What was found
FL 240, LEVEL 2, SEL ELEV FEEL LIM, SEL FLAP LIM OVRD IAS COMPARATOR FLAG. CAPT A/S CONTINUALLY 20 KTS LESS THAN F/O AND STBY SY STEM. RAN CHECKLIST FOR AIRSPEED LOSS OR ERRATIC. EMERGENCY DECLARED R & R NR 1 ADC IAW AMM 34-16-01-000-801. PERFORMED OPS CK OF NR 1 ADC IAW AMM 34-16-01-700-801. FOUND FAULT CODE "0" ON EXT SENSOR FAULT. PERFORMED PITOT/STATIC CK IAW MM 34-11-00-700-801, ADC NR 1 CKS OK. PERFORMED CAT 3, RTS IAW MM 34-16-01. IPC REF 34-16-01-01.