Narrative:

We were scheduled to operate ZZZ-ZZZZ on jan/fri/06. Approximately 40 mins into the flight; we elected to return to ZZZ with an abnormally operating engine. The engine continued to operate throughout and the landing was uneventful. After reaching cruise at FL330; we noticed that the right engine was not maintaining a constant N1. The autothrottles would set the proper target N1; after a few seconds the N1 would fall off by about 3%-5%; then the autothrottles would reset the target N1. This cycle continued to repeat itself. The frequency of the fluctuations stayed about the same; but the magnitude varied. Thrust lever stagger never developed and the target N1 was consistent with that of the left engine. Yaw was subtle but detectable; and in sync with the engine indications. At first; I thought it might be an autothrottle problem; so I disconnected it; but with no effect. There were no EICAS or status messages and all circuit breakers were in. The use of engine anti-ice had no effect either. We reviewed the QRH to see if any procedures applied to this situation. I did not feel that the engine limit/surge/stall checklist was appropriate because engine limits were never approached; there were no abnormal noises; all indications were consistent with thrust lever position; and the engine was responding to thrust lever movement. After talking to maintenance and being unable to clear the problem; we elected to return to ZZZ. We obtained an ATC clearance and were cleared to ZZZ via the zzzzz arrival with an initial descent to FL320. We advised the flight attendants and passenger of our situation. I felt that the safest course of action was to operate the engine normally. The fluctuations were not large enough to affect aircraft ctlability and they continued regardless of thrust lever position. We had already burned all the center tank fuel so jettisoning was not a factor. We were going to be overweight for landing; but I felt that it would be imprudent to extend the flight in order to reduce our landing weight. The descent; approach and landing on runway 31L were uneventful. During our taxi to the gate; the fluctuations persisted; even at idle thrust. Engine shutdown was normal. We documented the engine problem and the overweight landing; and briefed the mechanic. I checked in with our duty pilot in ZZZ1. Since we still had duty day available and the crew was fit to safely conduct the flight; we switched aircraft and operated the flight to ZZZZ. Callback conversation with reporter revealed the following information: the reporter stated a local maintenance technician said the cause of the N1 fluctuation was caused by a leaking fuel pressure transmitter. The reporter has not received any report from maintenance operations on the engine or engine component failure.

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Original NASA ASRS Text

Title: A B767 AT FL330 RETURNED TO THE DEP ARPT DUE TO CONSTANT 3%-5% FLUCTUATION OF R ENG N1. FELT IN SLIGHT YAW. AUTOTHROTTLE DISCONNECTED NO HELP.

Narrative: WE WERE SCHEDULED TO OPERATE ZZZ-ZZZZ ON JAN/FRI/06. APPROX 40 MINS INTO THE FLT; WE ELECTED TO RETURN TO ZZZ WITH AN ABNORMALLY OPERATING ENG. THE ENG CONTINUED TO OPERATE THROUGHOUT AND THE LNDG WAS UNEVENTFUL. AFTER REACHING CRUISE AT FL330; WE NOTICED THAT THE R ENG WAS NOT MAINTAINING A CONSTANT N1. THE AUTOTHROTTLES WOULD SET THE PROPER TARGET N1; AFTER A FEW SECONDS THE N1 WOULD FALL OFF BY ABOUT 3%-5%; THEN THE AUTOTHROTTLES WOULD RESET THE TARGET N1. THIS CYCLE CONTINUED TO REPEAT ITSELF. THE FREQ OF THE FLUCTUATIONS STAYED ABOUT THE SAME; BUT THE MAGNITUDE VARIED. THRUST LEVER STAGGER NEVER DEVELOPED AND THE TARGET N1 WAS CONSISTENT WITH THAT OF THE L ENG. YAW WAS SUBTLE BUT DETECTABLE; AND IN SYNC WITH THE ENG INDICATIONS. AT FIRST; I THOUGHT IT MIGHT BE AN AUTOTHROTTLE PROB; SO I DISCONNECTED IT; BUT WITH NO EFFECT. THERE WERE NO EICAS OR STATUS MESSAGES AND ALL CIRCUIT BREAKERS WERE IN. THE USE OF ENG ANTI-ICE HAD NO EFFECT EITHER. WE REVIEWED THE QRH TO SEE IF ANY PROCS APPLIED TO THIS SIT. I DID NOT FEEL THAT THE ENG LIMIT/SURGE/STALL CHKLIST WAS APPROPRIATE BECAUSE ENG LIMITS WERE NEVER APCHED; THERE WERE NO ABNORMAL NOISES; ALL INDICATIONS WERE CONSISTENT WITH THRUST LEVER POS; AND THE ENG WAS RESPONDING TO THRUST LEVER MOVEMENT. AFTER TALKING TO MAINT AND BEING UNABLE TO CLR THE PROB; WE ELECTED TO RETURN TO ZZZ. WE OBTAINED AN ATC CLRNC AND WERE CLRED TO ZZZ VIA THE ZZZZZ ARR WITH AN INITIAL DSCNT TO FL320. WE ADVISED THE FLT ATTENDANTS AND PAX OF OUR SIT. I FELT THAT THE SAFEST COURSE OF ACTION WAS TO OPERATE THE ENG NORMALLY. THE FLUCTUATIONS WERE NOT LARGE ENOUGH TO AFFECT ACFT CTLABILITY AND THEY CONTINUED REGARDLESS OF THRUST LEVER POS. WE HAD ALREADY BURNED ALL THE CTR TANK FUEL SO JETTISONING WAS NOT A FACTOR. WE WERE GOING TO BE OVERWT FOR LNDG; BUT I FELT THAT IT WOULD BE IMPRUDENT TO EXTEND THE FLT IN ORDER TO REDUCE OUR LNDG WT. THE DSCNT; APCH AND LNDG ON RWY 31L WERE UNEVENTFUL. DURING OUR TAXI TO THE GATE; THE FLUCTUATIONS PERSISTED; EVEN AT IDLE THRUST. ENG SHUTDOWN WAS NORMAL. WE DOCUMENTED THE ENG PROB AND THE OVERWT LNDG; AND BRIEFED THE MECH. I CHKED IN WITH OUR DUTY PLT IN ZZZ1. SINCE WE STILL HAD DUTY DAY AVAILABLE AND THE CREW WAS FIT TO SAFELY CONDUCT THE FLT; WE SWITCHED ACFT AND OPERATED THE FLT TO ZZZZ. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED A LCL MAINT TECHNICIAN SAID THE CAUSE OF THE N1 FLUCTUATION WAS CAUSED BY A LEAKING FUEL PRESSURE XMITTER. THE RPTR HAS NOT RECEIVED ANY RPT FROM MAINT OPS ON THE ENG OR ENG COMPONENT FAILURE.

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