Narrative:

Event occurred on descent during the RNAV arrival. Engine and airfoil anti-ice was on for the descent due to the weather. Conditions were IFR with moderate rain and moderate turbulence during the descent. Precipitation was evident on the windscreen but no ice buildup on the wipers was visible. About 13000 feet on the arrival we received 'IAS' alert indicating a comparison mismatch between captain and first officer (first officer) airspeed indication. As PF (pilot flying) I noticed the alert and queried first officer's airspeed and compared that to the standby airspeed indicator. Captain and first officer's airspeeds appeared to be within 5 knots of each other. However we were in moderate rain and moderate turbulence during the arrival descent. Captain's airspeed appeared to the one that matched the standby indicator closer at 290 knots. Very shortly after that we received level 2 alerts for sel elev feel man and sel flap lim ovrd. At this particular time I wasn't sure if we were losing input to the FCC's so I advised ATC we had possible navigation; and possibly airspeed and altitude issues. I requested from the controller ATC vectors to ZZZ. He gave me a heading and altitude. The autopilot and autothrottles subsequently disconnected. I advised the first officer that I had the airplane and radios and confirmed our airspeed; altitude; and heading with ATC. Then directed the first officer to run 'the most appropriate' checklist. We both agreed we suspected a pitot static issue. We had a few more alerts begin to pop up. As he worked through the QRH procedures for the alerts indicated and our aircraft condition we transferred to the 'airspeed unreliable' checklist based on the checklist notes we had for the indicated alert. ATC continued giving me vectors to the runway. Since we were comfortable that we had accurate airspeed on the captain's side as compared to what ATC indicated and the standby indicator; and the HUD was accurate; I elected to continue toward a landing. The first officer continued the 'airspeed unreliable' checklist to a logical conclusion. During this time we continued descent to 5000 toward final. ATC offered the center runway but I elected the left to have a visual PAPI backup. At some point the airspeeds seemed to be back with normal indications. The first officer and I agreed it was most likely the first officer pitot static system that had froze up and not had finally thawed out. I elected to turn the autopilot and autothrottle systems back on and all systems were normal. We cancelled the emergency with ATC and landed normally on xxl with 35 degree flaps.suspect the first officer pitot static system froze up during descent in moderate rain and IMC conditions. It thawed out and began functioning normally several minutes later while on a base to final. This was a system malfunction with the anti ice system. Fantastic job on ATC controllers part to provide me backup indications of airspeed; heading and altitude when we're flying in IMC conditions with degraded flight instruments. We did not get VMC conditions until about 4000 feet and on an extended base to final.

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Original NASA ASRS Text

Title: Cargo aircraft Captain reported an airspeed alert indicating a comparison mismatch between Captain and First Officer airspeed indication.

Narrative: Event occurred on descent during the RNAV arrival. Engine and airfoil anti-ice was on for the descent due to the weather. Conditions were IFR with moderate rain and moderate turbulence during the descent. Precipitation was evident on the windscreen but no ice buildup on the wipers was visible. About 13000 feet on the arrival we received 'IAS' alert indicating a comparison mismatch between Captain and F/O (First Officer) airspeed indication. As PF (pilot flying) I noticed the alert and queried F/O's airspeed and compared that to the standby airspeed indicator. Captain and F/O's airspeeds appeared to be within 5 knots of each other. However we were in moderate rain and moderate turbulence during the arrival descent. Captain's airspeed appeared to the one that matched the standby indicator closer at 290 knots. Very shortly after that we received Level 2 alerts for SEL ELEV FEEL MAN and SEL FLAP LIM OVRD. At this particular time I wasn't sure if we were losing input to the FCC's so I advised ATC we had possible navigation; and possibly airspeed and altitude issues. I requested from the controller ATC vectors to ZZZ. He gave me a heading and altitude. The autopilot and autothrottles subsequently disconnected. I advised the F/O that I had the airplane and radios and confirmed our airspeed; altitude; and heading with ATC. Then directed the F/O to run 'the most appropriate' checklist. We both agreed we suspected a pitot static issue. We had a few more alerts begin to pop up. As he worked through the QRH procedures for the alerts indicated and our aircraft condition we transferred to the 'Airspeed Unreliable' checklist based on the checklist notes we had for the indicated alert. ATC continued giving me vectors to the runway. Since we were comfortable that we had accurate airspeed on the Captain's side as compared to what ATC indicated and the standby indicator; and the HUD was accurate; I elected to continue toward a landing. The F/O continued the 'Airspeed Unreliable' checklist to a logical conclusion. During this time we continued descent to 5000 toward final. ATC offered the center runway but I elected the left to have a visual PAPI backup. At some point the airspeeds seemed to be back with normal indications. The F/O and I agreed it was most likely the F/O pitot static system that had froze up and not had finally thawed out. I elected to turn the autopilot and autothrottle systems back on and all systems were normal. We cancelled the emergency with ATC and landed normally on XXL with 35 degree flaps.Suspect the F/O pitot static system froze up during descent in moderate rain and IMC conditions. It thawed out and began functioning normally several minutes later while on a base to final. This was a system malfunction with the anti ice system. Fantastic job on ATC controllers part to provide me backup indications of airspeed; heading and altitude when we're flying in IMC conditions with degraded flight instruments. We did not get VMC conditions until about 4000 feet and on an extended base to final.

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.