Narrative:

Regular flight from atl to ZZZ1. WX was VFR and our clearance was atl.COKEM1 as filed. We ran normal checklists and taxied out to runway 26L. By this time I had checked in the FMS a couple of times to see that runway 26L was loaded into my FMS and watched my captain check his. Both were loaded correctly (universal FMS clearance page said 26L.COKEM1). Our cdis were both set up to receiving course information from our respective FMS (pink needles). While holding short of the runway; atl tower asked us to verify our first waypoint. I replied with crous. We taxied into position and were cleared for takeoff. During the climb; both flight directors commanded initially a 272 degree heading to comply with the climb out. At 500 ft AGL we engaged the autoplt with 'flaps up; climb thrust; low bank off; yaw damp on; and autoplt on.' about that time (about 8-10 miles before reaching crous) both flight directors commanded a hard turn towards the north away from the course. I disconnected the autoplt and began to hand fly the departure until we could figure out whether we should follow the course depicted on both of our mfds or the flight directors (which were still commanding a hard turn to the north). The captain made his initial call to atl departure. The controller responded and told us that we were 1.5 miles north of course. We then corrected back and proceeded direct to crous. We were later instructed to call atl departure control over the phone; so we did. We spent the remainder of the flight reviewing RNAV departure procedures outlined in our company's gom and poh. We also rechked the flight plan that was entered into both the FMS' to try and figure out why this had happened. We had set up the departure correctly; never changed it; engaged the autoplt at the appropriate time and for some reason the both flight directors commanded a turn to the north so autoplt began to turn off course well before crous (approximately 8 to 10 miles prior). The rest of this information is what was told to me by the captain; I did not speak to atl departure personally. When the captain called atl; he told them what had happened. Atl replied and said to him that they had been having a lot of problems with all embraer type aircraft (including the 145 type and the 170 type) leaving atl on the RNAV departures.

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Original NASA ASRS Text

Title: AN EMB135 DEPARTING ON THE ATL RWY 26L COKEM1; DEVIATED N WHEN THE DUAL FMCS ERRONEOUSLY COMMANDED TURNS OFF OF THE DEP.

Narrative: REGULAR FLT FROM ATL TO ZZZ1. WX WAS VFR AND OUR CLRNC WAS ATL.COKEM1 AS FILED. WE RAN NORMAL CHKLISTS AND TAXIED OUT TO RWY 26L. BY THIS TIME I HAD CHKED IN THE FMS A COUPLE OF TIMES TO SEE THAT RWY 26L WAS LOADED INTO MY FMS AND WATCHED MY CAPT CHK HIS. BOTH WERE LOADED CORRECTLY (UNIVERSAL FMS CLRNC PAGE SAID 26L.COKEM1). OUR CDIS WERE BOTH SET UP TO RECEIVING COURSE INFO FROM OUR RESPECTIVE FMS (PINK NEEDLES). WHILE HOLDING SHORT OF THE RWY; ATL TWR ASKED US TO VERIFY OUR FIRST WAYPOINT. I REPLIED WITH CROUS. WE TAXIED INTO POSITION AND WERE CLRED FOR TKOF. DURING THE CLB; BOTH FLT DIRECTORS COMMANDED INITIALLY A 272 DEG HDG TO COMPLY WITH THE CLB OUT. AT 500 FT AGL WE ENGAGED THE AUTOPLT WITH 'FLAPS UP; CLB THRUST; LOW BANK OFF; YAW DAMP ON; AND AUTOPLT ON.' ABOUT THAT TIME (ABOUT 8-10 MILES BEFORE REACHING CROUS) BOTH FLT DIRECTORS COMMANDED A HARD TURN TOWARDS THE N AWAY FROM THE COURSE. I DISCONNECTED THE AUTOPLT AND BEGAN TO HAND FLY THE DEP UNTIL WE COULD FIGURE OUT WHETHER WE SHOULD FOLLOW THE COURSE DEPICTED ON BOTH OF OUR MFDS OR THE FLT DIRECTORS (WHICH WERE STILL COMMANDING A HARD TURN TO THE N). THE CAPT MADE HIS INITIAL CALL TO ATL DEP. THE CTLR RESPONDED AND TOLD US THAT WE WERE 1.5 MILES N OF COURSE. WE THEN CORRECTED BACK AND PROCEEDED DIRECT TO CROUS. WE WERE LATER INSTRUCTED TO CALL ATL DEP CTL OVER THE PHONE; SO WE DID. WE SPENT THE REMAINDER OF THE FLT REVIEWING RNAV DEP PROCS OUTLINED IN OUR COMPANY'S GOM AND POH. WE ALSO RECHKED THE FLT PLAN THAT WAS ENTERED INTO BOTH THE FMS' TO TRY AND FIGURE OUT WHY THIS HAD HAPPENED. WE HAD SET UP THE DEP CORRECTLY; NEVER CHANGED IT; ENGAGED THE AUTOPLT AT THE APPROPRIATE TIME AND FOR SOME REASON THE BOTH FLT DIRECTORS COMMANDED A TURN TO THE N SO AUTOPLT BEGAN TO TURN OFF COURSE WELL BEFORE CROUS (APPROX 8 TO 10 MILES PRIOR). THE REST OF THIS INFO IS WHAT WAS TOLD TO ME BY THE CAPT; I DID NOT SPEAK TO ATL DEP PERSONALLY. WHEN THE CAPT CALLED ATL; HE TOLD THEM WHAT HAD HAPPENED. ATL REPLIED AND SAID TO HIM THAT THEY HAD BEEN HAVING A LOT OF PROBS WITH ALL EMBRAER TYPE ACFT (INCLUDING THE 145 TYPE AND THE 170 TYPE) LEAVING ATL ON THE RNAV DEPS.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.