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.