As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLT 6399 - IAD-BOS - DURING CRUISE, AIRCRAFT MADE AN UNSCHEDULED LANDING DUE TO CAPTS ASI OFF BY 15 TO 40 KNOTS. AIRCRAFT RETURNED TO GATE WITH NO FURTHER PROBLEMS. MAINTENANCE INSPECTION AND REMOVED AND REPLACED LEFT ADC, OPS CHECK OK. AIRCRAFT WAS RELEASED AND APPROVED FOR RETURN TO SERVICE. (M)