What was found
AT FL380, WITH THE ENGINE & WING ANTI-ICE ON, RECEIVED AN IAS COMPARITOR LIGHT. A/S STARTED DECAYING TOWARD THE LWR FOOT. STARTED AN IMMEDIATE DESCENT TO FL340. LVL 2 SEL ELEV FEEL MAN. QRH DIRECTED REFERENCE TO FLT INSTEUMENTS > EMERG NON-ALERT PROCEDURE - AIRSPEED UNRELIABLE. COMPLIED WITH QRH. CADC 2 - INOP. JT CREW DEBRIEF: WAS IN HEAVY ICING CONDITIONS AND SUSPECTED POSSIBLE ICE BLOCKAGE. CADC 2 PASSED OPS/BITE TEST. MSC AIR DATA HEATER TEST PASSED. PITOT MOISTURE TRAPS IN E/E WERE DRY. DISC 2 PITOT LINE AT CADC QD FITTING AND BLEW DRY NITROGEN BACK THROUGH SYSTEM. INITIALLY HAD NO FLOW, BUT AFTER A FEW SECONDS A COUPLE CC'S OF COLD WATER BLEW OUT OF PITOT TUBE AND HAD GOOD FLOW. VERIF'D PITOT DRAINS WERE CLEAR BY NITROGEN FLOW AND VISUALLY. VERIF'D NO DEBRIS IN PITOT TUBE. TURNED ON PITOT HEAT AND VERIF'D PITOT HEAT OPS NML, PROBE GETS HOT. PERF AUTOLAND RTS OF #2 FCC, ALL TESTS PASSED. LLM STATUS REMAINS THE SAME. NO HIST 90 DAYS. REF AMM 34-16-01-4, 22-00-00-2, 34-10-00-2.