ATCT LCL CTLR CLRED AN MDT FOR TKOF RWY 31 WITH AN ACR B727 ON 2 MI FINAL FOR RWY 22. AS THE MDT BECAME AIRBORNE HE SAW THE B727 START A GAR AROUND APPROX ONE QUARTER MI FROM THE RWY 22 THRESHOLD. CTLR THEN TOLD THE B727 TO GAR AND THE MDT TO LEVEL OFF AT 200 FT. THE MDT PASSED DIRECTLY UNDER THE B727 AND MISSED BY APPROX 200 FT.

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LGA TWR CLRED US INTO POS AND HOLD RWY 31; EXPECT IMMEDIATE TKOF. 20 SECONDS LATER WE WERE CLRED FOR TKOF 'NO DELAY; TFC ON 2 MI FINAL' RWY 22; AN ACR Y B727. WE ACKNOWLEDGED STRAIGHT OUT DEP FROM TWR. ONCE AIRBORNE; I NOTICED ACR Y INITIATING A GAR AT 1/4 MI FROM THRESHOLD. AS WE ASCENDED THROUGH 100 FT AGL; A COLLISION COURSE WAS IMMINENT. TWR THEN ISSUED GAR TO ACR Y THEN SILENCE. I KNEW THAT EVASIVE ACTION WITH A HIGH THREAT OF SEVERE WAKE ENCOUNTER WAS NECESSARY. TWR VAINLY REQUESTED THAT WE LEVEL AT 200 FT AGL. WE WERE LEVELED ALREADY THEN FLEW DIRECTLY BENEATH THE ACR Y B727 200 FT BELOW. LUCKILY NO WAKE WAS ENCOUNTERED AND WE RESUMED ASSIGNED HDG OF 360 DEGS. CONTRIBUTING FACTORS: TWR RUSHING TO GET US AIRBORNE. MISJUDGED DISTANCE OF LNDG TFC. FAILED TO ASK US TO ABORT TKOF. ACR Y; B727; INITIATED GAR WITH XING DEP AIRBORNE CREATING COLLISION AND WAKE TURB HAZARD. POOR JUDGEMENT FROM ACR Y CREW. SLOW TO CLB ON GAR. CORRECTIVE ACTION POSSIBILITIES: ATC -- BETTER DISTANCE JUDGEMENT OF ACFT. MAKE DECISION TO HOLD TKOF IF ANY QUESTION OF ADEQUATE SEPARATION EXISTS. ACR Y -- BETTER KNOWLEDGE OF ACFT TKOF RUNS. FASTER DECISIONS TO GAR ADVISE; ETC. KNOWLEDGE OF EVASIVE ACFT MANEUVERS.