FK100 FLIES OFF COURSE DUE TO DRIFTING FMS.

Narrative

I WAS OCCUPYING THE R SEAT OF ACR FLT KK FROM PBI TO RDU. OUR CLRNC INTO RDU WAS AN AR RTE; ILM AND THE BRADE ARR TO RDU. WE WERE FLYING AN FK100 (BBB) AND ALMOST SIMULTANEOUSLY AS WE OBSERVED THAT OUR FMS'S HAD GONE INDEPENDENT OF ONE ANOTHER AND THUS NECESSITATED OUR 'HARD-TUNING' OF VORS TO FIX OUR POS EXACT; ZDC QUERIED US AS TO OUR CLRNC AND SUBSEQUENTLY ISSUED VECTORS (TO THE L) WHILE STATING THAT WE WERE N OF COURSE. WE COMPLIED AND AFTER LNDG WITHOUT INCIDENT I SUMMONED MAINT TO MEET US. WE DISCUSSED THE ABOVE; CHKED THE FMS'S AT THE GATE AND FOUND THEM TO BE ALMOST '20 MI IN ERROR.' MAINT 'RESET' THE IRS'S; THEY GND CHKED OK AND I PHYSICALLY INFORMED THE NEW; ONCOMING CAPT AS TO OUR PLIGHT. AGAIN; I AM BRINGING THIS TO YOUR ATTN BY HARD COPY ON AN UNAUTH FORM AND ANY RECEIPT YOU WISH TO SEND ME OF THIS RPT WILL BE SUITABLE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR COULD NOT UNDERSTAND HOW THEY GOT SO FAR OFF COURSE. FMS SHOWED 20 MI XTRACK ERROR AT THE LNDG GATE. IRS WAS RESET; CAPT INFORMED NEW CAPT COMING ON BOARD; AND THE ACFT FLEW 3 MORE LEGS THAT EVENING WITH NO FAULTS. THIS SAME CAPT FLEW THE ACFT THE NEXT DAY AND THERE WERE NO PROBS WITH THE IRS OR FMS. THE NAV UNITS WERE NEVER REPLACED; NO OTHER DEVS WERE RPTED AFTER THIS INCIDENT. SUPPLEMENTAL INFO FROM ACN 351637: WHILE FLYING S BOSTON ARR TO RDU WE WERE GIVEN THE USUAL 250 KT SPD RESTR AT ALDAN INTXN (SBV 172 DEG RADIAL; 17 NM). I ROUTINELY INPUT THE CONSTRAINT AND MONITORED THE ACFT'S FMS PROFILE DSCNT; ARRIVING AT ALDAN AT PRECISELY 250 KTS. A FEW MOMENTS LATER; RDU APCH QUERIED WHAT OUR SPD WAS AND WE STATED; '250 KTS.' THEY THEN QUERIED; 'WERE YOU GIVEN 250 KTS AT ALDAN?' TO WHICH WE RESPONDED YES. I SUSPECTED SOMETHING WAS WRONG BUT DID NOT KNOW WHAT. IT IS ALSO IMPORTANT TO NOTE THAT AS A PLT I RELIGIOUSLY CHK THE IRU DRIFT AND FMS LATITUDE/LONGITUDE -- UPDATING WHENEVER I FEEL IT IS BEYOND ACCEPTABLE LIMITS. OUR COMPANY AND THE FMS MANUFACTURER; HONEYWELL; STATE THAT 3.5 NM DRIFT AND 15 KTS IS THE MAX ALLOWABLE SHIFT ACCEPTABLE FOR FLT. I PERSONALLY UPDATE ANY POS EXCEEDING 0.5 TO 0.7 NM. IT'S JUST A 'THING' I HAVE. I RARELY EVER SEE A PRECESSION OR DRIFT OF THE IRUS NEARING THIS AMOUNT AND IN THE 2 YRS I HAVE BEEN OPERATING THIS PARTICULAR FMS UNIT; HAVE NEVER SEEN DRIFT EXCEEDING THE COMPANY/HONEYWELL TOLERANCE OF 3.5 NM. AS USUAL; WHEN ARRIVING AT THE GATE AT RDU; I CHKED THE IRU DRIFT AND LATITUDE/LONGITUDE AND WAS SURPRISED TO SEE 6.4 MI. IF THE FMS DSCNT WAS PREDICATED ON SLOWING THE ACFT TO 250 KTS BY ALDAN; IT WAS MISCALCULATING THE DSCNT PROFILE TO SOME POINT PERHAPS NOT ANYWHERE NEAR ALDAN. THIS MAY HAVE BEEN EXACERBATED BY A WIND COMPONENT I NOTED OF 285 DEGS 79 KTS AT 10000 FT. SINCE ATC ONLY ASKED ME TO CONFIRM MY SPD I HAVE NO WAY OF KNOWING IF INDEED THE XING RESTR WAS OF CONCERN. BUT I BELIEVE IT IS SIGNIFICANT BECAUSE OF THE ERRORS WHICH SHOWED UP DURING MY LITTLE POSTFLT IRU CHK. THESE ERRORS ARE ATYPICAL. IN ADDITION TO AVIATION I AM TRAINED IN ELECTRONICS AND HAVE CONSIDERABLE RF EXPERIENCE. I CANNOT HELP FEELING THAT THE INORDINATE DRIFT COULD HAVE BEEN THE RESULT OF SPURIOUS RF INPUT SUCH AS THAT GENERATED BY A PAX LAPTOP COMPUTER OR THE MIXING OF 2 GND-BASED XMITTING SIGNALS THAT INTERFERED WITH THE FMS UPDATING SEQUENCE. IT IS MY HOPE THAT CONTINUING RESEARCH IS BEING DONE INTO THE POTENTIAL HAZARDS I BELIEVE EXIST DUE TO RF GENERATING DEVICES AND FMS SYS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR HAS A BACKGND IN ELECTRONICS AND IS CONVINCED THAT SOME INTERFERENCE WAS ON BOARD THE ACFT TO CAUSE THE TRACKING ERROR. HE FELT A LAPTOP COMPUTER; CD PLAYER; OR COMPUTER GAME COULD BE THE CAUSE OF THE NAV PROB. HE WENT ON TO FLY THIS SAME ACFT ANOTHER 3 LEGS THAT EVENING WITH NO OTHER PROBS WITH THE IRS OR FMC. BECAUSE THERE WERE NO OTHER PROBS; HE SURMISES THAT IT WAS A LCL PROB CAUSED BY A RADIO FREQ GENERATOR; SUCH AS THOSE LISTED ABOVE.