CRJ200 FLT CREW RADAR ALTIMETER FAILURE CAUSES HIGH VOLUME WARNING HORN TO SOUND DURING DSCNT INTERFERING WITH COMS.

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CAPT SIDE RADIO ALTIMETER WAS DEFERRED THE NIGHT BEFORE. SHOWED UP IN THE MORNING AND THE RA HAD BEEN FIXED AND MEL HAD BEEN CLRED. AFTER TKOF AT 7000 FT; I HEARD A VERBAL WARNING (MAYBE IT WAS TERRAIN) AND THEN NOTICED MY RA GOING TO 0. IT WENT TO 0 AND STAYED THERE AND SLIDE RULE INDICATING (R SIDE OF ALT INDICATOR) THAT WE WERE ON THE GND. JUST THOUGHT THAT MY RA FAILED AGAIN; RAN THE EMER CHKLIST ON IT (SAID NO FURTHER PLT ACTION; AND THOUGHT IT WAS A NO BIGGIE -- IT WASN'T GOING TO AFFECT THE FLT AND JUST WRITE IT UP WHEN WE GOT TO ZZZ.) WELL; WHEN WE WERE ON THE ARR INTO ZZZ; THEY ISSUED A DSCNT FROM FL230 TO 11000 FT. AS WE STARTED DSNDING; A HIGH VOLUME WARNING HORN SOUNDED CONTINUOUSLY. THE SOUND MADE EFFECTIVE COM IMPOSSIBLE. I HAD TO TURN UP THE VOLUME OF MY INTERCOM AND RADIO AND STILL COULDN'T MAKE OUT ANYTHING. I ASKED FOR AND RECEIVED A LEVELOFF FROM ATC AT FL200. ONCE WE LEVELED OFF; THE WARNING SOUND STOPPED. FINALLY WE COULD COMMUNICATE AGAIN. WE DISCUSSED WHAT WAS GOING ON AND WHAT THE SOUND WAS AND HOW TO STOP IT FROM HAPPENING AGAIN. I KNEW IT HAD TO DO WITH MY RA GOING BAD AND THERE WAS NO OTHER PROC IN THE EMER CHKLISTS UNDER THE RA OR LOUD HORN. WE BOTH HAD DISCUSSED THE GEAR HORN BUT I DIDN'T THINK IT WAS BECAUSE OF THE PWR SETTING (I SPECIFICALLY REMEMBER LOOKING AT THE PWR LEVER DURING THE DSCNT -- IT WAS WELL ABOVE IDLE; ABOUT 1/4 WAY IN ITS ARC OF TRAVEL). WE ALSO BRIEFLY DISCUSSED PUSHING IN THE DCU BUTTONS BUT I THOUGHT THAT WAS A BAD IDEA. I CONTACTED MAINT FOR GUIDANCE AND TOLD THEM OF THE PROB. ATC ISSUED ANOTHER DSCNT AND WE DSNDED DOWN TO FL180. I WANTED TO SEE IF THE HORN WOULD KEEP GOING OFF DURING THE NEXT DSCNT; WHICH IT DID. AGAIN; COM WAS NIL DURING THE WARNING SOUND. I TALKED TO MAINT AGAIN WHEN WE LEVELED OFF AND THEY ADVISED US WHAT CIRCUIT BREAKERS WOULD BE PULLED AFTER DEFERRING THE RA -- K7 AND L-SOMETHING. I LOOKED AT K7; WHICH SAID CAPT RA; AND KNEW THAT IF PULLED; THE SOUND WOULD GO AWAY. I KNOW WE ARE NOT SUPPOSED TO PULL CIRCUIT BREAKERS UNLESS IN THE CHKLIST; BUT I THOUGHT OF A COUPLE THINGS: THE SOUND WAS SERIOUSLY CAUSING MISCOM; PULLING THE CIRCUIT BREAKER WAS NECESSARY FOR THE SAFE CONTINUATION OF THE FLT; KNEW THE RA WAS ITS OWN LITTLE SYS AND THE SYS WASN'T REALLY NEEDED FOR A SAFE FLT (IT DIDN'T HAVE A LOT OF IMPACT ON OTHER SYS). PULLING THE BREAKER DID SILENCE THE WARNING SOUND AND THE FLT THEN ENDED WITHOUT INCIDENT. WHEN WE GOT IN TO THE GATE AND SHUT DOWN THE ENGS; WE BOTH HAD TIME TO THINK (NOT IN THE HEAT OF THE MOMENT; INFLT WITH A GLARING HORN GOING OFF). WE TALKED ABOUT THE HORN AND REALIZED THAT IT WAS PROBABLY THE GEAR HORN. UP IN THE AIR; I DIDN'T THINK IT WAS BECAUSE THE PWR WAS NOWHERE NEAR IDLE. PLUS I HAD FORGOTTEN ALL ABOUT THE HORN SAFETY SWITCH. IT'S GUARDED AND NEVER USED AND QUICKLY FORGOTTEN ABOUT AFTER YOU ARE OUT OF TRAINING FOR A WHILE (IT'S ONLY USED IN TRAINING AND THAT IS DURING ENG FAILURES WITH A PWR LEVER BEING AT IDLE). AND I ALSO REALIZED THAT THE SYS DIDN'T COMPLETELY FAIL (DIDN'T HAVE A RED; BOXED RA DISPLAYED). THE RA JUST INDICATED 0 SO IT THOUGHT IT WAS AT GND LEVEL OR SLIGHTLY ABOVE; AND THAT EVERY TIME YOU WERE IN A DSCNT; THE SYS THOUGHT YOU WERE JUST ABOVE THE GND WITHOUT THE GEAR EXTENDED. THE EMER CHKLIST HAD NO GUIDANCE FOR THIS OCCURRENCE. I THOUGHT WE DID THE BEST WE COULD DURING THE OCCURRENCE. IN THE HEAT OF THE MOMENT; WITH A HORN SOUNDING IN YOUR EARS; LITTLE TO NO COM; AND HAVING A HARD TIME THINKING BECAUSE OF THE SOUND; I THINK WE MADE THE BEST DECISION. IN HINDSIGHT; WHEN JUST SITTING THERE AT THE GATE AND HAVING PLENTY OF TIME TO TALK AND COMMUNICATE; IT IS A LOT EASIER TO FIGURE OUT WHAT IS GOING ON.