the safest course of action was to declare an emergency and return to ZZZ given the fact that we were having problems with the hydraulic system. I confirmed with the flight attendant the passenger count and reviewed our fuel on board. By this time we were heading back to ZZZ and the first officer was the PF. I called ATC and declared an emergency. I told them we had 31 souls on board; 2 hours of fuel and no hazmat. We were assigned runway xr. The WX was VFR and we decided that at the completion of the descent check we would switch controls and I would land the aircraft. I called operations and advised then to be ready to possibly tow the aircraft if we had any problems on landing. I did not have time to call dispatch back and advise them we had declared an emergency. I decided I would talk to them and maintenance when we were safely on the ground. I told approach to have the trucks standing by in case we had any problems on landing. The first officer and I thoroughly briefed the approach and discussed any potential problems and how we would handle them. It was a normal approach and landing with no problems. Once on the ground the hydraulic pump stopped cycling. We had all normal indications and taxied to the gate.

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Original NASA ASRS Text

Title: SF340 CAPT REPORTS MAIN ACCUMULATOR PRESSURE CYCLING BETWEEN 2000 AND 3000 PSI IN FLIGHT. FLT CREW ELECTS TO RETURN TO LAND; THE SYSTEM HAD BEEN PREVIOUSLY WRITTEN UP.

Narrative: WHILE REVIEWING THE MAINT LOG DURING MY PREFLT INSPECTION I NOTICED THAT THE PREVIOUS DAY A CREW WROTE UP THE MAIN HYD ACCUMULATOR FOR PRESSURE FLUCTUATIONS. MAINT REPLACED THE FLAP VALVE AND SIGNED IT OFF. WE LEVELED OFF AT 16000 FT. I NOTICED THAT THE MAIN HYD ACCUMULATOR STARTED CYCLING ON AND OFF EVERY TIME THE PRESSURE WOULD DROP DOWN TO 2000 PSI. IT WOULD GO UP TO 3000 PSI; BLEED DOWN TO 2000 PSI AND THEN CYCLE AGAIN. I BEGAN TO TIME THE CYCLE GIVEN THE HISTORY ON THE ACFT AND THAT IT WAS HAPPENING OFTEN. THE MAIN ACCUMULATOR WAS HAVING PRESSURE FLUCTUATIONS AND CYCLING EVERY 35 SECONDS. THE EMER; OUTBOARD; AND INBOARD ACCUMULATORS WERE STEADY AT 3000 PSI. THE HYD QUANTITY WAS FLUCTUATING BTWN 80-120 CUBIC INCHES. WE CHKED OUR CIRCUIT BREAKERS; NONE WERE TRIPPED. WE THEN CONSULTED THE NON NORMAL HYD CHKLIST AND THERE WAS NO PROC FOR THE PROB WE WERE HAVING. I CONTACTED DISPATCH AND MAINT AND EXPLAINED THE PROB. WE ALL AGREED THAT WE SHOULD RETURN TO ZZZ. DISPATCH GAVE US OUR AMENDED RELEASE INFO AND I THEN CONTACTED ATC AND TOLD THEM WE NEEDED TO RETURN TO ZZZ. I EXPLAINED TO THEM THAT WE WERE HAVING A HYD PROB. THEY ASKED ME IF I WAS DECLARING AN EMER AND I SAID NOT AT THIS TIME. THE REASON I WAITED A FEW MINS WAS BECAUSE I WANTED TO BRIEF MY FO; FLT ATTENDANT; AND THE PAX AND ALERT THEM OF THE SITUATION. I DID NOT FEEL THAT WE WERE IN EMINENT DANGER AS WE HAD HYD FLUID AND THE EMER ACCUMULATOR HAD 3000 PSI. THE FO AND I AGREED THAT THE SAFEST COURSE OF ACTION WAS TO DECLARE AN EMER AND RETURN TO ZZZ GIVEN THE FACT THAT WE WERE HAVING PROBS WITH THE HYD SYS. I CONFIRMED WITH THE FLT ATTENDANT THE PAX COUNT AND REVIEWED OUR FUEL ON BOARD. BY THIS TIME WE WERE HDG BACK TO ZZZ AND THE FO WAS THE PF. I CALLED ATC AND DECLARED AN EMER. I TOLD THEM WE HAD 31 SOULS ON BOARD; 2 HRS OF FUEL AND NO HAZMAT. WE WERE ASSIGNED RWY XR. THE WX WAS VFR AND WE DECIDED THAT AT THE COMPLETION OF THE DSCNT CHK WE WOULD SWITCH CTLS AND I WOULD LAND THE ACFT. I CALLED OPS AND ADVISED THEN TO BE READY TO POSSIBLY TOW THE ACFT IF WE HAD ANY PROBS ON LNDG. I DID NOT HAVE TIME TO CALL DISPATCH BACK AND ADVISE THEM WE HAD DECLARED AN EMER. I DECIDED I WOULD TALK TO THEM AND MAINT WHEN WE WERE SAFELY ON THE GND. I TOLD APCH TO HAVE THE TRUCKS STANDING BY IN CASE WE HAD ANY PROBS ON LNDG. THE FO AND I THOROUGHLY BRIEFED THE APCH AND DISCUSSED ANY POTENTIAL PROBS AND HOW WE WOULD HANDLE THEM. IT WAS A NORMAL APCH AND LNDG WITH NO PROBS. ONCE ON THE GND THE HYD PUMP STOPPED CYCLING. WE HAD ALL NORMAL INDICATIONS AND TAXIED TO THE GATE.

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.