Narrative:

At 37;000 feet (while IMC) we were given a climb to 39;000. As I spun the altitude and hit VNAV to start climbing; my airspeed rapidly went from 259 KIAS to 218 KIAS and began approaching the yellow bar on the airspeed tape. Initially I thought we were losing airspeed in the climb and were getting too slow. I tried to slow the climb rate and selected vertical speed and spun the dial inadvertently to a descent. We lost a couple hundred feet and I saw 36;800 on the altimeter. ATC did not mention this altitude deviation. I then received the airspeed and altitude unreliable messages and performed the memory items and cross checked the captain's side which was showing accurate data. I clicked off my autopilot B and selected the captain's autopilot a. We referenced the QRH and determined my pitot static probes may have frozen. The captain contacted mx thru dispatch and I continued to fly with the a autopilot on. We told ATC we were going to stay at 37;000. After a few minutes; ATC queried us about our altitude and said we were showing 37;400 for the last two minutes. We told them it was the transponder with the unreliable altitude on my side and I immediately switched the transponder to number one on the captain's side. ATC then said that fixed it and we were now showing level at 37;000 feet. They did not seem concerned. We told them we were non rvsm and descended to 27;000. My airspeed and altitude came back at the lower altitude. Rest of flight was uneventful.

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Original NASA ASRS Text

Title: B737-700 First Officer experiences unreliable airspeed indication at FL370 in IMC. The autopilot is switched from B to A and the flight continues at FL370 until ATC questions a Mode C readout of FL374. The transponder is switched to the Captain's pitot static system and the flight descends out of RVSM airspace.

Narrative: At 37;000 feet (while IMC) we were given a climb to 39;000. As I spun the altitude and hit VNAV to start climbing; my airspeed rapidly went from 259 KIAS to 218 KIAS and began approaching the yellow bar on the airspeed tape. Initially I thought we were losing airspeed in the climb and were getting too slow. I tried to slow the climb rate and selected Vertical Speed and spun the dial inadvertently to a descent. We lost a couple hundred feet and I saw 36;800 on the altimeter. ATC did not mention this altitude deviation. I then received the airspeed and altitude unreliable messages and performed the memory items and cross checked the Captain's side which was showing accurate data. I clicked off my autopilot B and selected the Captain's autopilot A. We referenced the QRH and determined my pitot static probes may have frozen. The Captain contacted MX thru Dispatch and I continued to fly with the A autopilot on. We told ATC we were going to stay at 37;000. After a few minutes; ATC queried us about our altitude and said we were showing 37;400 for the last two minutes. We told them it was the transponder with the unreliable altitude on my side and I immediately switched the transponder to number one on the Captain's side. ATC then said that fixed it and we were now showing level at 37;000 feet. They did not seem concerned. We told them we were non RVSM and descended to 27;000. My airspeed and altitude came back at the lower altitude. Rest of flight was uneventful.

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.