B737-800
Flight Crew Air Transport Pilot (ATP)
Distraction
Aircraft Equipment Problem Critical
Aircraft Equipment Problem Dissipated; Flight Crew Landed in Emergency Condition; Flight Crew Took Evasive Action; General Declared Emergency
Aircraft; Weather
Captain flying the RNAV arrival. Entered clouds as we leveled at 10;000 FT. Within a few seconds the CA PFD IAS suddenly decreased from 250 KTS to less than 200 KTS. I turned the autothrottles OFF; set power; adjusted pitch attitude and checked the FO's PFD IAS. Since the First Officer's PFD indicated 250 KTS I transferred the flight controls to him with the autopilot OFF. I was then the PM and advised Approach we had an emergency and requested ARFF. Approach Control gave us priority to Runway 16 ILS. As we descended below the 5;000 FT ceiling the Captain's IAS appeared to return to normal. The FO completed the approach and landed uneventfully with 30 flaps and autobrakes 2. The emergency vehicles followed us to the gate. I spoke with the Fire Chief and Dispatch. There were no braking issues (no overheat). So we completed the Parking Checklist and the Maintenance logbook and handed the airplane over to Maintenance after everyone deplaned.Prior to this event; before top of descent; we had an 'IAS DISAGREE' light illuminate in both the Captain PFD and First Officer PFD. This light was intermittent and would appear with a 5 KT difference and extinguish with a difference of 4 KTS or less. So we were cross-checking IAS frequently which helped us to quickly react to the sudden loss of Captain's IAS. After we completed the parking checklist; I went outside the airplane and checked the pitot tubes and static ports. They all appeared clear and normal.
A B737-800 Captain's Airspeed dropped to about 200 KTS as the aircraft entered clouds at about 10;000 FT but the First Officer's airspeed remained normal at 250 KTS. Descending through about 5;000 FT; the airspeed was again normal.