COMPASS SHIFTS 40 DEGS ON TKOF.

Narrative

ON TKOF ROLL AT APPROX 120 KTS COMPASS HDG ANNUNCIATOR CAME ON; COMPASS SPLIT 40 DEGS FROM #1 AND #2 AS WE ENTERED IMC CONDITIONS. I WAS NOT ABLE TO MAINTAIN ASSIGNED HDG BECAUSE OF THIS SIT -- WE ADVISED ATC; LEVELED WINGS AND SLAVED COMPASS TO MAGNETIC COMPASS. THIS HAS BEEN A PROB IN LGA; BECAUSE OF THE METAL PILING ON WHICH THE RWY 22 EXTENSION IS BUILT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED HE WAS FLYING AN MD88. THE MD88 HAS A GLASS COCKPIT BUT NO IRS. INSTEAD; THE MD88 HAS AN 'AHARS' WHICH IS THE ACRONYM FOR AUTOMATIC HDG AND ATTITUDE REF SYS AND ESSENTIALLY DOES SIMILAR CHORES TO WHAT THE INERTIAL REF SYS DOES. THE RPTR STATED THAT THOUGH THE ACFT WITH THE IRS; LIKE THE B767; ARE NOT IMMUNE TO THE MAGNETIC INTERFERENCE; THE MD88 SEEMS MORE SUSCEPTIBLE THAN THE OTHER ACFT SYS. THE PLTS AT HIS AIRLINE ARE MADE AWARE OF THIS PROB AND SIMILAR PROBS AT OTHER ARPTS LIKE RWY 01 AT LOUISVILLE; KY; AND RWY 22R AT NEWARK. ON THE MD88 THE FLT DIRECTOR BIASES OUT OF VIEW WHEN THERE IS A DISCREPANCY BTWN HDG INDICATORS. BEING THERE ARE ONLY 2; THE FLT DIRECTOR IS UNABLE TO DETERMINE WHICH IS CORRECT. WHEN COMPASS DEVS OCCURRED BEFORE; THE ACFT WAS STOPPED ON THE IRON PIER AND THE COMPASS COULD BE RESET. IN THIS CASE THE ANOMALY OCCURRED AT 120 KTS. DUE TO CONTINUOUS ATC ASSIGNED TURNS THE WINGS COULD NOT BE LEVELED LONG ENOUGH TO RECONCILE HDG INDICATIONS FROM THE MAGNETIC COMPASS; WHICH IS VIEWED THROUGH A MIRROR BEHIND THE COPLT; UNTIL ABOUT 12000 FT ON THE CLB. THERE IS NOTHING HE WOULD DO DIFFERENTLY AND IN ABSENCE OF CORRECTING THE PROB HE WOULD NOT CHANGE ANY ATC PROCS.