Narrative:

While waiting for takeoff at klga runway 13 the first officer side had a heading warning flag; first officer went to dr mode and as we were cleared to position and hold 13 the ca side got a heading warning flag. I put his into dr mode and we aligned the runway with the correct heading via mag compass. We were cleared for takeoff with no issues. On the departure (lga 4 whitestone climb) we executed the first and second turn in dr mode which was still correct with the magnetic compass. The captain called for the climb checklist [while] on the 040 heading approximately 4000-5000 MSL. As I was completing the checklist ATC issued us a 290 heading. While we were turning I was finishing the checklist where I put both compasses back to magnetic mode. Both primary compasses slewed while we were in the turn and when we rolled out we were almost 60 degrees off heading. ATC asked what our heading was [while] both primary indicators read 290; to which I responded 290 heading. ATC said they were showing 230 heading. We crosschecked against the magnetic standby compass which read about a 240 heading. The captain started an immediate turn to the right to correct while I put both primary heading indicators back to dg mode and slewed back to the proper heading. After about 5 minutes I attempted just my side heading indicator back to magnetic mode where it slewed about 20 degrees off. It took about 10 minutes to get both headings to match and stop precessing off actual heading.the magnetic variation issues at the departure end of lga runway 13 caused a larger issue with the heading indicator than we both realized. Only move one at a time and crosscheck against magnetic compass.

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Original NASA ASRS Text

Title: A CRJ-700 flight crew encountered magnetic heading anomalies after departure from Runway 13 at LGA.

Narrative: While waiting for takeoff at KLGA runway 13 the FO side had a HDG warning flag; FO went to DR mode and as we were cleared to Position and Hold 13 the CA side got a HDG Warning flag. I put his into DR mode and we aligned the runway with the correct heading via Mag compass. We were cleared for takeoff with no issues. ON the departure (LGA 4 Whitestone Climb) we executed the first and second turn in DR mode which was still correct with the Magnetic Compass. The captain called for the Climb Checklist [while] on the 040 heading approximately 4000-5000 MSL. As I was completing the checklist ATC issued us a 290 heading. While we were turning I was finishing the checklist where I put both compasses back to MAG mode. Both primary compasses slewed while we were in the turn and when we rolled out we were almost 60 degrees off heading. ATC asked what our heading was [while] both primary indicators read 290; to which I responded 290 heading. ATC said they were showing 230 heading. We crosschecked against the Magnetic Standby Compass which read about a 240 heading. The captain started an immediate turn to the right to correct while I put both primary HDG indicators back to DG mode and slewed back to the proper heading. After about 5 minutes I attempted just my side HDG indicator back to MAG mode where it slewed about 20 degrees off. It took about 10 minutes to get both headings to match and stop precessing off actual heading.The magnetic variation issues at the departure end of LGA runway 13 caused a larger issue with the HDG indicator than we both realized. Only move one at a time and crosscheck against magnetic compass.

Data retrieved from NASA's ASRS site 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.