REPORTER POINTS OUT SEA SUMMA SID REQUIRES HIGH CLIMB RATE TO MAKE CROSSING RESTRICTION. TWO ACR-LGT'S HAD POTENTIAL CONFLICT DUE TO PRECEEDING CLIMBING SLOW AND DELAYING TURN AND THEN COMING INTO CONFLICT WITH ACFT FOLLOWING.

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THE SEATTLE 'SUMMA 2 DEP' CONTAINS A MARGINAL XING RESTRICTION AT HEAVY GROSS WTS AND HIGH AMBIENT TEMPS WHEN USING RWY 34 DEPS. DELAYING THE RIGHT TURN TO 070 DEGS BEYOND THE 8 DME FIX IN ORDER TO REACH 4000' WOULD ALSO PLACE THE ACFT LATE IN ITS SBND TRACK AND IN POSSIBLE CONFLICT WITH A CLOSELY SPACED SECOND DEP WHO HAD FLOWN THE ROUTING CORRECTLY. IN ADDITION TO THE POSSIBLE CHART CONFUSION IT CONTAINS NO CLB MINIMUM CRITERIA--PERHAPS A REVIEW OF THIS PROC MAY BE IN ORDER. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR FLT HAD NO TROUBLE COMPLYING WITH SID. PROB BECAME APPARENT TO THEM WHEN AFTER MAKING THE TURN TO 070 DEGS. THEY SAW AN ACFT COMING FROM THEIR LEFT WHICH TURNED OUT TO BE THE PRECEEDING LGT WHICH HAD TAKEN OFF VERY HEAVY AND HAD CONTINUED NBOUND UNTIL HE MADE THE XING ALT; THEN TURNED BACK. WHEN RPTR QUESTIONED TFC; DEP CTLR QUICKLY GAVE HIM A 'MAINTAIN ALT;' AS HE APPARENTLY HAD NOT BEEN WATCHING THE TRACK OR ALT OF PRECEEDING ACFT AND NOW HAD POTENTIAL CONFLICT AS THE SECOND LGT WAS OUTCLBING THE FIRST. RPTR POINTS OUT SID PROBABLY REQUIRES ABOUT 1000 FPM CLB FOR JETS TO MAKE XING AND HEAVY LOADED LGT CANNOT DO IT. SUGGESTIONS: SHOULD BE NOTED ON SID SHOWING CLB RATE NEEDED AND NOTED TO ADVISE IF CANNOT COMPLY.