Narrative:

While holding in IMC at FL360 at 200 KIAS; we observed an increasing discrepancy between the #1 airspeed indicator and the #2 airspeed indicator; along with the appearance of the amber IAS airspeed comparator annunciation on the pfds. The #2 airspeed was relatively constant around 200 KIAS; while the #1 airspeed indicator (asi); indicated decreasing airspeed to around 180 KIAS; at which time the #1 airspeed data was lost (red X on #1 airspeed indicator). Simultaneously; the autopilot disconnected with an autopilot fail master warning as the aircraft was turning in the holding pattern; resulting in a loss of altitude. The captain leveled the aircraft while hand flying; and we coupled the flight guidance to the first officer side and I began hand flying with my airspeed #2 indication still functional. As we continued in the holding pattern and completed the flight manual procedure for the loss of #1 airspeed indication; the airspeed #2 indicator began to indicate decreasing airspeed. We initiated a descent and increased thrust; but the airspeed #2 indication continued to decrease while the integrated standby isis airspeed indication increased. We stabilized the descent with integrated standby instrument system (isis) indicating about 240 KIAS and airspeed #2 indicating below 180 KIAS. There were several other intermittent EICAS cautions related to integrated computers (ic) and weight on wheels (wow) faults. Also a pack #2 ovht caution and pack #2 vlv clsd message appeared while leveled at FL330. Descending through about FL280; while selecting the new landing elevation on the pressurization controller; the airspeed #2 indication suddenly increased from about 180 KIAS to match the isis displayed speed of 240 KIAS. While maneuvering for approach to landing; the airspeed #1 indication removed the red X and displayed a speed that coincided with the #2 and isis displayed speeds; and then also continued to operate normally. The aircraft ice detectors never indicated an ice condition throughout the incident; nor was ice accumulation observed from the cockpit. There were no EICAS messages associated with the heated probes. For reasons yet undetermined; the aircraft experienced the insidious failure of both primary airspeed indications within minutes of each other; each with the same presentation of decreasing airspeed that made it not immediately clear; what the actual airspeed condition of the aircraft was. The patterned failure of the two independent and redundant airspeed indicating systems caused me to question whether the single remaining standby airspeed indication (isis); was even accurate and reliable.

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Original NASA ASRS Text

Title: A Captain and First Officer report that while they were in a holding pattern at FL360; their EMB-145LR experienced failure of both primary airspeed indicators within minutes of each other; with the same presentation of decreasing airspeed; causing them to question whether the single remaining standby airspeed indication was even accurate and reliable. Aircraft diverted.

Narrative: While holding in IMC at FL360 at 200 KIAS; we observed an increasing discrepancy between the #1 Airspeed indicator and the #2 airspeed indicator; along with the appearance of the amber IAS airspeed comparator annunciation on the PFDs. The #2 airspeed was relatively constant around 200 KIAS; while the #1 airspeed indicator (ASI); indicated decreasing airspeed to around 180 KIAS; at which time the #1 airspeed data was lost (red X on #1 airspeed indicator). Simultaneously; the autopilot disconnected with an AUTOPILOT FAIL master warning as the aircraft was turning in the holding pattern; resulting in a loss of altitude. The Captain leveled the aircraft while hand flying; and we coupled the flight guidance to the First Officer side and I began hand flying with my airspeed #2 indication still functional. As we continued in the holding pattern and completed the flight manual procedure for the loss of #1 airspeed indication; the airspeed #2 indicator began to indicate decreasing airspeed. We initiated a descent and increased thrust; but the Airspeed #2 indication continued to decrease while the Integrated Standby ISIS airspeed indication increased. We stabilized the descent with Integrated Standby Instrument System (ISIS) indicating about 240 KIAS and airspeed #2 indicating below 180 KIAS. There were several other intermittent EICAS cautions related to integrated computers (IC) and weight on wheels (WOW) faults. Also a Pack #2 OVHT caution and PACK #2 VLV CLSD message appeared while leveled at FL330. Descending through about FL280; while selecting the new landing elevation on the pressurization controller; the airspeed #2 indication suddenly increased from about 180 KIAS to match the ISIS displayed speed of 240 KIAS. While maneuvering for approach to landing; the airspeed #1 indication removed the red X and displayed a speed that coincided with the #2 and ISIS displayed speeds; and then also continued to operate normally. The aircraft ice detectors never indicated an ice condition throughout the incident; nor was ice accumulation observed from the cockpit. There were no EICAS messages associated with the heated probes. For reasons yet undetermined; the aircraft experienced the insidious failure of both primary airspeed indications within minutes of each other; each with the same presentation of decreasing airspeed that made it not immediately clear; what the actual airspeed condition of the aircraft was. The patterned failure of the two independent and redundant airspeed indicating systems caused me to question whether the single remaining standby airspeed indication (ISIS); was even accurate and reliable.

Data retrieved from NASA's ASRS site as of April 2012 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.