BE90 PLT EXCEEDS ALT ASSIGNMENT OF 4000 FT ON DEP FROM ISP AND CONFLICTS WITH A BE55 ENRTE AT 5000 FT.

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THE KING AIR HAD FILED AN INITIAL IFR ROUTING. DEPARTED ISP RWY 6; ASSIGNED THE LONG ISLAND 3 DEP PROC CLBING TO 3000 FT. THE KING AIR RECEIVED A RADAR VECTOR (150 DEGS) TO JOIN THE AIRWAY. BTWN 2000-3000 FT; DEP CTL ASSIGNED 8000 FT. THE DEP CTLR REQUESTED A 'GOOD RATE' CLB SHORTLY AFTER THE ALT ASSIGNMENT. THE CLB RATE WAS INCREASED; IN EXCESS OF 1500 FPM; CLOSE TO 2000 FPM. AFTER PASSING 3000 FT; THE KING AIR RECEIVED A REASSIGNED ALT OF 4000 FT BY DEP CTL. A TA WAS MADE TO ANOTHER ACFT BY THE DEP CTLR SPECIFYING A KING AIR TYPE. THE DEP CTLR THEN PROVIDED A TA TO THE KING AIR OF A BARON AT 5000 FT (KING AIR ALT AT THE TIME WAS GREATER THAN 3500 FT). WX CONDITIONS WERE MIXED IMC/VMC BTWN LAYERS WITH AN ESTIMATED FLT VISIBILITY OF LESS THAN 3 MI; IN TURB. THE KING AIR PLT EXCEEDED THE ASSIGNED ALT WHILE LOOKING FOR TFC (4300 FT); AND INITIATED A DSCNT BACK TO 4000 FT. AFTER THE DSCNT WAS INITIATED; THE CTLR ADVISED THE KING AIR OF AN AUTOMATED ALERT. INITIAL LATERAL SEPARATION BTWN THE KING AIR AND THE BARON MIGHT BE ESTIMATED AT BTWN 5-10 NM (CONVERGING COURSE). THE REQUEST BY THE DEP CTLR FOR A 'GOOD RATE' OF CLB WOULD APPEAR TO BE AN ATTEMPT TO OBTAIN A VERT SEPARATION SOLUTION; BEFORE THE LOSS OF LATERAL SEPARATION BTWN THE KING AIR AND THE BARON. THE 4000 FT ALT REASSIGNMENT BY DEP CTL WOULD APPEAR TO BE AS A RESULT OF THE REALIZATION THAT THE 8000 FT 'GOOD RATE' PLAN WOULD NOT PROVIDE FOR POSITIVE SEPARATION FROM THE BARON AT 5000 FT. THE LOSS OF SEPARATION IS A RESULT OF THE INITIAL 8000 FT ASSIGNMENT ('BETTING ON THE COME'); AND SUBSEQUENT REQUESTS TO MAKE IT WORK ('FORCING THE BET'). THE AUTOMATED ALERT IS A RESULT OF AN ALT EXCURSION WHILE ATTEMPTING TO COMPLY WITH THE DEVELOPING CONFLICT RESOLUTION REQUESTS MADE BY ATC. HAD THE ALERT NOT OCCURRED; A POTENTIALLY DANGEROUS ATC OPERATIONAL PRACTICE WOULD NOT HAVE COME TO LIGHT. A LATE ALT REASSIGNMENT (WITHIN 1000 FT OF THE NEW ALT) IS NOT A GOOD OPERATIONAL PRACTICE; PARTICULARLY AFTER A 'GOOD RATE' REQUEST. THE LIKELIHOOD OF AN EXCURSION; PARTICULARLY IN TURB; IS THE CONSIDERATION. HAD COMS BEEN INTERFERED WITH ('STEPPED ON') THE PROB BECOMES MORE COMPLEX. IT HIGHLIGHTS WHY THE SEPARATION ERROR OCCURRED WITH THE INITIAL 8000 FT ASSIGNMENT TO THE KING AIR. TA'S ISSUED TO ACFT APCHING ASSIGNED ALTS SHOULD BE AVOIDED. THE PLT WILL BE DISTR DURING A CRITICAL PHASE (LEVELOFF) BY THE NEED TO IDENT CONFLICTING TFC VISUALLY. THE TA WITH THE ALT REASSIGNMENT WOULD HELP TO ALLEVIATE THIS PROB.