HELPING A FO UNFAMILIAR WITH THE LGA EXPRESSWAY VISUAL FLY; A DISTR A320 CAPT FAILED TO CONTACT TWR PRIOR LNDG.

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I WAS THE CAPT ON A FLT BOUND FOR LGA. THE FO WAS THE PF. WHILE IN RADIO CONTACT WITH NEW YORK APCH; WE RECEIVED 3 RWY CHANGES WITHIN 40 MI OF LGA. I UPDATED THE MCDU WHILE THE FO FLEW THE ACFT. THE FINAL RWY CHANGE POINTED US TO RWY 31. THE XING RESTRS WERE SET AS APCH CTL CLRED US FOR THE EXPRESSWAY VISUAL APCH TO RWY 31. THE FO WAS UNFAMILIAR WITH THE ARPT; SO I PROCEEDED TO WATCH FOR TFC AND POINT OUT THE RWY. HE ASKED IF I WOULD CALL HIS TURN TO FINAL. I CALLED HIS TURN. DURING THE TURN TO FINAL APCH; HE ACQUIRED THE RWY; THEN ROLLED OUT OF THE TURN SO AS TO FLY THROUGH THE FINAL. I TOLD HIM TO CONTINUE HIS TURN AS HE WAS FLYING THROUGH FINAL. HE STEEPENED UP HIS BANK ANGLE AND WAS DECELERATING. I TOLD HIM TO WATCH HIS SPD; BANK ANGLE AND ALT. AT 500 FT AGL; THE ACFT WAS STABLE FOR LNDG. I CONTINUED TO MONITOR THE FO'S PROGRESS UNTIL 200 FT AGL WHEN I REALIZED I HAD NOT SWITCHED THE RADIO FREQ OR CONTACTED LGA TWR FOR A CLRNC TO LNDG. I CHKED THE XING RWY ALONG WITH OUR LNDG RWY AND SAW NO TFC; SO I ELECTED TO CONTINUE AND LET THE FO LAND. ON THE GND I SWITCHED THE FREQ FROM APCH TO TWR AND CONTACTED THE CTLR AND APOLOGIZED. HE STATED IT WAS NOT A PROB AND INSTRUCTED US TO TAXI TO OUR GATE. IF NY APCH HAD TOLD US TO CONTACT THE TWR; WE WOULD HAVE DONE IT SOONER. BECAUSE THIS IS A TIGHT TURN TO A 3 MI FINAL APCH; THE TWR NEEDS TO BE CONTACTED MUCH EARLIER. NEXT TIME I WILL INCLUDE THAT IN THE BRIEFING SINCE THE LATTER PORTION OF THIS APCH IS VERY BUSY; ESPECIALLY WHEN THE PF IS UNFAMILIAR WITH THE ARPT.