AN ACR MLG FOULED UP ITS LDA 18 APCH TO DCA.

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FLT WAS CLRED FOR ROSLYN LDA 18 APCH AT DCA FROM A HIGH AND FAST POS RESULTING FROM DELAYED ATC LET DOWN IN TERMINAL AREA. THE WX WAS RPTED 4000 BROKEN 5 HAZE 89/70 2110. THE LDA WAS CAPTURED AND THE FLT MODE ANNUNCIATOR INDICATED 'LOC TRK' AS WE DSNDED TOWARDS THE GS IN THE 'IAS' PITCH MODE WITH THROTTLES CLAMPED. ACFT NEVER SHALLOWED TO CAPTURE GS BUT INSTEAD REMAINED IN 'IAS' AND PITCHED DOWN THROUGH THE GS AT ABOUT 4 1/2 MI LDA DME. AUTOPLT WAS DISCONNECTED AND PWR APPLIED (AUTOTHROTTLE OVERRIDDEN) TO RETURN TO GS. ACFT FLEW OVER THE BUILDINGS IN ROSLYN AS WE TURNED TO THE N BANK OF THE RIVER FOR THE VISUAL PORTION OF THE APCH. CTRLINE WAS OVERSHOT BUT SAFELY REGAINED; RWY 18 WITH VASI CLRLY IN SIGHT. LNDG WAS IN THE TOUCHDOWN ZONE ABOUT 1500 FT FROM THRESHOLD. THE ROSLYN LDA 18 APCH IS A POOR APCH AT BEST; BUT HIGH AND FAST FURTHER EXAGGERATES THE PROB. WE FEEL THE PROB WITH THE GS MAY HAVE RESULTED FROM DSNDING IN IAS WITH THROTTLES CLAMPED. ALSO; THE CAPTURE POINT WAS PROBABLY APCHING THE AREA OF POOR GS SIGNAL AS THE APCH PLATE POINTS OUT 'GS UNUSABLE BELOW 1100.' WAITING TO 'FLY THE RIVER' AT 3.5 DME (OVER ROSLYN) IS A GOOD SET UP FOR AN OVERSHOOT WITH A R XWIND. WITH THE HAZY CONDITIONS AT THE TIME; IT MAY NOT BE THE BEST IDEA TO CONSTRUCT AN APCH CONTINGENT ON A VISUAL 'MANEUVER' REQUIRING A 30 DEG COURSE CHANGE AND RWY SET UP IN 3.5 MI. I CONSIDER DCA A MARGINAL ARPT AT BEST; BUT CONCEDE THAT THE APCH WOULD HAVE BEEN MORE SUCCESSFUL WITH AN EARLIER SET UP.