Date: 201202
Local time of day: 1801-2400
Location: ATL.Airport, GA
Flight conditions: VMC
Aircraft: EMB ERJ 145 ER/LR
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Climb
Airspace: Class B ATL
ATC / advisory: Tower ATL
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Communication Breakdown; Confusion; Situational Awareness
Second reporter function: Pilot Flying; First Officer
Anomaly: ATC Issue All Types; Aircraft Equipment Problem Critical; Conflict Airborne Conflict; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Air Traffic Control Separated Traffic

Narrative: After verifying the correct runway and RNAV departure (8R; SUMMT 5) in ATL; and selecting the flight guidance panel to sub-mode; FMS and NAV; coupled to the First Officer's side. We were cleared for takeoff with a clearance to RNAV to RONII. I read back that clearance and we departed. At 500 FT we engaged the autopilot and the aircraft made a slight turn to the right; nothing excessive. The airplane then tracked towards RONNI and at acceleration altitude we cleaned up the airplane. Soon thereafter we were told by the Tower to make an immediate 15 degree left turn. We did. The Tower then asked us if we were showing to be off course. I told them that we were showing us on course. We were given another heading and then a re-intercept back to the RNAV departure. Remainder of the flight was uneventful.I believe there is a difference in our software and what ATC desires from us. The RNAV departures are telling us to fly a heading to an intercept and that's what the airplane will do; but; it's my belief that ATC wants us to track that heading as if it were a course. Not sure; but that's my thought. Another issue is that it's not uncommon in these airplanes to make an initial slight turn in the opposite direction in order to make a course intercept. Not very good when your trying to fly RNAVs.

Callback: The Captain twice repeated his belief that the FMS in his E145 fleet predictably makes a turn toward the course when programmed to intercept rather than continue on the heading until intercepting as required by the SID. He suggested a modest rear quartering crosswind from the Southwest may have exacerbated that tendency by drifting them to the north of the runway centerline during the period they were flying runway heading. It should; however; be noted the First Officer reported a crosswind from the north in his narrative.

Second narrative: In the vicinity of acceleration altitude; Tower told [Company 123; not our callsign] to turn left 15 degrees. We did not reply to or follow these initial instructions due to high workload and the wrong callsign being used. About 5-10 seconds later we were given the same instructions with the correct callsign and we were informed that we were south of course. The FMS; however; showed us to be on course. We complied; and Tower asked us our position. We said we showed on course and going to RONII. She said we were 'well south of course' and issued us a 070 heading; and we complied... We queried Departure about our position; and he showed us on course. We monitored green needles closely for the duration of the flight.This could have been prevented had we known our equipment was not going to work; thereby leading us to avoid flying an RNAV Departure. In the future; I will request that the PIC refuse RNAV departures.


Synopsis: An E145 flight crew climbing on a heading of 092 degrees to intercept course 079 into RONII per the SUMMT RNAV SID from ATL was advised of a course deviation by the Tower despite the fact their Nav Display showed them to be following the procedure as published.