Narrative:

During cruise at FL360 in VMC conditions southeast of airport ZZZ prior to descent; we noticed IAS disagree illuminated on both captains and first officer's airspeed tapes. Initially the air speeds were approximately 6 knots difference between each other. We ran the QRH for IAS disagree which sent us to the airspeed unreliable checklist. We determined using that checklist that the captain's side was unreliable. That checklist also had us reference the flight with unreliable airspeed in the performance chapter. While referencing that; the captain's and first officer's altimeters began to disagree by approximately 300 ft. We ran the altitude disagree checklist as well and still believed the captain's side to be faulty. Just about that time; the controller asked us to start our descent. Airspeed and altitude differences started getting much larger after a short period of time once we began our descent. The captains airspeed indicator started falling back to the lowest possible value of 45 knots and altimeter was off now by about 1;000 feet. The airspeed low aural warning started sounding in the cockpit. The unreliable airspeed checklist warned us of this possibility. The first officer's airspeed and altimeter agreed with the standby instruments; however we still backed up the flight for added safety by referencing pitch and power settings in the performance chapter to make sure that we had some reliable information. After starting our descent from FL360 both engine eec altitude lights illuminated. We ran the QRH for eec alternate mode. Shortly after that; the speed trim fail light illuminated and the QRH was run for speed trim fail. Next; VNAV disconnected and we went into cws pitch mode. We looked down at the box and noticed we had lost all performance data in the box. This explained the VNAV disconnect. We engaged level change and that worked fine. About this time ATC changed our arrival and we noticed that even though we were still in LNAV; the airplane was not tracking LNAV. [ATC was notified] and we received priority handling in the form of a heading direct to the airport and then vectors onto the visual approach for runway xy. I was originally flying the airplane; but we quickly got so emerged in checklists and busy tasks; I decided to follow the guidance in the fom on managing [the current condition] and gave pilot flying duty to the first officer so I could run checklists and manage the [situation]. I accomplished all checklists; then called the flight attendants. I explained to them what had happened and the type of [condition]; told them there were no special instructions; and that we had approximately 15-20 minutes until landing. I then made a PA to the passengers and told them we had a downgrade of some of our flight instruments; but had the situation under control and we're working on backup instruments. I told them that as a precaution; we had requested [priority] handling from ATC and because of that as a precaution; they would see fire trucks on the side of the runway when we landed. I then texted my dispatcher and told them the situation. They copied and asked if there was anything they could do to help. I told them no thanks. The [arff] equipment was rolled. Around 11;000 feet; my airspeed was still reading 45 knots and my altimeter was 1;000 feet off from our altitude. All of a sudden; the airspeed and altimeter snapped back to normal and airspeed and altitude disagree messages went away. Since the first officer was doing such a great job flying the airplane; and his instruments could be trusted more than mine; I let him fly the approach and landing. We still were referencing pitch and power settings in the performance section of the QRH for unreliable airspeed for added safety. We landed without incident and taxied off the runway and to the gate. The probe heat was on for the entire flight per SOP and we never got a caution light saying any probe heat was inoperative.

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Original NASA ASRS Text

Title: B737 Captain reported receiving an IAS Disagree warning. Crew followed procedures and flew a safe approach and landing.

Narrative: During cruise at FL360 in VMC conditions southeast of Airport ZZZ prior to descent; we noticed IAS Disagree illuminated on both Captains and FO's airspeed tapes. Initially the air speeds were approximately 6 knots difference between each other. We ran the QRH for IAS Disagree which sent us to the Airspeed Unreliable checklist. We determined using that checklist that the captain's side was unreliable. That checklist also had us reference the Flight with unreliable airspeed in the performance chapter. While referencing that; the Captain's and FO's altimeters began to disagree by approximately 300 FT. We ran the ALT disagree checklist as well and still believed the Captain's side to be faulty. Just about that time; the controller asked us to start our descent. Airspeed and Altitude differences started getting much larger after a short period of time once we began our descent. The Captains airspeed indicator started falling back to the lowest possible value of 45 knots and altimeter was off now by about 1;000 feet. The airspeed low aural warning started sounding in the cockpit. The unreliable airspeed checklist warned us of this possibility. The first officer's airspeed and altimeter agreed with the standby instruments; however we still backed up the flight for added safety by referencing pitch and power settings in the performance chapter to make sure that we had some reliable information. After starting our descent from FL360 both engine EEC ALT lights illuminated. We ran the QRH for EEC Alternate mode. Shortly after that; the Speed Trim Fail light illuminated and the QRH was run for Speed Trim Fail. Next; VNAV disconnected and we went into CWS pitch mode. We looked down at the box and noticed we had lost all performance data in the box. This explained the VNAV disconnect. We engaged level change and that worked fine. About this time ATC changed our arrival and we noticed that even though we were still in LNAV; the airplane was not tracking LNAV. [ATC was notified] and we received priority handling in the form of a heading direct to the airport and then vectors onto the visual approach for Runway XY. I was originally flying the airplane; but we quickly got so emerged in checklists and busy tasks; I decided to follow the guidance in the FOM on managing [the current condition] and gave pilot flying duty to the FO so I could run checklists and manage the [situation]. I accomplished all checklists; then called the flight attendants. I explained to them what had happened and the type of [condition]; told them there were no special instructions; and that we had approximately 15-20 minutes until landing. I then made a PA to the passengers and told them we had a downgrade of some of our flight instruments; but had the situation under control and we're working on backup instruments. I told them that as a precaution; we had requested [priority] handling from ATC and because of that as a precaution; they would see fire trucks on the side of the runway when we landed. I then texted my dispatcher and told them the situation. They copied and asked if there was anything they could do to help. I told them no thanks. The [ARFF] equipment was rolled. Around 11;000 feet; my airspeed was still reading 45 knots and my altimeter was 1;000 feet off from our altitude. All of a sudden; the airspeed and altimeter snapped back to normal and airspeed and ALT disagree messages went away. Since the First Officer was doing such a great job flying the airplane; and his instruments could be trusted more than mine; I let him fly the approach and landing. We still were referencing pitch and power settings in the performance section of the QRH for unreliable airspeed for added safety. We landed without incident and taxied off the runway and to the gate. The probe heat was on for the entire flight per SOP and we never got a caution light saying any probe heat was INOP.

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.