S80 FLC MAKES A GAR WHEN ACFT DEPARTING DOES NOT LIFT-OFF SOON ENOUGH. ON SECOND APCH CTLR INDICATES A LOW ALT ALERT WHEN FLC IS ON GS.

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ATC FACILITY -- DCA TWR. LOCATION -- RWY 18 FINAL DCA. FLYING INTO DCA ON THE ROSSLYN LDA RWY 18 APCH. LIGHT RAIN SHOWERS IN THE AREA. A B727 WAS CLRED FOR IMMEDIATE TKOF IN FRONT OF US. AT APPROX 10-20 FT THE B727 HAD NOT QUITE LIFTED OFF SO WE WERE INSTRUCTED TO GAR. CAPT; PF; EXECUTED A GAR; DURING WHICH TIME THE MAIN WHEELS TOUCHED THE GND. ABOUT THAT TIME TWR TOLD US; NO; NO CLRED TO LAND; CLRED TO LAND. AT THIS POINT; THE PWR WAS BACK UP AND THE CAPT WISELY CONTINUED THE GAR. WE CAME BACK AROUND IN DETERIORATING WX FOR ANOTHER ROSSLYN LDA RWY 18. JUST INSIDE MINIE TWR ADVISED LOW ALT ALERT; MINIMUM ALT IS XXX WE WERE ON THE GS ABOVE 1100 FT AND WERE BEGINNING TO MAKE VISUAL GND CONTACT AND CONTINUED. LATER INSIDE 3.5 DME WHILE FLYING VISUALLY TWR ASKED IN A CONCERNED WAY DO YOU HAVE THE RWY IN SIGHT? WE LANDED UNEVENTFULLY AND DID NOT HAVE THE OPPORTUNITY TO QUESTION HIM LATER. THE TWO CONCERNS I HAVE ARE SQUEEZING OUT DEPS WITHOUT ROOM AND TELLING US TO LAND AFTER INSTRUCTING A GAR. SHOULD WE HAVE TRIED TO LAND WE COULD HAVE HAD OUR HANDS FULL ON THAT SHORT A RWY. ALSO THE LOW ALT ALERT; WHY WHEN WE WERE ON LOC AND GS?