A REGIONAL CARRIER E120 FLC EXPERIENCED A DUAL ELECTRONIC INFO DISPLAY FAILURE. THE BACKUP SYS WAS SELECTED; AND FAILED; ALSO; IN VMC.

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THE EMB120 BRASILIA IS EQUIPPED WITH AN ELECTRONIC DISPLAY FOR EADI AND EHSI ON BOTH L AND R SIDES. 2 SEPARATE COMPUTERS OPERATE THEM TO PROVIDE THE DISPLAY PICTURE AND HDG/ATTITUDE INFO. IF NEEDED THE R COMPUTER CAN PROVIDE ATTITUDE/HDG INFO TO L SIDE AND VICE-VERSA. DISPLAY BACKUP TO EITHER SIDE CAN BE SELECTED FROM CTR MULTI-FUNCTION DISPLAY (MFD). AFTER TKOF FROM GPT INTO HVY RAIN; R EADI/EHSI SYMBOLS BEGAN TO DISTORT AND COMPRESS. I INSTRUCTED THE COPLT TO TURN OFF THE AUTOPLT AND SELECT DISPLAY FROM MFD. THIS WAS SUCCESSFUL. AFTER ENTERING HVY RAIN FOR APPROX 20 MINS WE ENTERED VFR CONDITIONS OVER MGM; AL; AT 25000 FT. UPON DSCNT INTO ATL; L SIDE EADI/EHSI BEGAN TO DISTORT AND BECOME COMPRESSED. NOW ONLY THE R SIDE (FEEDING FROM THE MFD) WAS USABLE. 10 MINS LATER THE R SIDE EHSI/EADI BECAME UNUSABLE DUE TO DISTORTION; AND NOW WHILE STILL VFR AND BEING VECTORED FOR A VISUAL APCH TO RWY 27L AT ATL; THE MFD SYMBOL DISPLAY FAILED. I LANDED THE ACFT USING VISUAL CLUES AND STANDBY GYRO FOR BACKUP ALONG WITH RMI (MECHANICAL). WE LOST BOTH PRIMARY DISPLAYS FOR EADI/EHSI AND THE BACKUP MFD. HAD THIS HAPPENED IN IFR THE OUTCOME COULD HAVE BEEN SERIOUS AS NO NAV INFO OTHER THAN MECHANICAL RMI FOR ADF AND VOR COULD BE USED. RMI WILL NOT DISPLAY LOC INFO. I HAVE BEEN FLYING THE E120 FOR 8 YRS AND HAVE ON OCCASION LOST 1 DISPLAY SOURCE; BUT NEVER ALL 3. MAINT DETERMINED EXCESSIVE WATER IN AVIONICS BAY CAUSED THE PROB. WHENEVER AN ACFT DESIGNER MENTIONS ELIMINATING THE STANDBY GYRO AND MECHANICAL RMI; SHOW THIS TO THEM. WITHOUT THESE BACKUP INSTS IN AN IFR SIT; I COULD NOT HAVE CTLED THE ACFT. FAILURES WITH AN 'EXTREMELY REMOTE' (10-9) FAILURE PROBABILITY (SEE AC 25.13091A) DO HAPPEN. DC10 HYD SYS; PITCH CTL DISCONNECTION; ETC. DON'T LET THEM ADD TO THE LIST. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR FEELS THAT DESIGN OF THE HYDS BAY ALLOWED WATER TO ENTER; THAT THIS WAS NOT A RESULT OF FAULTY MAINT. FLT IN HVY PRECIPITATION; ANYTIME; MAY SUBJECT THE ACFT TO THIS SORT OF FAILURE BECAUSE MOISTURE IN THE AVIONICS BAY CAN TAKE OUT THE PRIMARY AND REDUNDANCY EQUIP ALL AT ONCE. A LOCATION CHANGE OF THE BACKUP SYS MIGHT BE THE SOLUTION. IN LOW IFR CONDITIONS; ALL THAT REMAINS WITH THIS TYPE OF FAILURE IS A STANDBY GYRO AND A FIXED CARD ADF FOR AN INST APCH.