B737 Next Generation Undifferentiated
Flight Crew Last 90 Days 180; Flight Crew Total 15472; Flight Crew Type 3672
Confusion; Distraction; Time Pressure; Troubleshooting
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Provided Assistance; Flight Crew Overcame Equipment Problem; Flight Crew Requested ATC Assistance / Clarification
Aircraft; Weather
We were at level cruise in the last 1/3rd of our flight when we noticed a variance of 150 feet between the Captain's and the Copilot's altimeters. We were discussing the problem and comparing the CA (Captain) and FO (First Officer) side with the STBY instruments when the variance increased to over 200 feet triggering an ALT DISAGREE message. We followed the QRH procedure and notified ATC that we are unable RVSM. The variance between the CA and FO pitot static system slowly increased and we soon got an IAS DISAGREE message. The FO and STBY instruments were in close agreement. The IAS DISAGREE QRH lead us to the AIRSPEED UNRELIABLE checklist and we began to run that procedure. The Captain transferred control of the aircraft to me because I had the reliable instruments. At this point; there was a 400 foot difference in altitude between the FO/STBY instruments and the CA instruments. We switched the transponder to my side which triggered a query from ATC because Mode C now showed us 400 feet high. I descended smoothly back down to the assigned altitude. We were in a heavy rain shower with high winds prior to departing. Therefore; we suspected ice in the Captain's pitot static system. The Captain requested a lower altitude. Air traffic control was not immediately able to accommodate our request due to traffic and coordination. The Captain [advised ATC]. During the descent; the Captain's instruments showed air speeds as low as 40 knots and altitude errors of thousands of feet. This triggered one airspeed low aural warning. Once we descended to an altitude that yielded a TAT of plus 5 deg C; the Captain's instruments returned to normal indications. The Captain canceled the emergency with ATC. I flew a normal approach and landing [at the destination airport] without using the auto throttles. The Captain communicated with Maintenance and Dispatch after we arrived at the gate.
B737 pilot reported a significant discrepancy between the Captain's and First Officer's airspeed and altitude indications; possibly due to ice contamination in the pitot-static system.