Narrative:

On takeoff from runway 26L in atl, assigned maintain 275 degree heading. Compasses checked on runway. Attitude/heading reference system (ahrs) was aligned at the gate and the runway was updated prior to taking the active. Rotating was normal and called for heading select. The nd yelled for slight left turn to 275 degrees. I looked out as I saw an aircraft departure, parallel runway 27R, as I followed the flight director. Obviously this was wrong, so I corrected back to the right as tower called up and asked us to turn right to 275 degrees. Both my nd and RMI were agreeing on the same heading, which I believe to be about 10 degrees off. I asked the first officer to check the wet compass and we were not able to see it due to cockpit glare. Mirrors and compass lights worked normally, but we could not see the heading. My belief is that both the ahar's were off as indicated by pilot's RMI and nd agreeing in an obvious 7-10 degree compass error. There were no flags and no erratic movement of compasses. We wrote the system up but could not duplicate! Normally if there is an error, it's pretty easy to find the correct system--not so in this case. Assigned heading on takeoff is important at all times. In this case, we were getting erroneous information with no flags and an inadequate wet compass.

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Original NASA ASRS Text

Title: REPORTER ALLEGES 7-10 DEGREE ERROR IN HIS COMPASS SYSTEM ON TKOF AT ATL.

Narrative: ON TKOF FROM RWY 26L IN ATL, ASSIGNED MAINTAIN 275 DEG HDG. COMPASSES CHKED ON RWY. ATTITUDE/HDG REF SYS (AHRS) WAS ALIGNED AT THE GATE AND THE RWY WAS UPDATED PRIOR TO TAKING THE ACTIVE. ROTATING WAS NORMAL AND CALLED FOR HDG SELECT. THE ND YELLED FOR SLIGHT LEFT TURN TO 275 DEGS. I LOOKED OUT AS I SAW AN ACFT DEP, PARALLEL RWY 27R, AS I FOLLOWED THE FLT DIRECTOR. OBVIOUSLY THIS WAS WRONG, SO I CORRECTED BACK TO THE RIGHT AS TWR CALLED UP AND ASKED US TO TURN RIGHT TO 275 DEGS. BOTH MY ND AND RMI WERE AGREEING ON THE SAME HDG, WHICH I BELIEVE TO BE ABOUT 10 DEGS OFF. I ASKED THE F/O TO CHK THE WET COMPASS AND WE WERE NOT ABLE TO SEE IT DUE TO COCKPIT GLARE. MIRRORS AND COMPASS LIGHTS WORKED NORMALLY, BUT WE COULD NOT SEE THE HDG. MY BELIEF IS THAT BOTH THE AHAR'S WERE OFF AS INDICATED BY PLT'S RMI AND ND AGREEING IN AN OBVIOUS 7-10 DEG COMPASS ERROR. THERE WERE NO FLAGS AND NO ERRATIC MOVEMENT OF COMPASSES. WE WROTE THE SYS UP BUT COULD NOT DUPLICATE! NORMALLY IF THERE IS AN ERROR, IT'S PRETTY EASY TO FIND THE CORRECT SYS--NOT SO IN THIS CASE. ASSIGNED HDG ON TKOF IS IMPORTANT AT ALL TIMES. IN THIS CASE, WE WERE GETTING ERRONEOUS INFO WITH NO FLAGS AND AN INADEQUATE WET COMPASS.

Data retrieved from NASA's ASRS site as of August 2007 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.