A BE20 CPR FLC WAS PERFORMING THE KENNEDY 7 DEP (IDLEWILD CLB) AND FOUND THAT THEY WERE UNABLE TO STAY WITHIN THE 2 PT 5 DME RESTR WHILE ALSO AVOIDING TURNING BEFORE 400 FT OR THE ARPT BOUNDARIES.

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CLRED TO DEPART RWY 31L WITH THE JFK 7 DEP; IDLEWOOD CLB. THE SID REQUIRED A R TURN TO 090 DEGS WITHIN 2.5 DME OF JFK VOR. AFTER LIFTOFF; WE CLBED RWY HDG TILL 400 FT AGL; THEN BEGAN A R TURN. UPON COMMENCING THE TURN; IT BECAME APPARENT WE WOULD NOT MAKE THE 2.5 DME RESTR AND ADVISED ATC. THE CTLR GAVE US AN IMMEDIATE L TURN DIRECT TO A VOR; THEN CANCELED THAT CLRNC AND DIRECTED US TO MAKE AN IMMEDIATE R TURN TO 110 DEGS WITH A GOOD RATE OF TURN. THE CTLR PROCEEDED TO WARN US TO LET CLRNC DELIVERY KNOW IF WE CANNOT MAKE THAT RESTR ON ANY FUTURE FLTS. IN THE INTEREST OF SAFETY AND OPERATIONAL PROCS; WE DO NOT INITIATE ANY TURNS BELOW 400 FT AGL. IN THIS CASE; BECAUSE THE VOR IS AT THE APCH END OF RWY 31L; THE ACFT IS ALMOST TO 2.5 DME BY THE TIME YOU REACH 400 FT AGL. IT IS A VERY DIFFICULT RESTR TO COMPLY WITH AND IF WE HAD REALIZED JUST HOW DIFFICULT IT WAS; WE MOST CERTAINLY WOULD HAVE ADVISED CLRNC DELIVERY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CAPT SAID THAT HE DID NOT REALIZE THAT THE SID WAS PRACTICALLY IMPOSSIBLE TO PERFORM WITHOUT VIOLATING ONE RULE OR ANOTHER SO HE DECIDED TO FLY AS PER HIS UNDERSTANDING OF THE ACFT'S NORMAL OPERATING PROC. HE ALSO DISCUSSED THE NORMAL REQUIREMENT OF NO TURNS BEFORE XING THE ARPT BOUNDARIES AND NOT OVERFLYING ACFT OR TERMINALS AND HE FEELS NOW THAT THE 2.5 DME RESTR SIMPLY CANNOT BE MET AND SHOULD BE REMOVED. HE SAID THAT IF HE RECEIVES THIS DEP IN THE FUTURE HE WILL; ROUTINELY; ADVISE ATC THAT HE CANNOT MEET THE 2.5 DME REQUIREMENT.