Narrative:

In cruise flying as the pilot monitoring we got a a/P P/rst warning light and saw that I had an IAS disagree message on my pfd. Looked at the first officer's pfd and saw that my airspeed was increasing and the altitude was indicating a climb. The first officer's airspeed and altitude indications were normal and within limits of the standby instruments. I checked that the probe heat was on and there were no visible icing indications. Sat had previously been checked at less than -40 degrees celsius. The first officer continued to fly the aircraft and the autopilot was still engaged. We had a jumpseating captain [onboard] and I asked him to monitor the descent and act as a third set of eyes for us. I asked center for a descent to FL320 and was given clearance to descend. In the descent in IFR conditions my airspeed indicator started to increase and the overspeed clacker started to sound. At the same time the autopilot clicked off and the first officer lost the roll command on his flight director. He also had the IAS disagree warning on his pfd and the airspeed hook was cycling up and down on his pfd. At this time I declared an emergency with center and and explained that we were showing erroneous airspeed and altitude indications. We descended using the fpv and aoa as our main indications crosschecking with center for our groundspeed. The first officer held 3 degrees nose down on the fpv and between 3 and 4 o'clock on the aoa. I asked for a descent to VFR conditions and vectors for the nearest airport. We were given clearance to 11;000 feet and a turn to head direct to ZZZ. At this point I rang the number 1 [flight attendant] and told her that we were having problems with our flight instruments and that I had declared an emergency and we were proceeding to ZZZ for a normal landing in approximately 15 minutes; and that I would talk to the passengers and let her know if any thing changed. In the descent and passing approximately FL200 the clacker quit sounding and my pfd started to show reasonable indications of airspeed and altitude. The first officer regained his FD and and his airspeed hook disappeared. I had received a message from dispatch asking if conditions would allow a divert to ZZZ1. We leveled at 11;000 feet and made the decision to proceed to ZZZ1 based on the facts that we were VFR; our airspeeds and altitudes agreed and appeared normal at this time; and both engines were functioning normally. I asked and we were given a vector to ZZZ1 for landing on [runway] xl. I called the number 1 [flight attendant] again to tell her we were now proceeding to ZZZ1 for a normal [landing] and that all indications were normal at this time. PA was made to passengers informing them of our diversion. Performing the landing checklist and pushing the recall it was discovered that the left engine had gone to the alternate mode. Following the QRH both engines were put into the alternate mode for landing. A normal 30 flaps landing was made in ZZZ1 with all indications normal. After landing we had the arff make sure the aircraft looked normal to them and we taxied to the gate for a normal shutdown. The only comment I will make is the simulator training on unreliable airspeed was invaluable. At no time did we as a crew feel that our aircraft was not in complete control.

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Original NASA ASRS Text

Title: B737-800 Captain experiences a pitot system failure at cruise altitude resulting in unreliable airspeed and altitude indications on his PFD. An emergency is declared and flight diverts; with pitot system indications returning to normal upon exiting IMC.

Narrative: In cruise flying as the Pilot Monitoring we got a A/P P/RST warning light and saw that I had an IAS disagree message on my PFD. Looked at the First Officer's PFD and saw that my airspeed was increasing and the altitude was indicating a climb. The First Officer's airspeed and altitude indications were normal and within limits of the standby instruments. I checked that the probe heat was on and there were no visible icing indications. SAT had previously been checked at less than -40 degrees Celsius. The First Officer continued to fly the aircraft and the autopilot was still engaged. We had a jumpseating Captain [onboard] and I asked him to monitor the descent and act as a third set of eyes for us. I asked Center for a descent to FL320 and was given clearance to descend. In the descent in IFR conditions my airspeed indicator started to increase and the overspeed clacker started to sound. At the same time the autopilot clicked off and the First Officer lost the roll command on his flight director. He also had the IAS disagree warning on his PFD and the airspeed hook was cycling up and down on his PFD. At this time I declared an emergency with Center and and explained that we were showing erroneous airspeed and altitude indications. We descended using the FPV and AOA as our main indications crosschecking with Center for our groundspeed. The First Officer held 3 degrees nose down on the FPV and between 3 and 4 o'clock on the AOA. I asked for a descent to VFR conditions and vectors for the nearest airport. We were given clearance to 11;000 feet and a turn to head direct to ZZZ. At this point I rang the number 1 [Flight Attendant] and told her that we were having problems with our flight instruments and that I had declared an emergency and we were proceeding to ZZZ for a normal landing in approximately 15 minutes; and that I would talk to the passengers and let her know if any thing changed. In the descent and passing approximately FL200 the clacker quit sounding and my PFD started to show reasonable indications of airspeed and altitude. The First Officer regained his FD and and his airspeed hook disappeared. I had received a message from Dispatch asking if conditions would allow a divert to ZZZ1. We leveled at 11;000 feet and made the decision to proceed to ZZZ1 based on the facts that we were VFR; our airspeeds and altitudes agreed and appeared normal at this time; and both engines were functioning normally. I asked and we were given a vector to ZZZ1 for landing on [Runway] XL. I called the number 1 [Flight Attendant] again to tell her we were now proceeding to ZZZ1 for a normal [landing] and that all indications were normal at this time. PA was made to passengers informing them of our diversion. Performing the Landing checklist and pushing the recall it was discovered that the left engine had gone to the alternate mode. Following the QRH both engines were put into the alternate mode for landing. A normal 30 flaps landing was made in ZZZ1 with all indications normal. After landing we had the ARFF make sure the aircraft looked normal to them and we taxied to the gate for a normal shutdown. The only comment I will make is the simulator training on unreliable airspeed was invaluable. At no time did we as a crew feel that our aircraft was not in complete control.

Data retrieved from NASA's ASRS site as of July 2013 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.