What was found
DIVERSION/DECLARED EMERGENCY-------------------------------------------------------------AT FL400 INDICATED AIR SPEED DISAGREE FLAG ON PRIMARY FLIGHT DISPLAY +- 5-6 KTS BETWEEN CAPTAIN/FIRST OFFICER/STANDBY AIR SPEED. DURING DESCENT CAPTAINS AIR SPEED GRADUALLY DECREASED TO 45KT, FIRST OFFICERS STANDBY INDICATED 250KT. CAPTAINS ALTITUDE 300-500FT HIGH. ELECTRONIC ENGINE CONTROL SYSTEM / SHIFTED TO ALTITUDE MODE. CAPTAIN AIR SPEED & ALTITUDE REMAINED ERRONEOUS UNTIL 5000-8000FT WHEN THEY SEEMED NORMAL. / FOUND FAULTY LEFT AIR DATA INERTIAL REFERENCE UNIT PER AMM.