ATX-SMT DEVIATED FROM CLRNC ROUTE CAUSING POTENTIAL LESS THAN STANDARD SEPARATION WITH UNK.

Narrative

AS WE COMPLETED THE BEFORE TKOF CHKLIST AT THE DEP ARPT; THE #2 HSI WAS DISCOVERED TO HAVE SLAVED MORE THAN 40 DEGS OFF THE CORRECT MAGNETIC HDG. THE COPLT SLEWED THE CARD TO ALIGN IT WITH THE #1 HSI AND IT APPEARED TO FUNCTION NORMALLY. AT OUR CRUISE ALT OF FL280; ON INITIAL CONTACT WITH ZID; WE WERE ASSIGNED A HDG NW OF OUR COURSE FOR TFC SEP. THE ACFT WAS ON AUTOPLT AND THE CRUISE CHK HAD BEEN COMPLETED. AFTER A PERIOD OF MINUTES WE QUERIED THE CTLR AND WERE GIVEN DIRECT CVG. THE FMS WAS PROGRAMMED AND QUICKLY SHOWED THE DIRECT COURSE TO CVG ABOUT 30-40 DEGS LEFT OF PRESENT HDG. I SWUNG THE HDG BUG ON THE HSI TO THE SPECIFIC COURSE AND THE ACFT BEGAN TO TURN AT THE HALF STANDARD RATE WHICH HAD BEEN SELECTED ON THE AUTOPLT. THE NAV SYS IS LOCATED ON THE EXTREME LEFT SIDE OF THE INSTRUMENT PANEL; SO WHILE THE ACFT WAS TURNING TO CVG; I DECIDED TO COMPLETE THE PROGRAMMING OF OUR ALTERED ROUTE TO IND. AS THE COPLT CALLED OUT COORDINATES; I ENTERED THEM AND NOTICED WE HAD 20 DEGS OF TURN TO REACH CVG. THE COPLT THEN SCANNED HIS HSI AND NOTICED A LARGE HDG DISCREPANCY WITH THE #1 EHSI. I TENDED TO DISCOUNT THE #2 HSI INDICATION; REMEMBERING ITS TROUBLES AT OUR DEP POINT. THE #1 EHSI WAS INDICATING 300 DEGS AND THAT WE WERE DIRECT CVG. THE #2 INDICATED 240 DEGS AND THAT CVG WAS WELL TO THE RIGHT. THE MAGNETIC COMPASS INDICATED 270 DEGS DUE TO THE FACT THE WINDSHIELD HEAT WAS ON; THUS MAKING IT UNRELIABLE. SECS AFTER WE NOTED THE DISCREPANCY; CENTER ASKED US OUR HDG. WE HESITATED ANSWERING BECAUSE WE STILL HAD NOT DETERMINED WHICH COMPASS SYS WAS AT FAULT. HE ASKED AGAIN; AND I REPLIED THAT WE HAD A NAV FAILURE AND ASKED HIM TO STANDBY. HE ANSWERED THAT HE COULD NOT; AND TOLD US TO ALTER COURSE 30 DEGS TO THE RIGHT; WITH WHICH WE COMPLIED AND NOTICED THE EHSI WAS TURNING AT A RATE OF ABOUT 1/4 THE ACTUAL RATE. NOW WE WERE SURE THE #2 WAS ACCURATE. AS WE ROLLED OUT OF THE TURN; WE WERE TOLD TO DSND TO FL260 OR PERHAPS FL240. SHORTLY AFTERWARD; THE OBVIOUSLY ANNOYED ATC GENTLEMAN HANDED US OFF TO ANOTHER ZID FREQ. UPON STUDYING THE INCIDENT; WE DETERMINED THAT ALTHOUGH THE PHASE OF FLT WAS EXTREMELY ROUTINE; THAT SINCE THE NAV SYS WAS ON THE LEFT SIDE OF THE INSTRUMENT PANEL; THE SIC SHOULD HAVE BEEN MONITORING THE ACFT WHILE I SOLELY PROGRAMMED THE EQUIPMENT. A CONTRIBUTING FACTOR WAS THE EHSI. I HAD SELECTED THE ARC MODE WHICH DISPLAYS ONLY A 60 DEG WEDGE OF THE EHSI AND USES THE REST OF THE SCREEN TO DISPLAY DME-TYPE INFO. THIS DISPLAY; SINCE IT WAS SO ENLARGED; I FEEL MASKED THE GREATLY SLOWER RATE; AND SINCE NO FLAGS WERE AT ALL EVIDENT; ONE NEEDED TO SCAN THE RMI TO DISCOVER THE DISCREPANCY. SINCE EFIS HAS NO MOVING PARTS; ITS RELIABILITY SHOULD BE INCREASED. HOWEVER; IT IS IMPORTANT TO REALIZE THAT EFIS IS STILL SUBJECT TO UNDETECTABLE ERRORS/FAILURES WHENEVER IT INTERFACES WITH ELECTROMECHANICAL COMPONENTS (SUCH AS IN THIS CASE; THE #1 COMPASS SYS).