A CRJ700 HAD A TRACK DEVIATION AFTER TKOF WHEN THE FMS DID NOT FOLLOWED THE RNAV DEPARTURE BECAUSE THE FMS INHIBIT AND NAV MODES WERE NOT SELECTED.

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WE WERE FLYING THE RNAV DEP FLYING THE INITIAL HDG OF XYZ DEGS. THE FLYING CALLED FOR NAV MODE AT 400 FT AGL. I SELECTED NAV MODE AND NAV MODE WAS ARMED. I WAS BUSY TUNING THE RADIO AND TALKING TO DEP; RECONFIGURING THE ACFT; AND COMPLETING THE CLB CHKLIST. AS I RECALL WE WERE PASSING THROUGH ABOUT 4000 FT MSL; WHEN THE FO INFORMED ME THAT THE FMS WAS NOT NAVING. I LOOKED UP FROM MY CHKLIST AND SAW THAT WE WERE STILL ON THE INITIAL HDG AND HAD FLOWN THROUGH THE COURSE LEADING TO THE FIRST WAYPOINT. THE FLT DIRECTOR WAS STILL COMMANDING THE INITIAL HDG. I IMMEDIATELY INSTRUCTED THE FO TO TURN THE ACFT BACK TOWARD THE PROPER COURSE. I THEN IMMEDIATELY CALLED ATC AND INFORMED THEM THAT WE WERE UNABLE TO FLY THE RNAV DEP. ATC GAVE US A HDG OF 277 DEGS TO FLY AND WE COMPLIED. ATC ASKED US IF WE HAD AN EQUIP PROB. I REPLIED YES. LATER; ANOTHER VOICE ON THE DEP FREQ ASKED IF THE EQUIP PROB WAS DISCOVERED ON THE GND OR INFLT. WE WERE INSTRUCTED TO JOIN A RADIAL OFF THE VOR. WE DID SO IN GREEN NEEDLES. WERE THEN TURNED OVER TO CTR. ALTHOUGH THE FMS WAYPOINTS WERE NOT SEQUENCING; AND WE WERE NOT RECEIVING COMMAND BAR GUIDANCE; WE COULD SEE FROM THE MOVING MAP THAT WE WERE OFF COURSE. I INSTRUCTED THE FO TO MANUALLY CORRECT OUR COURSE. I THEN NOTIFIED ATC THAT WE WERE UNABLE TO FLY THE FMS DEP. I WAS LOOKING FOR A CAUSE AS TO WHY THE FMS WAYPOINTS WERE NOT SEQUENCING. I FOUND THAT THE FMS SEQUENCING WAS IN THE INHIBIT MODE. THERE IS NO WAY THAT I AM AWARE OF THAT WOULD HAVE CAUSED THIS OTHER THAN PUSHING THE INHIBIT BUTTON. THIS IS A BUTTON THAT IS NEVER USED AND CERTAINLY WAS NOT USED AT THIS TIME. IN ADDITION; I FOUND THE HDG MODE WAS SELECTED RATHER THAN THE NAV MODE. I AM CERTAIN THAT THE NAV MODE HAD BEEN SELECTED AND THAT IT HAD BEEN ARMED. WHATEVER GLITCH IN THE FMS THAT CAUSED IT TO GO INTO THE INHIBITED MODE ALSO CAUSED IT TO GO INTO THE HDG MODE. WHEN I RE-SELECTED THE AUTO SEQUENCE MODE; THE WAYPOINTS THAT WERE ALREADY BEHIND US DID NOT AUTOMATICALLY SEQUENCE. I HAD TO MANUALLY TAKE THE NEXT WAYPOINT THAT WAS AHEAD OF US AND MOVE IT TO THE ACTIVE WAYPOINT LINE. ALTHOUGH WE WERE NOW FLYING RAW DATA (GREEN NEEDLES); WE WATCHED TO SEE IF THE FMS WOULD RESUME NORMAL OP. IT DID. CLOSER VIGILANCE ON THE PART OF THE PF COULD HAVE RESULTED IN THE PROB BEING IDENTED SOONER. I DO NOT BELIEVE THERE WAS ANY LOSS OF SEPARATION.