A BEECHCRAFT 400A AT FL370 EXPERIENCED LOSS OF ALL PRIMARY AND SUPPORTING FLT DISPLAYS. ACFT FLOWN TO FL200 ON STANDBY ATTITUDE INDICATOR AND STANDBY ALTIMETER. CAUSE UNKNOWN.

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I WAS THE PIC OF A BE400. WE WERE ENRTE FROM LIT TO GYY. THIS WAS LEG 5 OF 6 FOR THE DAY. THE ENTIRE TRIP WAS LAF-GYY-LIT-GYY-LAF. AT FL370; WHILE GOING AROUND SOME HVY TSTMS; WE EXPERIENCED LOSS OF ALL PRIMARY AND SOME SUPPORTING FLT DISPLAYS. THE INCIDENT STARTED WITH A YELLOW IAS ANNUNCIATOR ON BOTH PFD'S. IT TOOK US A FEW SECONDS TO NOTICE THAT THERE WAS A 20 KIAS DIFFERENCE BTWN THE 2 PFD AIRSPD INDICATORS. AT THE TIME; WE HAD BEEN IN IMC FOR 30+ MINS; WITH ENG ANTI-ICE ON. THE ONLY TEMP I REMEMBER AT FL370 WAS ISA +3 DEGS C. THE ONLY PRECIP WE HAD BEEN IN WAS A HVY RAIN SHOWER LEAVING GYY. AT NO TIME DID WE OBSERVE STRUCTURAL ICING OR DID THE 'ICING' LIGHT ILLUMINATE. SOON; BOTH AIRSPD INDICATORS WERE SHOWING A DECREASE IN SPD. ABOUT THIS TIME; WE WERE GIVEN A DSCNT TO FL330. DURING THE DSCNT; THE AIRSPD INDICATORS BOTH CONTINUED TO SHOW A DECREASE IN SPD; EVEN THOUGH I HAD LOWERED THE NOSE WITHOUT TOO MUCH OF A PWR REDUCTION. I DO NOT RECALL THE STANDBY AIRSPD INDICATIONS AT THIS POINT IN TIME. WHILE TRYING TO SORT THIS OUT; THE FOLLOWING HAPPENED IN FAIRLY RAPID SEQUENCE. I CHKED TO SEE THAT THE PITOT HEAT WHITE LIGHTS WERE ILLUMINATED. THEY WERE. BOTH AIRSPD INDICATORS ON THE PFD'S WENT TO ZERO. (ACTUALLY; LOW SCALE WITH THE RED FENCE SHOWING.) THE STANDBY AIRSPD INDICATOR WENT OFF SCALE LOW. THE ANGLE OF ATTACK GOT AN 'OFF' FLAG AND WENT OFF SCALE. THE DIGITS ON THE PFD'S FOR SHOWING AIRSPD AND ALT DISAPPEARED. BOTH PFD ATTITUDE INDICATOR PICTURES DISAPPEARED. THE HSI STILL DISPLAYED; BUT I'M NOT SURE IT WAS WORKING CORRECTLY. WE WERE GIVEN A LOWER ALT; FL240; BUT I FORGOT WHAT IT WAS SINCE THE DIGITS ON THE ALT PRESELECT ALSO DISAPPEARED AND I DSNDED TO AROUND FL225. THE AUTOPLT AND YAW DAMPER DISCONNECTED. THE DISCONNECT HORN SOUNDED CONTINUOUSLY AND COULD NOT BE SILENCED BY PUSHING ON THE TRIM BUTTON ON THE CTL WHEEL. I PULLED THE CIRCUIT BREAKER. DURING THIS TIME; THE CO-PLT WAS TRYING TO FIND THE LOSS OF ALL AIRSPD INDICATOR CHKLIST. THE ONLY FLT INSTS I WAS SURE WERE WORKING OK WERE THE STANDBY ATTITUDE INDICATOR AND THE STANDBY ALTIMETER. ATC DID NOT MENTION A LOSS OF SEPARATION. AROUND FL200; WE BROKE OUT OF THE CLOUDS AND; SHORTLY THEREAFTER; ALL SYS RETURNED TO NORMAL AND REMAINED THAT WAY FOR THE REST OF THE FLT. AFTER SYS RETURNED TO NORMAL; WE NOTICED THAT THE FLT TIMER ON THE PERFORMANCE PAGE SHOWED AN INCORRECT TIME FOR TKOF. THE TIME DISPLAYED WAS THE TIME THAT THE SYS RETURNED TO NORMAL; NOT THE TKOF TIME FROM LIT. SUBSEQUENT INVESTIGATION REVEALED NO LIGHTNING STRIKE DAMAGE; ALL PITOT; STATIC; AND ANGLE OF ATTACK HEATERS WORKED NORMALLY; NO WATER WAS FOUND IN THE PITOT/STATIC DRAINS; AND NO PROBS/ABNORMALITIES WERE FOUND IN THE AVIONICS SECTIONS IN THE NOSE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THIS ACFT IS EQUIPPED WITH 1/2 MILLION DOLLARS OF ELECTRONICS AND THEY ALL FAILED EXCEPT FOR THE STANDBY ATTITUDE INDICATOR AND STANDBY ALTIMETER. THE RPTR SAID MAINT CHKED EVERYTHING FROM WATER IN THE PITOT STATIC SYS TO PROBE AND STATIC PORT HEATING AND NO FAULTS WERE FOUND. THE RPTR STATED THE ELECTRONIC SYS WERE CHKED AND NO FAULTS FOUND. THE RPTR SAID THE MANUFACTURER IS COMING OUT WITH A NEW DESIGN PITOT STATIC PROBE BUT NOT SURE IF THIS COULD BE TO CORRECT PROBS LIKE THEY EXPERIENCED OR JUST A MORE ADVANCED DESIGN.