MLG DEPARTS WITH UNCORRECTED COMPASS SYNCHRONIZATION.

Narrative

DEPARTED LGA GATE. SHORT TAXI TO RWY 31; #1 FOR IMMEDIATE TKOF; ACFT ON 3 MI FINAL APCH TO RWY 31. WIND OUT OF THE N AT ABOUT 15 KTS; R XWIND. DID NOT NOTE HSI RWY ALIGNMENT. TOOK OFF WITH ATC CLRNC TO TURN R TO 340 DEG AT APPROX 800 FT. DEP INSTRUCTED US TO TURN R TO 340 DEG. FO WAS FLYING ACFT. I LOOKED AT HSI READ 340 DEGS AND XCHKED RNI AT 340 DEGS; AND TOLD DEP THAT WE WERE HEADING 340 DEGS. ATC ADVISED THAT HE SHOWED US ON CTR LINE. I THEN CHKED MAGNETIC COMPASS WHICH INVOLVED USING A REARVIEW MIRROR MOUNTED ON GLARE SHIELD TURNING ON STANDBY COMPASS LIGHT; ALIGNING BOTH TO READ STANDBY CMPASS. READ 310 DEGS ON STANDBY COMPASS; RESET CAPT'S RMI; THEN INSTRUCTED FO TO MATCH HIS RMI TO MINE BY MATCHING RMI TO HIS HSI. THEN INSTRUCTED FO TO TURN R TO 340 DEGS AND NOTIFIED ATC THAT WE HAD A COMPASS MISALIGNMENT AND HAD CORRECTED PROB AND WERE NOW HDG 340 DEGS. ATC NOTED NEW HDG END OF EVENT. THERE IS A NOTE IN COMPANY ARPT ADVISORY PAGE WARNING AGAINST MAGNETIC IRREGULARITIES AFFECTING COMPASS HDGS DURING TKOF; PARTICULARLY FROM GATES A AND B ON SHORT TAXIES. OUR SHORT TAXI TIME DID NOT ALLOW ENOUGH TIME FOR THE COMPASSES TO RESYNCHRONIZE TO CORRECT HDG. IN COMMUNICATING WITH THE CHIEF PLT I DISCOVERED THAT THE JETWAYS NEED TO BE DEMAGNETIZED. THIS SHOULD BE DONE WITHOUT DELAY. I WOULDN'T WANT THIS SCENARIO TO BE REPEATED ON A COLD DARK WINTER NIGHT IN ICING CONDITIONS. NEEDLESS TO SAY I WILL TAKE THE TIME IN THE FUTURE TO CONFIRM COMPASS ALIGNMENT WITH RWY PRIOR TO TKOF. (SINCE BOTH COMPASSES SLEWED 30 DEGS TO THE R OF MAGNETIC HDG THE ACFT INST COMPARATOR SYS DID NOT ANNUNCIATE THE PROB).