Narrative:

We were in cruise at FL360. We were contacted by dispatch with a proposed reroute due to weather. We discussed the reroute; decided to accept it and altered course eastbound to avoid the weather. ATC altered the requested reroute and gave us a route would have taken us through heavy precipitation so we requested vectors further eastbound to avoid it. The initial plan was to pass through two areas of moderate precip then continue on course. While vectoring through the weather we realized the precip was building fast and would require us to pass closer by it than originally anticipated. In preparation we contacted flight attendant; and slowed the aircraft to .63 mach. Light turbulence and rain were encountered and airspeed indications seemed to indicate the aircraft was slowly and steadily decelerating. I added power to increase speed. The aircraft did not seem to respond as well as usual for the low speed at altitude. The poor/slow increase in IAS prompted me to select climb mode for more thrust. What initially seemed like just sluggishness for a given weight/speed/altitude started to look like a more serious problem. Possible windshear with decreasing performance crossed my mind. Airspeed continued to roll back or decrease and I instinctively pushed the thrust levers forward past the detents. Airspeed continued to slowly decrease so I instructed the first officer to request lower so we could reduce our pitch angle and could get the situation sorted out. As airspeed continued to slow during the first officer's radio call to request lower I felt it necessary for safety to drop the nose to avoid a stall. ATC denied the request for lower so I insisted the first officer request lower once again but it was again denied by ATC for traffic. I had still not regained airspeed and continued to lower the nose to about 500 FPM while indicating only 170 KTS. ATC asked if we were declaring an emergency I responded 'yes' to the first officer and told her I would continue to descend and to get a turn to avoid traffic from ATC. I cannot recall what the isis [integrated standby instrument system] was indicating at this point but just then the high speed warning sounded and I looked over to see the first officer's indications were appropriate for the warning. I leveled off; assumed the #2 air data computer was correct; coupled the aircraft to the first officer's side and instructed her to be the flying pilot. As I began to cross check the instrumentation I wasn't sure which was correct. All three; the captain's; first officer's and standby systems read differently. We had no positive cross check. We wanted to rely on the isis but soon realized it was frozen at .78M with an incorrect altitude.once we thought we had a handle on which instrumentation was correct we contacted ATC and notified them of an equipment malfunction and that we no longer met requirements for rvsm airspace. It seemed that the #2 side was correct but I cannot be sure of this; any of the 3 indications seemed to be wrong at some point. About 10 to 15 minutes later; around FL200; all three seemed to match up. 'Sps advanced' and amber 'altitude' and amber 'IAS' were present intermittently throughout the ordeal. At some points deviations were erratic and obviously false but the initial decrease to .63M and further were smooth and seemed trustworthy. Altitudes varied as well during the confusion so the actual deviation in altitude is just an estimate. At one point in time the isis seemed to be locked at redline .78M and the altitude steadily decreasing as both #1 and #2 altimeters indicated level flight. Maintenance was notified and the culprit was believed to be excessive water in the pitot systems causing erroneous readings. I was told by the captain who was supposed to fly the next leg on the plane that maintenance found no water in the system. The next the day we flew the same aircraft again and it seemed to operate normally.

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Original NASA ASRS Text

Title: While deviating around weather and in moderate precipitation an EMB145 flight crew encountered widely divergent airspeed indications among the Captain's; First Officer's and Standby airspeed indicators. They initially were aware only that the Captain's speed was gradually decreasing and; only after receiving a mach overspeed warning; did they cross check and notice the First Officer's was touching the Barber Pole. They wrote up the event but failed to note the maintenance response when they flew the airplane the following day--when it operated normally.

Narrative: We were in cruise at FL360. We were contacted by Dispatch with a proposed reroute due to weather. We discussed the reroute; decided to accept it and altered course eastbound to avoid the weather. ATC altered the requested reroute and gave us a route would have taken us through heavy precipitation so we requested vectors further eastbound to avoid it. The initial plan was to pass through two areas of moderate precip then continue on course. While vectoring through the weather we realized the precip was building fast and would require us to pass closer by it than originally anticipated. In preparation we contacted Flight Attendant; and slowed the aircraft to .63 Mach. Light turbulence and rain were encountered and airspeed indications seemed to indicate the aircraft was slowly and steadily decelerating. I added power to increase speed. The aircraft did not seem to respond as well as usual for the low speed at altitude. The poor/slow increase in IAS prompted me to select CLB mode for more thrust. What initially seemed like just sluggishness for a given weight/speed/altitude started to look like a more serious problem. Possible windshear with decreasing performance crossed my mind. Airspeed continued to roll back or decrease and I instinctively pushed the thrust levers forward past the detents. Airspeed continued to slowly decrease so I instructed the First Officer to request lower so we could reduce our pitch angle and could get the situation sorted out. As airspeed continued to slow during the First Officer's radio call to request lower I felt it necessary for safety to drop the nose to avoid a stall. ATC denied the request for lower so I insisted the First Officer request lower once again but it was again denied by ATC for traffic. I had still not regained Airspeed and continued to lower the nose to about 500 FPM while indicating only 170 KTS. ATC asked if we were declaring an emergency I responded 'yes' to the First Officer and told her I would continue to descend and to get a turn to avoid traffic from ATC. I cannot recall what the ISIS [Integrated Standby Instrument System] was indicating at this point but just then the high speed warning sounded and I looked over to see the First Officer's indications were appropriate for the warning. I leveled off; assumed the #2 ADC was correct; coupled the aircraft to the First Officer's side and instructed her to be the flying pilot. As I began to cross check the instrumentation I wasn't sure which was correct. All three; the Captain's; First Officer's and standby systems read differently. We had no positive cross check. We wanted to rely on the ISIS but soon realized it was frozen at .78M with an incorrect altitude.Once we thought we had a handle on which instrumentation was correct we contacted ATC and notified them of an equipment malfunction and that we no longer met requirements for RVSM airspace. It seemed that the #2 side was correct but I cannot be sure of this; any of the 3 indications seemed to be wrong at some point. About 10 to 15 minutes later; around FL200; all three seemed to match up. 'SPS advanced' and amber 'ALT' and amber 'IAS' were present intermittently throughout the ordeal. At some points deviations were erratic and obviously false but the initial decrease to .63M and further were smooth and seemed trustworthy. Altitudes varied as well during the confusion so the actual deviation in altitude is just an estimate. At one point in time the ISIS seemed to be locked at redline .78M and the altitude steadily decreasing as both #1 and #2 altimeters indicated level flight. Maintenance was notified and the culprit was believed to be excessive water in the pitot systems causing erroneous readings. I was told by the Captain who was supposed to fly the next leg on the plane that Maintenance found no water in the system. The next the day we flew the same aircraft again and it seemed to operate normally.

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.