AN ACR MD-80 CAPT RPTS MAGNETIC DISTURBANCES WHEN DEP RWYS 13 AND 22 AT LGA.

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WE WERE CLRED FOR A MASPETH CLB ON DEP FROM RWY 13 AT LGA. DURING TAXI OUT THAT WAS CHANGED TO A WHITESTONE CLB. AFTER CHKING RWY HDG WHILE IN POS ON RWY 13; MY HSI COMPASS HDG WAS REASONABLY NORMAL. DURING TKOF ROLL; ABOUT 1/2 WAY DOWN THE RWY; OUR HDG COMPARATOR LIGHT CAME ON. I THEN NOTICED BRIEFLY WHAT APPEARED TO BE A 15 DEG TO 20 DEG HDG SPLIT. WITH OUR FLT DIRECTORS AND AUTOPLT UNUSABLE AT THIS TIME; WE CONTINUED OUR TKOF AND DEP. WITH THIS WARNING LIGHT STILL ON; WE CONTINUED OUR DEP; AND ONCE AGAIN OUR DEP WAS AGAIN CHANGED. DURING THE TKOF ROLL; AN ABORTED TKOF WAS CONSIDERED BUT WITH GOOD WX; AND THIS PROB ONLY EXPECTED TO BE BRIEF; WE CONTINUED. WE COULD GENERALLY SEE OUR PLANNED DEP BY ORIENTING SOME GND CHK POINTS. ATC NEVER QUESTIONED ANY HDGS. SOME FACTORS ARE: ALL TKOFS FROM RWY 13 AT LGA; THAT I HAVE MADE RECENTLY; HAVE HAD COMPASS PROBS - MAGNETIC DEVS. OUR PROCS WARN US OF THIS PROB. I MAY HAVE HAD TO CONSIDER AN ABORT IF THE WX WAS 800 RVR? I AM SURE THAT I AM NOT THE ONLY ACFT TO HAVE HAD THESE PROBS. COMPLICATED DEP ROUTINGS. DEP CHANGED 3 TIMES. WORKLOAD - - AUTOPLT AND FLT DIRECTOR UNUSABLE WITH MISALIGNED COMPASSES. UNDER BAD WX/VISIBILITY; THIS CAN BE DANGEROUS. CAN THIS STEEL PIER MAGNETIC PROB BE CORRECTED? RAPID RATE OF CONTINUOUS TURNS NECESSARY ON THIS RWY 13 DEP DOES NOT ALLOW COMPASS SYS TO RESET ITSELF QUICKLY. (5 MINS IN OUR CASE). ATC MUST UNDERSTAND THIS PROB AND ALLOW FOR IT; EXACT COMPASS READING/ALIGNMENT SHOULD POSSIBLY BE MADE JUST BEFORE DEP ON THIS RWY. GENERALLY CORRECT; AS WE DID; MAY NOT BE GOOD ENOUGH! CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR IS A CAPT ON AN MD-80. HE HAS FLOWN SEVERAL TYPES OF ACFT INTO LGA INCLUDING THE DC-10 AND B727. HE STATES THAT ALL OF THESE ACFT HAVE SHOWN A SIMILAR TYPE OF MAGNETIC ANOMALY WHEN DEPARTING RWY 13 AND RWY 22. BOTH OF THESE RWYS START ON PIERS BUILT OUT INTO THE BAY. THE PIERS ARE CONSTRUCTED OF STEEL AND REINFORCED CONCRETE. THE RPTR'S ACR PUBLISHES A PAGE FOR HIS FLT MANUAL THAT CONTAINS A WARNING REGARDING POSSIBLE MAGNETIC DISTURBANCE. NEITHER THE COMMERCIAL; NOAA; NOR DEPT OF DEFENSE MANUALS CONTAIN ANY WARNING REGARDING MAGNETIC DISTURBANCE ON THESE RWYS AT LGA. THE RPTR WAS SURPRISED AND DISAPPOINTED TO FIND THAT THERE HAVE BEEN NO OTHER RPTS TO ASRS ABOUT THIS PHENOMENON. THE RPTR BELIEVES THAT THE MD-80 MAY BE MORE SUSCEPTIBLE TO THIS THAN OTHER ACFT THAT HE HAS FLOWN THROUGH LGA.