Narrative:

On takeoff runway 13 at lga, we got a heading annunciation about 1/2 way into the roll. A minor split was noticed on the headings, which became more pronounced after liftoff (approximately 15-20 degree difference). We split the difference for the initial heading of 175 degrees, I then intercepted the VOR radial on the coney climb. After about 2 mins, the heading difference resolved itself. Contributing factors were: this was an ahrs airplane that does not react well when taking off on a steel runway on piers. This problem is documented in the commercial charts. But it doesn't help us on takeoff roll! I believe our company slowed the slewing rate on the flux valves, but we still get heading errors on the pier. The only other options are to not fly ahrs aircraft into lga or to not require precise headings on departure -- an initial 'fly runway heading' would be preferable, since the flight directors command a heading hold after liftoff. Callback conversation with reporter revealed the following information: reporter stated the aircraft was a full glass cockpit MD88. However, the flux valves for the compass are in the wings, therefore, they are subject to the metal pilings in the runway. This type of MD88 is known to have ahrs installation for navigation. The ahrs stands attitude, heading, reference system. The compass heading always drifts off 5-20 degrees when using runway 13 or runway 22 at lga when using an aircraft with this system. It is sensitive to the metal in the runway. The aircraft has a slew switch to help align the compass, but the flight crew isn't always certain what the exact heading is to be. As the aircraft becomes airborne the magnetic compass system returns to normal in 2-5 mins. Reporter is concerned that a heading indicator that is not exact may let them drift off of the assigned departure.

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Original NASA ASRS Text

Title: AN MD88 ACFT WHICH IS AHRS EQUIPPED, SUFFERS A HDG CHANGE IN ITS COMPASS SYS AT LGA, NY. THIS IS DUE TO STEEL PIERS WHICH ARE BURIED IN THE RWY.

Narrative: ON TKOF RWY 13 AT LGA, WE GOT A HDG ANNUNCIATION ABOUT 1/2 WAY INTO THE ROLL. A MINOR SPLIT WAS NOTICED ON THE HDGS, WHICH BECAME MORE PRONOUNCED AFTER LIFTOFF (APPROX 15-20 DEG DIFFERENCE). WE SPLIT THE DIFFERENCE FOR THE INITIAL HDG OF 175 DEGS, I THEN INTERCEPTED THE VOR RADIAL ON THE CONEY CLB. AFTER ABOUT 2 MINS, THE HDG DIFFERENCE RESOLVED ITSELF. CONTRIBUTING FACTORS WERE: THIS WAS AN AHRS AIRPLANE THAT DOES NOT REACT WELL WHEN TAKING OFF ON A STEEL RWY ON PIERS. THIS PROB IS DOCUMENTED IN THE COMMERCIAL CHARTS. BUT IT DOESN'T HELP US ON TKOF ROLL! I BELIEVE OUR COMPANY SLOWED THE SLEWING RATE ON THE FLUX VALVES, BUT WE STILL GET HDG ERRORS ON THE PIER. THE ONLY OTHER OPTIONS ARE TO NOT FLY AHRS ACFT INTO LGA OR TO NOT REQUIRE PRECISE HDGS ON DEP -- AN INITIAL 'FLY RWY HDG' WOULD BE PREFERABLE, SINCE THE FLT DIRECTORS COMMAND A HDG HOLD AFTER LIFTOFF. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THE ACFT WAS A FULL GLASS COCKPIT MD88. HOWEVER, THE FLUX VALVES FOR THE COMPASS ARE IN THE WINGS, THEREFORE, THEY ARE SUBJECT TO THE METAL PILINGS IN THE RWY. THIS TYPE OF MD88 IS KNOWN TO HAVE AHRS INSTALLATION FOR NAV. THE AHRS STANDS ATTITUDE, HDG, REF SYS. THE COMPASS HDG ALWAYS DRIFTS OFF 5-20 DEGS WHEN USING RWY 13 OR RWY 22 AT LGA WHEN USING AN ACFT WITH THIS SYS. IT IS SENSITIVE TO THE METAL IN THE RWY. THE ACFT HAS A SLEW SWITCH TO HELP ALIGN THE COMPASS, BUT THE FLC ISN'T ALWAYS CERTAIN WHAT THE EXACT HDG IS TO BE. AS THE ACFT BECOMES AIRBORNE THE MAGNETIC COMPASS SYS RETURNS TO NORMAL IN 2-5 MINS. RPTR IS CONCERNED THAT A HDG INDICATOR THAT IS NOT EXACT MAY LET THEM DRIFT OFF OF THE ASSIGNED DEP.

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.