ACR PIC 'FESSES UP TO SWAPPING OIL INDICATOR GAUGES TO SEE IF A FAULT LIES WITHIN THE ERRANT GAUGE.

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FLT XXX A CHARTER FLOWN BY XYZ AIRLINES; FROM EWR TO NASSAU. ABOUT 30 MINS AFTER TKOF; THE SO INFORMED ME THAT THE #1 ENG OIL QUANTITY WAS DECREASING SLOWLY. WE CONSULTED THE FLT MANUAL WHICH REFERRED NO ACTION UNTIL OIL TEMP OR PRESSURE REACTED. SINCE ALL ENGS WERE SVCED PRIOR TO DEP; WE SUSPECTED A BAD OIL QUANTITY GAUGE OR BAD SENDING UNIT. AT THIS TIME THE SO AND I DECIDED A POTENTIALLY SERIOUS PROBLEM OR A CATASTROPHIC ENG LOSS COULD BE AVERTED IF WE KNEW WHETHER THE GAUGE WAS BAD. WITH MY OK; HE SWITCHED THE #1 AND #2 OIL QUANTITY GAUGES; A SIMPLE PROC INVOLVING 1 SCREW AND A KNURLED KNOB CONNECTOR. THIS TROUBLESHOOTING INDICATED THAT THE GAUGE WAS ACCURATE AND WE WERE INDEED LOSING OIL QUANTITY. GAUGES WERE RETURNED TO THEIR ORIGINAL POS AND SHORTLY THEREAFTER WE BEGAN PREPARATION FOR LOSS OF THAT ENG. BECAUSE WE WERE ABLE TO ELIMINATE THE POSSIBILITY OF A BAD GAUGE WE WERE NOT SURPRISED WHEN THE OIL PRESSURE DROPPED AND FOLLOWING FLT MANUAL PROCS THE ENG WAS SHUTDOWN. AN EMER WAS DECLARED AND AN UNEVENTFUL 2 ENG LNDG WAS MADE. MY POINT OF THIS RPT IS THAT POSSIBLY SWAPPING OIL QUANTITY GAUGES MAY BE CONSTRUED BY THE FAA AS UNAUTHORIZED MAINT WHEN OUR INTENT WAS MERELY TO ANALYZE THE SITUATION AND TO BE PREPARED IN THE EVENT OF LOSS OF THE ENG. I FEEL OUR TROUBLESHOOTING WAS CORRECT AND NEVER AN ATTEMPT AT UNAUTHORIZED MAINT.