Date: 201508
Local time of day: 1201-1800
Location: ATL.Airport, GA
Flight conditions: VMC
Weather elements / visibility: 20
Light: Daylight
Aircraft: Large Transport; Low Wing; 2 Turbojet Eng
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Climb
Airspace: Class B ATL
ATC / advisory: Tower ATL
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Flight Instructor
Human factors: Human-Machine Interface; Distraction; Confusion; Troubleshooting
Second reporter function: Pilot Flying; Captain
Anomaly: Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented
Contributing factors: Aircraft; Procedure
Primary problem: Aircraft

Narrative: After a normal take off from Runway 8R at KATL; I instructed the Captain who was PM to select 'NAV' mode at 400 feet AGL; as per our procedures. I verified NAV TRK mode engaged. Shortly after; Tower asked if we were direct to the first RNAV waypoint on the departure; 'RONII'. I had the RNAV progress page up which displays the aircrafts lateral deviation from the course. It showed .2 NM right; not uncommon. The Captain indicated we were direct to the point. Tower then gave us heading to the northeast; and switched us to departure control. Just before initiating the turn; the lateral deviation moved to .3 NM right. Departure asked us if we were RNAV capable; and Captain replied we were. We stayed on the heading for a couple minutes; and eventually we were cleared to a waypoint further down the departure. No mention was ever made of possible deviation or that we were off course. Honestly; from our standpoint; it appeared to be a normal departure. ATC is indicating we were off course. Perhaps the RNAV was in error.Our aircraft do not have GPS; and use only IRU's and ground based NAVAIDs. Maybe GPS would enhance the capability of the RNAV and provide more precise information. Also; pilots should strive to maintain 0 cross track error.


Second narrative: On departure from Runway 8R in ATL; we were given 'RNAV to RONII; cleared for takeoff' (SUMMT9 RNAV departure). We performed the normal 'NAV (navigation) select' at 400 feet AGL; and then 'climb power; flaps up; VNAV' at 1;000 feet AGL.At that point; the Tower Controller asked if we were direct to RONII. I responded that we were direct to RONII and looked down at the FMS; PROGRESS PAGE 2 which was displayed on both my side and my first officer's side. It showed that the lateral deviation was 0.2 NM right of course. That is normal operation in this aircraft to fly the command bars and actually be off 0.2NM. The Tower Controller then instructed us to turn left to a 060 degree heading and then switched us to Atlanta Departure. During the turn to 060 degrees; the FMS; PROGRESS PAGE 2 showed only 0.3 NM right of course; then counted down to 0.0 NM when we crossed the original course; and then counted up to left of course as we crossed the course that was direct to RONII. When I checked in with Atlanta Departure; I informed the controller that we were given a heading. He asked if we were RNAV capable; and I responded that we were capable and earlier were showing direct to RONII. We were perplexed and only assumed that the tower controller gave us a preventative vector.


Synopsis: A non-GPS RNAV equipped aircraft departed on the ATL SUMMT9 RNAV flying with VNAV engaged and indicating a 0.2 NM lateral deviation. ATL asked if they were direct RONII; the FMS indicated they were but a vector was given.