Date: 202104
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Weather elements / visibility: Icing
Light: Night
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pitot/Static Ice System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Second reporter function: Captain; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Aircraft Equipment Problem Dissipated; Flight Crew Took Evasive Action; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Landed As Precaution; Flight Crew Diverted; General Maintenance Action; General Flight Cancelled / Delayed
Contributing factors: Aircraft; Weather
Primary problem: Aircraft

Narrative: After descending from cruise flight at 360; we leveled off at FL300. We entered the clouds on descent and activated the engine anti ice system. Once level for a few minutes at 300 I noticed a rapid drop of airspeed on my side. I pushed the throttles up and informed the Captain. At that point we realized we had unreliable airspeed on the First Officer side. The Captain assumed control of the aircraft. We had encountered a small amount of rime icing and decided to climb back up. At this point we also realized the Captains altimeter was off. We [advised] through ATC; ran the QRC and QRH checklists. We diverted to ZZZ. We advised Dispatch and the flight attendants of our situation. Passing 15;000 feet our instruments began to return to normal and were equal upon landing.  We also accomplished they overweight landing checklist. We landed at ZZZ without incident. I believe the icing conditions could have contributed to the event. Check the pitot static system. Or computers. We had no indication of any heat malfunction.


Second narrative: After descending from FL360 to FL300 the First Officer; who was the Pilot Flying; noticed his airspeed indication decaying. In the descent we had entered IMC; and the engine heat was selected on due to the OAT. After cross-checking the pilots; standby; and copilots instruments I assumed control of the aircraft and we accomplished all QRC and QRH checklists for unreliable airspeed. We also determined the pilots altimeter was unreliable. [We advised] ATC and we diverted to the nearest suitable airport; which was ZZZ. The flight attendants; passengers and Dispatch were informed of our decision to divert. As we descended and the air became warmer the copilots airspeed and the Captains altimeter started to agree with the correct indications. By approximately 15;000 MSL all instruments were in agreement. Due to our weight we also accomplished the overweight landing checklist. We landed uneventfully at ZZZ and taxied to the gate. An AML entry was made and we entered crew rest at ZZZ.The icing conditions encountered in the descent to FL300 seem to have overwhelmed the pitot-static heating system. There was no indication of anything more than light icing from the pilots perspective. The pitot-static heating system on the 737 needs to be evaluated to ensure this situation doesn't happen to other aircrews.


Synopsis: B737-800 flight crew reported icing conditions resulted in unreliable airspeed indications; followed by a descent and diversion to a precautionary landing.