Date: 201906
Local time of day: 0601-1200
Location: ZZZ.ARTCC, US
Flight conditions: IMC
Weather elements / visibility: Icing
Light: Daylight
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pitot/Static Ice System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Troubleshooting; Confusion; Distraction
Anomaly: Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Overcame Equipment Problem
Contributing factors: Weather; Aircraft
Primary problem: Ambiguous

Narrative: On the arrival into ZZZ; in IMC conditions but in smooth air; without precipitation; at about 27;000 feet MSL; autopilot on; autothrottles on; engines anti ice on; being the Captain the PF (Pilot Flying); suddenly the autopilot pitched down followed by 'AIRSPEED LOW' verbal message. On both PFDs (Primary Flight Display) we had 'IAS DISAGREE' and 'ALT DISAGREE' cautions messages. The CA's (Captain) PFD was indicating about 170 KIAS and there was a discrepancy in the altitude; I do not remember the exact difference right now. My first instinct was to check the stand-by instrument and compare speeds between PFDs. The CA disconnected the autopilot and took manual control of the aircraft. I run the QRC for UNRELIABLE AIRSPEED and once finished this check list; seeing that my side showed the same information as the stand by instrument we transferred controls. We [requested priority handling] with Center and we were given priority into the airport. The CA continue with the checklist and setting up the approach; while I was hand flying.I remember checking the overhead panel to see if there were any indications on the pitot-static panel and all lights were extinguished; indicating normal operation of the pitot heaters. Both ENG went into soft alternate mode as a consequence of not having good air data information; we decided to disregard this issue and concentrate ourselves into flying the aircraft; knowing it will not affect the safety of the flight at that moment.Reaching about 15;000 feet MSL all indications were back to normal; only the engines remained in alt mode; which is normal until shutdown; after discussing the issue we transferred controls back again; being the CA the PF again. We proceeded to do a normal approach and landing into Runway X without further issues.We; the CA and I; suspect of some kind of icing issue on the pitot system. What puzzles us is the fact that everything was set the way it supposed to be; without any indications of something not functioning correctly. We were in IMC but not in a thunderstorm; so we believe it could be some kind of crystal icing inside the pitot system; just a theory.The initial pitch down of the aircraft took us by surprise. We normally practice unreliable airspeed with the aircraft stabilized at some altitude; so will be interesting to do this scenario in the simulator where the initial problem is the pitch down.




Synopsis: B737 pilot reported a discrepancy with the airspeed readouts in the cockpit; possibly due to ice contamination in the pitot/static system.