Narrative:

We were level at FL250; 280 knots indicated; 30 north of mtata on the piglt 4 arrival mco. OAT positive four degrees total air temperature; icing; light turbulence; IMC night conditions. Out of nowhere captain airspeed; altitude; vsi started fluctuating wildly and randomly: airspeed 160-300 knots; altitude plus/minus 800 feet; vertical speed indicator plus/minus 1500 FPM. Autopilot immediately tried to compensate; aircraft was oscillating. I transferred my scan to first officer (first officer) instruments as he was off intercom making a PA. First officer instruments fluctuating as well; but not as bad as captain instruments. Standby instruments fluctuating but less so than first officer instruments. Observed first officer altimeter setting display randomly switch from standard 29.92 to local altimeter setting on its own.I initially worked against the autopilot to attempt to keep aircraft at what I believed was level altitude as there were numerous other aircraft in the vicinity. VNAV disconnect message popped up on CDU as all fluctuations continued. I maintained my scan on first officer and standby instruments and broke out autopilot to cws pitch with LNAV guidance and turned off auto throttles. Averaged out airspeed and pitch fluctuations to maintain what I felt was close to level flight and normal airspeed. Concentrated on keeping the HUD and ADI flight path markers on the horizon with mid-range power setting. Captain instruments continued enormous and alarming fluctuations. I was concerned about a high speed in flight break up when I saw it accelerating rapidly. Also when airspeed indications started decreasing; I was concerned about a high altitude stall and departure from controlled flight. Fought all instincts to overcompensate. Finally; I disconnected autopilot completely; followed the airspeed unreliable procedure as the fluctuations continued. By this time the first officer was back in the loop trying to assess what was going on. Our transponder was in the number one position; since it was captains leg and reporting our wildly changing altitude indications. Obviously; jax center was not happy about that. I reached down and switched it to number two; as its altitude reporting would be less wild. With the OAT near freezing; st. Elmo's fire present; there was a bunch of static on the VHF radios making communication with jax center a challenge. Also there were numerous rerouted aircraft on the frequency dealing with the effects of [a nearby storm]. I directed the first officer to get clearance for an immediate descent to warmer air and perhaps VMC conditions; he also briefly explained our malfunctions to jax center. I eased the aircraft down carefully not to overcompensate. As soon as the OAT increased two degrees the fluctuations began to dampen out. I was still very alarmed and didn't trust my instruments; air data computer or the fmcs; but I felt confident I had a solid IRS platform so we continued descent with new jax frequency and conditions steadily improved as we headed toward piglt. Did not have time to [advise ATC of situation] but came close. I briefly considered a divert to jax but we eventually arrived in VMC conditions with warm air and the aircraft began operating normally so we opted to continue to mco. Did not get any TCAS alerts. Remainder of flight; approach and landing was normal. Notified dispatch; [chief pilot]; ATC liaison in the operations center; as well as maintenance at mco. Logbook entry made. Very alarming event.add heaters to the static ports. Change airspeed unreliable checklist to read: altitude / airspeed unreliable.

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Original NASA ASRS Text

Title: B737NG flight crew reported erratic airspeed and altitude indications at FL250 in icing conditions. The Captain was able to maintain control by referencing the First Officer and standby instruments and as the flight descended to warmer air the instruments returned to normal.

Narrative: We were level at FL250; 280 knots indicated; 30 north of MTATA on the PIGLT 4 Arrival MCO. OAT positive four degrees Total Air Temperature; icing; light turbulence; IMC night conditions. Out of nowhere Captain airspeed; altitude; VSI started fluctuating wildly and randomly: airspeed 160-300 knots; altitude plus/minus 800 feet; Vertical Speed Indicator plus/minus 1500 FPM. Autopilot immediately tried to compensate; aircraft was oscillating. I transferred my scan to First Officer (FO) instruments as he was off intercom making a PA. FO instruments fluctuating as well; but not as bad as Captain instruments. Standby instruments fluctuating but less so than FO instruments. Observed FO altimeter setting display randomly switch from standard 29.92 to local altimeter setting on its own.I initially worked against the autopilot to attempt to keep aircraft at what I believed was level altitude as there were numerous other aircraft in the vicinity. VNAV disconnect message popped up on CDU as all fluctuations continued. I maintained my scan on FO and Standby instruments and broke out autopilot to CWS Pitch with LNAV guidance and turned off auto throttles. Averaged out airspeed and pitch fluctuations to maintain what I felt was close to level flight and normal airspeed. Concentrated on keeping the HUD and ADI flight path markers on the horizon with mid-range power setting. Captain instruments continued enormous and alarming fluctuations. I was concerned about a high speed in flight break up when I saw it accelerating rapidly. Also when airspeed indications started decreasing; I was concerned about a high altitude stall and departure from controlled flight. Fought all instincts to overcompensate. Finally; I disconnected autopilot completely; followed the Airspeed Unreliable procedure as the fluctuations continued. By this time the FO was back in the loop trying to assess what was going on. Our transponder was in the number one position; since it was Captains leg and reporting our wildly changing altitude indications. Obviously; JAX Center was not happy about that. I reached down and switched it to number two; as its altitude reporting would be less wild. With the OAT near freezing; St. Elmo's Fire present; there was a bunch of static on the VHF radios making communication with JAX Center a challenge. Also there were numerous rerouted aircraft on the frequency dealing with the effects of [a nearby storm]. I directed the FO to get clearance for an immediate descent to warmer air and perhaps VMC conditions; he also briefly explained our malfunctions to JAX Center. I eased the aircraft down carefully not to overcompensate. As soon as the OAT increased two degrees the fluctuations began to dampen out. I was still very alarmed and didn't trust my instruments; air data computer or the FMCs; but I felt confident I had a solid IRS platform so we continued descent with new JAX frequency and conditions steadily improved as we headed toward PIGLT. Did not have time to [advise ATC of situation] but came close. I briefly considered a divert to JAX but we eventually arrived in VMC conditions with warm air and the aircraft began operating normally so we opted to continue to MCO. Did not get any TCAS alerts. Remainder of flight; approach and landing was normal. Notified Dispatch; [Chief Pilot]; ATC liaison in the Operations Center; as well as Maintenance at MCO. Logbook entry made. Very alarming event.Add heaters to the static ports. Change Airspeed Unreliable Checklist to read: Altitude / Airspeed Unreliable.

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.