B737 pilot reported a significant discrepancy between the captain's and first officer's airspeed and...

Narrative:

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 first officer (first officer) side with the stby instruments when the variance increased to over 200 feet triggering an altitude disagree message. We followed the QRH procedure and notified ATC that we are unable rvsm. The variance between the ca and first officer pitot static system slowly increased and we soon got an IAS disagree message. The first officer 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 first officer/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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site 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	1574752
Time
Date	201809
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B737 Next Generation Undifferentiated
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Pitot-Static System
Person 1
Function	First Officer Pilot Not Flying
Experience	Flight Crew Last 90 Days 180 Flight Crew Total 15472 Flight Crew Type 3672
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Inflight Event / Encounter Weather / Turbulence

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.