Narrative:

While on an IFR flight plan level in cruise at FL190 near the cloud tops in IMC; negative ice; -28C; all is normal. Approximately one hour earlier I entered IMC and had turned on the pitot heat. Suddenly; I noticed the air speed readouts on the aspen pfd 1000 began degrading. I immediately check the pitot heat was on; cross checked to the backup sandia 340 quattro on the other side of the panel and both confirmed the airspeed slowly bleeding off from 140 KIAS (eventually reaching 0 KIAS). I distinctly recalled thinking about the analysis of [a similar] accident about pitot tube icing and immediately began cross checking the airframe for ice (which there was none) but I was in cloud. I then began checking instrumentation for changes in pitch attitude (straight and level indicated); altitude (no change from FL190); vsi (zero indication); then the jpi 900 for engine performance (all parameters normal) and finally crosschecked the aspen and avidyne ifd 540/440 GPS nav/coms (all indicating GPS ground speed of around 175KT). To validate the autopilot wasn't changing pitch attitude to hold altitude; as the asis fell thru 83KIAS; I clicked off the autopilot altitude hold (stec 30 with altitude hold) with GPS Nav1 engaged; pulled power back to 19.5' mp to test for any sudden changes in pitch which there were none. I then re-applied power; re-engaged the autopilot altitude hold and pulled out one of my in-ear radio/com headphones and closely listened for any changes to the air flow over the airframe and noted no changes. At that point confused; I just could not figure out how or if the pitot tube had iced up with the pitot heat on and no airframe icing present. Eventually I began suspecting the pitot tube heater had somehow failed since I was in cloud and even a little moisture present it had slowly iced up although no visible signs were present.I called center and advised them of the situation and asked for a climb to FL210 in the hopes of getting on top VFR to further assess the situation. I was immediately cleared to FL210 and began the climb carefully monitoring the aspen vsi and altimeter. Since this aircraft was turbo normalized; I was able to climb to FL210 with no issue. At FL210 however; I was still in and out of the cloud tops; negative ice; but clear above. About a minute at FL210 the aspen's primary ai and HSI failed completely displaying the large red X's thru those display features with warning boxes enunciating 'attitude fail check pitot heat' and 'direction indicator fail'. I could not comprehend why the aspen was failing instruments that are not connected to airspeed so I contacted center and [notified them of the situation]. The reason I did this is I was now partial panel and late in the day I wanted all the help I could get not knowing what the next thing that was about to fail. I did not want to attempt a decent into ZZZ in the dark; in IMC; partial panel. The good news is the backup ai was still online; the aspen was still displaying baro altitude; vsi and more importantly still receiving and feeding GPS heading information from the avidyne 540 to the autopilot so I still had the ability to navigate via GPS and descend thru cloud with a functional autopilot. I then asked center to check for the best solution for the nearest airport with the best weather options for an immediate decent to land hopefully VFR. I could hear them asking other aircraft for conditions etc. And they came back and advised ZZZ was clear below 12;000 but unknown above that. In the mean time I was checking my onboard avidyne based adsb weather and noted ZZZ was closer only about 50nm away and reporting clear below (best I recall) 10;000 feet I suspected overcast above 12;000 so I advised center that ZZZ was likely my best closest option. I was promptly cleared to ZZZ and reprogrammed the avidyne 540 with the autopilot still engaged and receiving GPS commands I began a controlled decent in IMC to ZZZ.descending and passing thru 15;000 feet; surprisingly the airspeed began to slowly coming back on line confirmed by both the aspen pfd the sandia backup. About 45 seconds later the aspen's primary asi and HSI came back on line and full functionality returned to normal. I advised center that my primary flight instruments had come back on line and upon exiting the cloud bases and having the airport in sight advised center that I was requested switching over to VFR. They granted the request but asked me to stay on my squawk code and call them when on the ground. I landed without incident and promptly called FSS and asked them to relay the message center since I was never provided a phone number. Center subsequently called the FBO and verified I was there which the confirmed. Not sure about the delay in message relay between FSS and center.on the ground; after shut down; I immediately checked the pitot heat and the probe was hot to the touch. I then went into the FBO and conversed with the local on-site a&P mechanic and he could not recall ever having any issues with and intermittent pitot tube behavior like I described. He thought perhaps water could have somehow formed in the line and froze between the heated pitot tube probe and the cockpit or more likely the pitot tube heating element had gone intermittent. I then returned to the plane and after letting it cool down for 30 minutes and I tested the pitot heat and it behaved normally warming to the touch in about 1 minute. With the weather reporting VFR I fueled and proceeded to my final destination without further issue.

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Original NASA ASRS Text

Title: Cessna 177RG pilot reported a failure of airspeed indications while in cruise.

