C208 PLT; UNFAMILIAR WITH NEW FMS SYSTEM -- FOR WHICH THE GPS FUNCTION WAS INOPERATIVE -- ENCOUNTERS NUMEROUS PROBLEMS WITH NAVIGATION AND ACFT ATTITUDE CONTROL.

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I HAD AN ACFT FOR MY FLT TONIGHT WHICH HAD A NEW FMS SYS (GARMIN 530'S AND MFD'S). I HAD OUR TRAINING ON THIS SYS WHICH WAS EXTENSIVE (8 HRS CLASS; 2 HRS INFLT). MY FIRST FLT WITH THIS ACFT WAS HAMPERED BY THE FACT THAT THE GPS PART OF THE GARMINS WAS DOWN; MAKING THEM MUCH LESS HELPFUL (IE; VIRTUALLY USELESS). I WAS HAVING TO NAV VIA VORS AND I DID A FINE JOB THROUGH THE FIRST 1/2 OF MY FLT. AS THE FLT DEVELOPED; A FUEL IMBAL WENT UNDETECTED. THE FUEL IMBAL WAS NOT TOO BIG (ACTUAL VALUE 300 LBS -- CARAVAN MAX 200) BUT IT APPARENTLY CAUSED ENOUGH OF AN ACFT OUT OF TRIM SITUATION TO WHERE THE ACFT AUTOPLT STOPPED VOR NAV TRACKING WHILE REMAINING ROUGHLY ON HDG. I WAS SLOW TO RECOGNIZE BOTH THE FUEL IMBAL/OUT OF TRIM AND NAV ERRORS. WHEN ATC QUERIED ME AS TO WHY I WAS APPROX 40 DEGS OR SO OFF COURSE; I ASKED FOR A VECTOR WHILE I SORTED OUT GETTING BACK ON MY FLT PATH. HAVING NEW EQUIP THAT WAS ONLY PARTIALLY FUNCTIONAL AND BEING NIGHT IFR MADE THIS CHALLENGING. UNFORTUNATELY THIS WAS NOT THE END OF MY PROBS ON THIS FLT. WHILE I WAS ATTEMPTING TO COPE WITH THE FUEL IMBAL AND TRIM PROBS (AND NAV PROBS); I NOTICED MY PRIMARY AND SECONDARY (PLT/COPLT) ATTITUDE INDICATION SYS HAD MAJOR DISAGREEMENTS. THE PRIMARY ATTITUDE INDICATOR WAS APPROX 20 DEGS OFF IN PITCH. THIS BECAME MY TRUE PRIMARY PROB AT THIS POINT. ALL MY ATTN HAD TO BECOME FOCUSED ON RECTIFYING THE TRIM PROB AND THEREBY SOLVING THE ATTITUDE INDICATION PROBS. BEING 5 MINS OR SO INTO THIS PROB; ATC HAD MADE ADDITIONAL INQUIRIES INTO MY STATUS. I HAD TO PUT OFF THEIR INQUIRIES WHILE I GOT THE ACFT BACK ON LEVEL/CONSTANT HDG FLT. THEY WERE SOMEWHAT MIFFED. GIVEN THE ATTITUDE INDICATION PROBS; I WAS CLOSE TO DECLARING AN EMER. I DIDN'T BECAUSE AS I STARTED TO RESOLVE THE FUEL IMBAL/TRIM MY ATTITUDE INDICATORS CAME BACK INTO AGREEMENT. AS I WAS GETTING CLOSER TO MY DEST; I ALSO FINALLY GOT RELIABLE NAV RESTORED OFF THE VOR OF MY HOME FIELD. MY PRIMARY PROBS ON THIS FLT WERE NAV DEFICIENCIES AND FAILURE TO PROMPTLY RESPOND TO ATC QUERIES. THE ROOT CAUSE WAS MY ALLOWING NORMALLY BENIGN PROBS (FUEL IMBAL) TO DEVELOP ON A NIGHT DURING WHICH I ALREADY HAD MAJOR SYS LIMITATIONS AND REALLY COULDN'T COPE WITH MUCH ELSE.