MDT HAS PITOT STATIC AIR DATA COMPUTER FAILURE.

Narrative

ACFT WAS CLRED FOR ENRTE DSCNT FROM 11000 FT TO 10000 FT BY GSO APCH CTL. AUTOPLT WAS ENGAGED. 10000 FT WAS SELECTED AND 1000 FPM DSCNT RATE SELECTED. AS ACFT APCHED 10000 FT ALTIMETER SUDDENLY WOUND DOWN TO 8700 FT AND PLT'S AIRSPD INDICATOR WENT TO 0 KIAS. XCHK OF COPLT'S INSTS SHOWED 230 KIAS 12700 FT MSL. XCHKED STANDBY ALTIMETER -- IT SHOWED APPROX 9000 FT MSL. QUERIED GSO APCH AS TO ALT. THEY INDICATED WE WERE AT 8700 FT MSL. WE ADVISED GSO OF OUR PROB AND CLBED BACK TO ASSIGNED ALT OF 10000 FT. WE DECIDED THAT WE HAD A PITOT STATIC AIR DATA COMPUTER FAILURE. CTL OF ACFT WAS TURNED OVER TO COPLT AS HE HAD OPERATING AIRSPD AND RELIED ON THE STANDBY ALTIMETER FOR ALT INFO. NO FURTHER PROBS ENCOUNTERED ENRTE TO CLT. AT APPROX 1500 FT AGL; SYS RETURNED TO NORMAL READINGS. PLT'S ALTIMETER HAD BEEN REPLACED AT MAINT BASE PREVIOUS NIGHT AND WAS SIGNED OFF AFTER SATISFACTORY STATIC SYS CHK. AS OF THIS TIME MAINT PERSONNEL HAVE NOT DETERMINED CAUSE OF PROB. VERY UNUSUAL TO LOSE AIRSPD ON ONE SIDE AND ALTIMETER ON THE OTHER. VERY DISTRACTING INITIALLY.