A B737-800 captain's airspeed dropped to about 200 KTS as the aircraft entered clouds at about 10;00...

Narrative:

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 first officer'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 first officer 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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1097912
Time
Date	201306
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.TRACON
State Reference	US
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Component
Aircraft Component	Speed (Rate Sensing)
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Critical

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.