B737-700
Flight Crew Last 90 Days 219
Confusion; Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude
Air Traffic Control Issued Advisory / Alert; Aircraft Equipment Problem Dissipated; Flight Crew Took Evasive Action
Aircraft; Weather
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.
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.