TFC SPACING IS SO TIGHT THAT A GAR IS MADE WHEN THE PRECEDING ACFT DOES NOT CLR THE RWY AT THE EXPECTED TURN OFF.

Narrative

AT LGA; ON ARR WE WERE SEQUENCED FOR APCH BEHIND ANOTHER CARRIER'S AIRBUS; AND TOLD TO MAINTAIN 180 KTS TO OM. SPACING WAS ABOUT 2 1/2 MI ACCORDING TO TCASII (A GREAT TOOL TO USE FOR THIS). THIS SPACING DROPPED TO JUST A LITTLE OVER 2 MI. THIS; HOWEVER; IS TYPICAL THESE DAYS TO 'ENHANCE CAPACITY.' ON SHORT FINAL WE COULD SEE THAT THE PRECEDING ACFT HAD MISSED THE LAST TXWY AND WAS ROLLING TO THE END OF THE RWY. WE DISCUSSED AND READIED FOR A GAR. ON VERY SHORT FINAL; COMING UP OVER THE END OF THE RWY; WE COULD NO LONGER SEE ANYONE ON THE RWY; AND CONTINUED. AT ABOUT 50 FT IN THE FLARE; PWR REDUCING; THE TWR CTLR TOLD US TO GO AROUND. WE EXECUTED THE REJECTED LNDG; TOUCHING DOWN BRIEFLY PRIOR TO GOING AROUND. SUBSEQUENT CLB; APCH AND LNDG WERE UNEVENTFUL. TWR APOLOGIZED FOR LATE GAR INSTRUCTION. I THINK '2 MI' ON FINAL IS CAPACITY DRIVEN AND IS A BIT TOO CLOSE. ACFT ARE NOT VERY WELL ILLUMINATED AT NIGHT AND ARE DIFFICULT OR IMPOSSIBLE TO SEE A MI OR MORE AWAY (FOR PLTS AND CTLRS). THE RWY 'LOOKED CLR.' IN FACT; I PRESUME WE JUST COULDN'T SEE THE OTHER ACFT THAT FAR AWAY.