DC9 ACFT ON ILS APCH IN IMC; JUST PRIOR TO MARKER GS DEFLECTION; FULL DOWN AND THEN FULL UP. FLC DISCONNECTED AUTOPLT IMMEDIATELY AND MAINTAINED DSCNT RATE. FLC COMPLAINED TO TWR WHO SAID THE CAUSE WAS AN ACFT TAXIING IN FRONT OF THE XMITTER. GND CTLR ADVISED FLC THAT THE ILS XMITTER NEED NOT BE PROTECTED WHEN WX WAS AT LEAST 800 FT AND 2 MI VISIBILITY.

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I WAS THE CAPT OF AN ACR AIRLINES DC9 FROM MCI TO CLE. IT WAS MY LEG. THE ATIS RPTED THE WX TO BE WINDS 250 DEGS AT 19 KTS GUSTING TO 27 KTS; CEILING 9 OVCST; VISIBILITY 4 WITH MIST. THE APCH WAS THE ILS TO RWY 23L. APCH HAD INSTRUCTED US TO MAINTAIN 170 KTS TO THE MARKER AND CONTACT TWR THERE. JUST OUTSIDE THE MARKER; WE SWITCHED TO TWR FREQ; BUT SOMEONE WAS TALKING SO WE WERE UNABLE TO CHK IN. AT THE MARKER I CALLED FOR 'GEAR DOWN; LNDG CHKLIST.' IT WAS THEN THAT WE GOT FULL DOWN DEFLECTION OF THE GS FOLLOWED BY AN IMMEDIATE FULL DEFLECTION UP OF THE GS. IT THEN CTRED. FORTUNATELY; I DISCONNECTED THE AUTOPLT AS SOON AS THE GS STARTED TO TAKE ITS DIVE. I MAINTAINED MY RATE OF DSCNT DURING THE 3 SECONDS OR SO THAT THE SLOPE WAS ERRATIC. THERE WERE NO FLAGS. SINCE THIS HAPPENED AT 1600 FT AGL; YOU CAN UNDERSTAND HOW IT GOT OUR ATTN. WHEN WE WERE ABLE TO CHK IN WITH TWR TO OBTAIN LNDG CLRNC; WE COMPLAINED ABOUT IT AND THEY SAID IT HAPPENED BECAUSE SOMEONE TAXIED IN FRONT OF THE XMITTER. AFTER LNDG; I ASKED GND ABOUT THIS; AND THEY SAID THEY DON'T HAVE TO PROTECT THE ILS UNLESS THE WX IS LESS THAN 800 FT 2 MI. I CONSIDER THIS PRACTICE TO BE DANGEROUS! JUST BECAUSE IT'S LEGAL; DOESN'T MAKE IT RIGHT. AT A MINIMUM; THEY SHOULD WARN YOU THAT IT MIGHT HAPPEN. FIXING THIS KIND OF PROB IS RIGHT UP YOUR ALLEY. I WOULD BE GLAD TO DISCUSS THIS WITH ANYONE AT ANY TIME. I ALSO WAIVE MY ANONYMITY.