Narrative:

Upon departure from svmi the N1 tachometer instrument from engine #2 was not indicating the expected reading on idle thrust. Circuit breaker was recycled and instrument continued erratic. No N1 rotation alternate procedure was conducted and ground crew confirmed N1 rotation from #2 engine ok. Since we were still at the tarmac, MEL was consulted and maintenance was called upon the aircraft. The item was deferred for ZZZ, our next destination, and the weight penalties described on the MEL were applied for our takeoff performance out of svmi. The payload was not affected, as the aircraft was basically ferried with 3500 kgs only to pick-up the cargo in ZZZ. The N1 during takeoff roll got back to normal reading, and we discovered then, that the instrument was faulty only on idle thrust. The flight from svmi to ZZZ was uneventful and caution was taken in using other engine parameters to protect the engine for possible exceedances should the N1 instrument go inoperative. In ZZZ maintenance liaised with XXX maintenance office to replace the N1 instrument and the aircraft spares cabinet was opened for that contingency. The N1 instrument was replaced and then swapped, and the engine #2 was cranked to verify the N1 instrument reading. The N1 instrument on idle thrust was again unreliable and was again deferred (dmi) to maintenance station mia. During the takeoff roll out of ZZZ the N1 instrument returned to normal and was written on the tech-log once again for proper inspection in mia. After review of procedures in-flight, the flight engineer and I realized that the takeoff penalties listed on the MEL were not applied for the takeoff performance out of ZZZ. We basically overlooked the N1 instrument weight penalty limitations on the premise that the instrument was not inoperative but rather erratic. I feel the N1 instrument problem and the maintenance action taken in ZZZ lead to ambiguity. On the one hand the company dispatch did not make any mention on the cfp (computer flight plan ZZZ-mia) and the rationale behind it was that by maintenance changing the instrument in ZZZ, we all thought the problem was over. The N1 instrument on idle thrust was always erratic and unreliable. Other engine parameters were crosschecked and ground crew was always consulted for confirmation of N1 rotation. Having said the above, I felt I should have taken the conservative approach of declaring the N1 instrument inoperative, and therefore applying the weight penalties discussed from the MEL section under N1. The takeoff performance from ZZZ to mia did not take into consideration this weight penalty of around 6000 kgs from the mrtogw.

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Original NASA ASRS Text

Title: A B747-200 WAS DISPATCHED IN NON COMPLIANCE WITH #2 ENG N1 INDICATOR DEFERRED INOP, BUT MEL WT LIMITS NOT OBSERVED.

Narrative: UPON DEP FROM SVMI THE N1 TACHOMETER INST FROM ENG #2 WAS NOT INDICATING THE EXPECTED READING ON IDLE THRUST. CIRCUIT BREAKER WAS RECYCLED AND INST CONTINUED ERRATIC. NO N1 ROTATION ALTERNATE PROC WAS CONDUCTED AND GND CREW CONFIRMED N1 ROTATION FROM #2 ENG OK. SINCE WE WERE STILL AT THE TARMAC, MEL WAS CONSULTED AND MAINT WAS CALLED UPON THE ACFT. THE ITEM WAS DEFERRED FOR ZZZ, OUR NEXT DEST, AND THE WT PENALTIES DESCRIBED ON THE MEL WERE APPLIED FOR OUR TKOF PERFORMANCE OUT OF SVMI. THE PAYLOAD WAS NOT AFFECTED, AS THE ACFT WAS BASICALLY FERRIED WITH 3500 KGS ONLY TO PICK-UP THE CARGO IN ZZZ. THE N1 DURING TKOF ROLL GOT BACK TO NORMAL READING, AND WE DISCOVERED THEN, THAT THE INST WAS FAULTY ONLY ON IDLE THRUST. THE FLT FROM SVMI TO ZZZ WAS UNEVENTFUL AND CAUTION WAS TAKEN IN USING OTHER ENG PARAMETERS TO PROTECT THE ENG FOR POSSIBLE EXCEEDANCES SHOULD THE N1 INST GO INOP. IN ZZZ MAINT LIAISED WITH XXX MAINT OFFICE TO REPLACE THE N1 INST AND THE ACFT SPARES CABINET WAS OPENED FOR THAT CONTINGENCY. THE N1 INST WAS REPLACED AND THEN SWAPPED, AND THE ENG #2 WAS CRANKED TO VERIFY THE N1 INST READING. THE N1 INST ON IDLE THRUST WAS AGAIN UNRELIABLE AND WAS AGAIN DEFERRED (DMI) TO MAINT STATION MIA. DURING THE TKOF ROLL OUT OF ZZZ THE N1 INST RETURNED TO NORMAL AND WAS WRITTEN ON THE TECH-LOG ONCE AGAIN FOR PROPER INSPECTION IN MIA. AFTER REVIEW OF PROCS INFLT, THE FE AND I REALIZED THAT THE TKOF PENALTIES LISTED ON THE MEL WERE NOT APPLIED FOR THE TKOF PERFORMANCE OUT OF ZZZ. WE BASICALLY OVERLOOKED THE N1 INST WT PENALTY LIMITATIONS ON THE PREMISE THAT THE INST WAS NOT INOP BUT RATHER ERRATIC. I FEEL THE N1 INST PROB AND THE MAINT ACTION TAKEN IN ZZZ LEAD TO AMBIGUITY. ON THE ONE HAND THE COMPANY DISPATCH DID NOT MAKE ANY MENTION ON THE CFP (COMPUTER FLT PLAN ZZZ-MIA) AND THE RATIONALE BEHIND IT WAS THAT BY MAINT CHANGING THE INST IN ZZZ, WE ALL THOUGHT THE PROB WAS OVER. THE N1 INST ON IDLE THRUST WAS ALWAYS ERRATIC AND UNRELIABLE. OTHER ENG PARAMETERS WERE XCHKED AND GND CREW WAS ALWAYS CONSULTED FOR CONFIRMATION OF N1 ROTATION. HAVING SAID THE ABOVE, I FELT I SHOULD HAVE TAKEN THE CONSERVATIVE APCH OF DECLARING THE N1 INST INOP, AND THEREFORE APPLYING THE WT PENALTIES DISCUSSED FROM THE MEL SECTION UNDER N1. THE TKOF PERFORMANCE FROM ZZZ TO MIA DID NOT TAKE INTO CONSIDERATION THIS WT PENALTY OF AROUND 6000 KGS FROM THE MRTOGW.

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.