B737-800
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Flight Crew Overcame Equipment Problem
Aircraft; Weather
During cruise at FL370 while deviating for thunderstorms in IMC conditions we encountered an airspeed unreliable condition. After performing memory items we ran the Airspeed Unreliable checklist. While going through the checklist we may have deviated from our assigned FL370 however we don't think we did. While running the checklist the event seemed to end. We continued through the entire checklist to make sure we were aware of all of the possible actions required if the event returned. This event occurred on the same aircraft about 3 weeks prior and maintenance addressed it at that time.I don't have any suggestions as we followed SOP and checklist usage. We don't think we had an altitude deviation but we aren't sure.
During Airspeed Unreliable event at FL370 and IMC; CA (Captain) was pilot flying. Both CA and FO (First Officer) airspeed and altitude were unreliable for at least a minute or more; Airspeed Low and IAS Disagree alerts. Accomplished procedures; and hand flew based on Standby Flight Display. As best we could tell; stayed within +- 200 ft of FL370. Exiting IMC; indications returned to normal. ATC made no comment about altitude.Factors include equipment malfunction and icing. Equipment malfunctions unavoidable
B737-800 flight crew reported unreliable airspeed and altimeter in IMC conditions. Systems returned to normal after exiting IMC.