Narrative: While on an IFR flight plan level in cruise at FL190 near the cloud tops in IMC; negative ice; -28C; all is normal. Approximately one hour earlier I entered IMC and had turned on the pitot heat. Suddenly; I noticed the air speed readouts on the Aspen PFD 1000 began degrading. I immediately check the pitot heat was on; cross checked to the backup Sandia 340 Quattro on the other side of the panel and both confirmed the airspeed slowly bleeding off from 140 KIAS (eventually reaching 0 KIAS). I distinctly recalled thinking about the analysis of [a similar] accident about pitot tube icing and immediately began cross checking the airframe for ice (which there was none) but I was in cloud. I then began checking instrumentation for changes in pitch attitude (straight and level indicated); altitude (no change from FL190); VSI (zero indication); then the JPI 900 for engine performance (all parameters normal) and finally crosschecked the Aspen and Avidyne IFD 540/440 GPS Nav/coms (all indicating GPS ground speed of around 175KT). To validate the autopilot wasn't changing pitch attitude to hold altitude; as the ASIs fell thru 83KIAS; I clicked off the autopilot altitude hold (STEC 30 with altitude hold) with GPS Nav1 engaged; pulled power back to 19.5' MP to test for any sudden changes in pitch which there were none. I then re-applied power; re-engaged the autopilot altitude hold and pulled out one of my in-ear radio/com headphones and closely listened for any changes to the air flow over the airframe and noted no changes. At that point confused; I just could not figure out how or if the pitot tube had iced up with the pitot heat on and no airframe icing present. Eventually I began suspecting the pitot tube heater had somehow failed since I was in cloud and even a little moisture present it had slowly iced up although no visible signs were present.I called Center and advised them of the situation and asked for a climb to FL210 in the hopes of getting on top VFR to further assess the situation. I was immediately cleared to FL210 and began the climb carefully monitoring the Aspen VSI and altimeter. Since this aircraft was turbo normalized; I was able to climb to FL210 with no issue. At FL210 however; I was still in and out of the cloud tops; negative ice; but clear above. About a minute at FL210 the Aspen's primary AI and HSI failed completely displaying the large red X's thru those display features with warning boxes enunciating 'ATTITUDE FAIL CHECK PITOT HEAT' and 'DIRECTION INDICATOR FAIL'. I could not comprehend why the Aspen was failing instruments that are not connected to airspeed so I contacted Center and [notified them of the situation]. The reason I did this is I was now partial panel and late in the day I wanted all the help I could get not knowing what the next thing that was about to fail. I did not want to attempt a decent into ZZZ in the dark; in IMC; partial panel. The good news is the backup AI was still online; the Aspen was still displaying baro altitude; VSI and more importantly still receiving and feeding GPS heading information from the Avidyne 540 to the autopilot so I still had the ability to navigate via GPS and descend thru cloud with a functional autopilot. I then asked Center to check for the best solution for the nearest airport with the best weather options for an immediate decent to land hopefully VFR. I could hear them asking other aircraft for conditions etc. and they came back and advised ZZZ was clear below 12;000 but unknown above that. In the mean time I was checking my onboard Avidyne based ADSB weather and noted ZZZ was closer only about 50nm away and reporting clear below (best I recall) 10;000 feet I suspected overcast above 12;000 so I advised Center that ZZZ was likely my best closest option. I was promptly cleared to ZZZ and reprogrammed the Avidyne 540 with the Autopilot still engaged and receiving GPS commands I began a controlled decent in IMC to ZZZ.Descending and passing thru 15;000 feet; surprisingly the airspeed began to slowly coming back on line confirmed by both the Aspen PFD the Sandia backup. About 45 seconds later the Aspen's primary ASI and HSI came back on line and full functionality returned to normal. I advised Center that my primary flight instruments had come back on line and upon exiting the cloud bases and having the airport in sight advised Center that I was requested switching over to VFR. They granted the request but asked me to stay on my squawk code and call them when on the ground. I landed without incident and promptly called FSS and asked them to relay the message Center since I was never provided a phone number. Center subsequently called the FBO and verified I was there which the confirmed. Not sure about the delay in message relay between FSS and Center.On the ground; after shut down; I immediately checked the pitot heat and the probe was hot to the touch. I then went into the FBO and conversed with the local on-site A&P mechanic and he could not recall ever having any issues with and intermittent pitot tube behavior like I described. He thought perhaps water could have somehow formed in the line and froze between the heated pitot tube probe and the cockpit or more likely the pitot tube heating element had gone intermittent. I then returned to the plane and after letting it cool down for 30 minutes and I tested the pitot heat and it behaved normally warming to the touch in about 1 minute. With the weather reporting VFR I fueled and proceeded to my final destination without further issue.

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.