CAPT COMPLAINT ABOUT THE WING FLAP POS LIMITATION IMPOSED BY AN ENG 'HUSH KIT' ON A DC9 ACFT.

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DURING PREFLT I DISCUSSED THE WX AT CID WITH MY DISPATCHER. WX AT MINIMUMS; SNOW; BLOWING SNOW; FOG; WIND 040 DEGS AT 12 KTS; MODERATE ICING BELOW 8000 FT. RWY RPTEDLY PLOWED AND SANDED. AS AN EXPERIENCED DC9 PLT; I TOLD MY DISPATCHER THAT A FLAPS 50 DEGS LNDG SHOULD BE PLANNED AND HE AGREED. UNFORTUNATELY OUR ACFT WAS THE FIRST AND ONLY FLEET DC9-30 WITH NEW STAGE III HUSH KIT; REQUIRING USE OF FLAPS 40 DEGS. ALSO; A CURRENT FIF BULLETIN INDICATED THAT FLAPS 50 DEGS COULD BE USED IN AN EMER. DISPATCHER AND I DISCUSSED THE FACT THAT IT WOULD BE RIDICULOUS TO DISPATCH A FLT INTO A KNOWN EMER; AND HE CALLED PLANNING DEPT TO GET US A DIFFERENT DC9. NO ALTERNATE ACFT WAS AVAILABLE AND WHEN MAINT WAS COMPLETED; I AGREED THE FLT COULD BE DISPATCHED SAFELY PLANNING LNDG FLAPS 40 DEGS. ICING CONDITIONS ENRTE AND ON APCH; WE THOROUGHLY BRIEFED FOR APCH TO MINIMUMS; INCLUDING POSSIBLE GAR AFTER TOUCHDOWN. WE LANDED WITHOUT INCIDENT; HOWEVER; THIS EVENT RAISED MANY SERIOUS QUESTIONS. WASN'T THE DC9 DESIGNED WITH FLAPS 50 DEGS FOR A REASON? ARE THESE REASONS NO LONGER VALID? DID EXPERIENCED LINE PLTS HAVE ANY INPUT INTO HUSH KIT DESIGN COMPROMISES? IN MY VIEW; DC9 RWY PERFORMANCE STANDARDS ARE CLRLY COMPROMISED IF NOISE CERTIFICATION STANDARDS TAKE AWAY THE PLT'S ABILITY TO SELECT FULL 50 DEGS FLAPS IN MANY CASES. PLTS SHOULD BE ABLE TO KEEP THE 50 DEGS FLAP OPTION WHEN NEEDED REGARDLESS OF STAGE III NOISE STANDARDS.