A BE40 CREW; ON A MAINT FLT; BECAUSE DISTR AND DIVERTED FROM ASSIGNED RTE.

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OUR FLT (MYSELF; COPLT AND MAINT TECHNICIAN) DEPARTED OUR MAINT FACILITY AT ZZZ; US; FOR TEST FLT AFTER A MAJOR INSPECTION. OUR FILED RTE WAS DIRECT TO GQO VOR; THEN THE ARR BACK INTO ZZZ. AS WE WERE FLYING DIRECT TO GQO AT APPROX FL250; WE WERE ALL PREOCCUPIED WITH TROUBLESHOOTING CORRECTED ITEMS; ESPECIALLY BEING CONCERNED ABOUT THE POSSIBILITY OF A THRUST REVERSER DEPLOYING. A MALE ATC GAVE US 'DIRECT GQO.' I COMPLIED AND PUSHED THE 'NAV' BUTTON ON MY SIDE OF THE FMS. IN A FEW MINS; I NOTICED OUR HDG WAS SLIGHTLY OFF. AS I WAS CORRECTING (TO DIRECT GQO AGAIN) ATC QUERIED OUR HDG. WE RESPONDED; 'DIRECT TO GQO.' NOTHING MORE WAS SAID FROM ATC AND HE HANDED US OFF TO ANOTHER ATC CTLR TO CONTINUE OUR CLB. (I BELIEVE MY COPLT PUSHED MY 'NAV' BUTTON AFTER I DID; WHICH CANCELED OUT MY COMMAND TO THE AUTOPLT. FORTUNATELY; I CAUGHT IT AND CORRECTED IMMEDIATELY.) NOW; WE WERE WITH A DIFFERENT CTLR; STILL DIRECT TO GQO. WE WERE FLYING NW; GQO WAS THE 'TIP' OF THE TRIANGLE ON OUR FILED RETURN TO GO BACK TO ZZZ. I NOTICED THAT THE AUTOPLT BEGAN A L TURN PRIOR TO REACHING GQO (PROBABLY BECAUSE THE ANGLE OF THAT CORNER OF THE TRIANGLE WAS SO TIGHT; IT REQUIRED TOO TIGHT OF A RADIUS OF TURN AND THE AIRPLANE WAS TRYING TO STAY ON THE FLT PLAN. WHEN I SAW THE AUTOPLT TURNING THE AIRPLANE; I IMMEDIATELY TOOK OFF THE AUTOPLT AND TURNED BACK TOWARD GQO. SIMULTANEOUSLY; THE CTLR QUERIED OUR HDG; MY COPLT ADVISED CTLR WHAT I JUST EXPLAINED ABOVE; AND WE WERE CORRECTING. CTLR SAID OK; AND ASKED IF VECTORS COULD GIVEN TO US FOR OUR CLB TO FL310. WE ADVISED WE WERE HAPPY TO COMPLY. WE WERE JUST A MAINT FLT. ALTHOUGH ATC NEVER REQUESTED US TO CALL; I BELIEVE SUBMITTING THIS RPT IS PRUDENT. A 'SIMPLE' TEST FLT CAN BE VERY DISTRACTING. ALSO; I HAD NEVER SEEN SUCH A TIGHT TURN ON A FLT PLAN. NOW; I HAVE LEARNED HOW TO 'INHIBIT' THE AUTOPLT IN FMS MODE; IN ORDER NOT TO 'LEAD' THE TURN.