CAPT OF AN ACR MLG ACFT QUESTIONED THE SAFETY OF THE TWR SEPARATION STANDARD FOR INTERSECTING RWYS IN ALLOWING ACFT APCHING ON A 90 DEG ANGLE TO LAND IF THE OTHER ACFT HAS CLRED THE RWY INTXN.

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WE WERE CLRED FOR A VISUAL APCH TO RWY 22 AT LGA. AFTER WE CALLED THE ARPT IN SIGHT; WE WERE TOLD BY APCH TO KEEP OUR SPD UP. ABOUT 1000 FT FROM THE THRESHOLD OF RWY 22 I LOOKED TO THE R AND SAW A WDB ON SHORT FINAL TO RWY 13. I COULD HARDLY BELIEVE WHAT I SAW. I ADDED PWR BECAUSE IT LOOKED LIKE WE WERE GOING TO PASS IN FRONT OF HIM. AS WE CROSSED THE THRESHOLD THE ACR PLT ASKED 'IS ACR CLRED TO LAND?' WE TOUCHED DOWN A LITTLE LONG AND PASSED THE INTXN OF 13 AND 22. ATIS WAS RPTING LNDG ON RWY 22 TAKING OFF ON RWY 13. THE TWR OR APCH CTLR DID NOT TELL US ABOUT THE ACR LNDG TFC. ON THE GND AT THE GATE I TALKED TO THE WDB PLT AND HE SAID HE SAW US AND WAS PREPARED TO GAR (THE TWR DID CLR THEM TO LAND AFTER THE INQUIRY ON SHORT FINAL). THE TWR SUPVR TOLD ME THE CRITERIA FOR A LEGAL PROC AT LGA WAS THAT THE ACFT LNDG ON RWY 13 COULD NOT HAVE REACHED THE LNDG THRESHOLD UNTIL THE ACFT LNDG ON RWY 22 HAD CLRED THE INTXN OF THE 2 RWYS. THAT MEANS THEY CAN ALLOW ACFT TO APCH THE ARPT ON A 90 DEG COLLISION COURSE AS LONG AS ONE PASSES IN FRONT OF THE OTHER BY AT LEAST 1200 FT (THE DISTANCE FROM THE THRESHOLD OF 13 TO THE INTXN OF RWY 22). THE PROB HERE WAS NOT JUST THE CTLR'S FAILURE TO ADVISE US OF THE LNDG TFC ON RWY 13; THE PROB IS THE MINIMUM CLRNC THE CTLRS ARE ALLOWED. 1200 FT IS NOT ENOUGH WHEN THAT DISTANCE CAN BE COVERED IN 5 SECONDS AT 130 KTS. EVEN IF THE CTLR HAD ADVISED US; WE WOULD HAVE BEEN TOO FOCUSED ON THE OTHER ACFT TO SAFELY FLY THE APCH AND LNDG. THE SAME APPLIES TO THE OTHER ACFT. ALSO; THE ATTN OF THE CTLR IS HAMPERED BY SUCH CLOSE TOLERANCES. THAT IS WHY I BELIEVE THE CTLR DIDN'T TELL US ABOUT THE WDB.