Narrative:

Aircraft was parked at gate X. We departed on time at XA51 EDT, pushed off gate, and since there were no delays, started both engines and proceeded to runway 31 for immediate takeoff. Initial heading was 340 degrees. Heading bugs were set to 340 degrees and shortly after takeoff, first officer who was making the takeoff, started a right turn, but then came back left. I looked at my compass and saw the heading bug was at 340 degrees and it was over on the left, not right. My RMI also showed within 5 degrees of my compass, so assumed (incorrectly) that we were on the correct heading, as both of our compasses were within 5 degrees of each other. Just then ny departure asked where we were going. I replied that we were on a 340 degree heading, as assigned. He then stated that no, we were well left of that course and heading for ewr airspace. We started an immediate right turn and followed the hudson river until given a subsequent left turn to the southwest. By looking down at the runway confign at ewr I was able to determine that both our compasses were about 30 degrees off. First officer and I manually slewed the compasses to the correct heading and they both operated normally for the remainder of the flight to phl. Item was written up in aircraft log and maintenance was unable to find any problem with the compasses. Flight operated normally to atl. We were not suspecting any problems at flight originations, as we had been flying this same aircraft all day with no problems. This morning, at atl, I mentioned this to the captain who was going to take this same aircraft out, and he advised me that he had had the same problem at this gate at lga on about 3 previous flts, with different aircraft. He had made a similar report to the chief pilot at bos. We believe it is something to do with the gate X and gate Y area of the lga ramp. Something is causing the compasses to slew off course by 30 degrees, and if not enough taxi time is encountered to allow the compasses to reset themselves, we all need to be aware of this and to manually check and slew the compasses to the correct heading, if necessary, before takeoff. Perhaps a warning placed on the jetway or terminal building, clearly visible to the flight crew, and/or a note on the 10-T page of the manual would be a help. Apparently this has happened numerous times before. I don't know if it is a problem on the medium large transport, and medium large transport aircraft alone, or fleet-wide, but I would like to know if other types of aircraft have encountered this problem.

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Original NASA ASRS Text

Title: MLG TAKES OFF WITH A LARGE COMPASS ERROR CAUSING A 30 DEG COURSE DEV.

Narrative: ACFT WAS PARKED AT GATE X. WE DEPARTED ON TIME AT XA51 EDT, PUSHED OFF GATE, AND SINCE THERE WERE NO DELAYS, STARTED BOTH ENGS AND PROCEEDED TO RWY 31 FOR IMMEDIATE TKOF. INITIAL HDG WAS 340 DEGS. HDG BUGS WERE SET TO 340 DEGS AND SHORTLY AFTER TKOF, FO WHO WAS MAKING THE TKOF, STARTED A R TURN, BUT THEN CAME BACK L. I LOOKED AT MY COMPASS AND SAW THE HDG BUG WAS AT 340 DEGS AND IT WAS OVER ON THE L, NOT R. MY RMI ALSO SHOWED WITHIN 5 DEGS OF MY COMPASS, SO ASSUMED (INCORRECTLY) THAT WE WERE ON THE CORRECT HDG, AS BOTH OF OUR COMPASSES WERE WITHIN 5 DEGS OF EACH OTHER. JUST THEN NY DEP ASKED WHERE WE WERE GOING. I REPLIED THAT WE WERE ON A 340 DEG HDG, AS ASSIGNED. HE THEN STATED THAT NO, WE WERE WELL L OF THAT COURSE AND HDG FOR EWR AIRSPACE. WE STARTED AN IMMEDIATE R TURN AND FOLLOWED THE HUDSON RIVER UNTIL GIVEN A SUBSEQUENT L TURN TO THE SW. BY LOOKING DOWN AT THE RWY CONFIGN AT EWR I WAS ABLE TO DETERMINE THAT BOTH OUR COMPASSES WERE ABOUT 30 DEGS OFF. FO AND I MANUALLY SLEWED THE COMPASSES TO THE CORRECT HDG AND THEY BOTH OPERATED NORMALLY FOR THE REMAINDER OF THE FLT TO PHL. ITEM WAS WRITTEN UP IN ACFT LOG AND MAINT WAS UNABLE TO FIND ANY PROB WITH THE COMPASSES. FLT OPERATED NORMALLY TO ATL. WE WERE NOT SUSPECTING ANY PROBS AT FLT ORIGINATIONS, AS WE HAD BEEN FLYING THIS SAME ACFT ALL DAY WITH NO PROBS. THIS MORNING, AT ATL, I MENTIONED THIS TO THE CAPT WHO WAS GOING TO TAKE THIS SAME ACFT OUT, AND HE ADVISED ME THAT HE HAD HAD THE SAME PROB AT THIS GATE AT LGA ON ABOUT 3 PREVIOUS FLTS, WITH DIFFERENT ACFT. HE HAD MADE A SIMILAR RPT TO THE CHIEF PLT AT BOS. WE BELIEVE IT IS SOMETHING TO DO WITH THE GATE X AND GATE Y AREA OF THE LGA RAMP. SOMETHING IS CAUSING THE COMPASSES TO SLEW OFF COURSE BY 30 DEGS, AND IF NOT ENOUGH TAXI TIME IS ENCOUNTERED TO ALLOW THE COMPASSES TO RESET THEMSELVES, WE ALL NEED TO BE AWARE OF THIS AND TO MANUALLY CHK AND SLEW THE COMPASSES TO THE CORRECT HDG, IF NECESSARY, BEFORE TKOF. PERHAPS A WARNING PLACED ON THE JETWAY OR TERMINAL BUILDING, CLRLY VISIBLE TO THE FLC, AND/OR A NOTE ON THE 10-T PAGE OF THE MANUAL WOULD BE A HELP. APPARENTLY THIS HAS HAPPENED NUMEROUS TIMES BEFORE. I DON'T KNOW IF IT IS A PROB ON THE MLG, AND MLG ACFT ALONE, OR FLEET-WIDE, BUT I WOULD LIKE TO KNOW IF OTHER TYPES OF ACFT HAVE ENCOUNTERED THIS PROB.

Data retrieved from NASA's ASRS site as of July 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.