Narrative:

While enroute between ZZZ and ZZZ1 at FL360; the #2 air data computer (air data computer) failed; initially resulting in erroneous altitude and airspeed indications. The standby instruments were referenced as the appropriate checklists and procedures were accomplished to determine the failed 2 air data computer and switch to the good 2 air data computer. The center controller was advised of the situation and confirmed altitude and ground speed. Weather conditions on the ground over a large area of the enroute portion were at or below landing minimums due to fog and reduced visibility; so the crew decided the best course of action was to continue the flight as we were in clear skies on top of the weather. Since the destination of ZZZ was in mountainous terrain and deteriorating weather conditions; we felt it was a prudent decision to change destinations to ZZZ1 where the weather was good VMC. ATC was advised and cleared our flight directly to ZZZ1. Altitude indications were accurate on the good air data computer; however; we were having airspeed fluctuations on both pfds. With the power setting and altitude constant; the indicated airspeeds were rising; and during the descent the airspeed was declining with altitude but on the standby the IAS was increasing. We believe there may have been a blockage somewhere in the pitot static system to create that situation. When we leveled at FL280 for a period of time; the IAS on the pfds came back to normal indications through the remainder of the flight. The crew did not declare an emergency; and there were no traffic conflicts. The aircraft was repaired at ZZZ1. Callback conversation with reporter revealed the following information: reporter stated this was the second time in a week; on the same cl-604; that an air data computer (air data computer) had failed. The left air data computer was previously changed after they encountered extremely heavy precipitation during climb out with a rising indicated airspeed (IAS) moving into red line and setting off the overspeed aural warning clacker. No circuit breaker exists in the cockpit to silence the overspeed clacker. The only inflight option to silence the clacker is to turn off one; or both; switches for the data concentrator units (DCU's); located on the co-pilot's sidewall. But; doing so also silences other aural warning systems. The pitot heat seemed to be functioning. Reporter stated the contract maintenance mechanic replaced the #2 air data computer; performed a pitot system test and believes the pitot static drains were also opened and cleared. No further problems have occurred since.

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Original NASA ASRS Text

Title: While enroute at FL360; pilot of a Challenger CL-604 noticed the #2 Air Data Computer (ADC) had failed; resulting in erroneous altitude indications and fluctuating airspeeds on both Primary Flight Displays.

Narrative: While enroute between ZZZ and ZZZ1 at FL360; the #2 Air Data Computer (ADC) failed; initially resulting in erroneous altitude and airspeed indications. The standby instruments were referenced as the appropriate checklists and procedures were accomplished to determine the failed 2 Air Data Computer and switch to the good 2 Air Data Computer. The Center Controller was advised of the situation and confirmed altitude and ground speed. Weather conditions on the ground over a large area of the enroute portion were at or below landing minimums due to fog and reduced visibility; so the crew decided the best course of action was to continue the flight as we were in clear skies on top of the weather. Since the destination of ZZZ was in mountainous terrain and deteriorating weather conditions; we felt it was a prudent decision to change destinations to ZZZ1 where the weather was good VMC. ATC was advised and cleared our flight directly to ZZZ1. Altitude indications were accurate on the good ADC; however; we were having airspeed fluctuations on both PFDs. With the power setting and altitude constant; the indicated airspeeds were rising; and during the descent the airspeed was declining with altitude but on the standby the IAS was increasing. We believe there may have been a blockage somewhere in the pitot static system to create that situation. When we leveled at FL280 for a period of time; the IAS on the PFDs came back to normal indications through the remainder of the flight. The crew did not declare an emergency; and there were no traffic conflicts. The aircraft was repaired at ZZZ1. Callback conversation with reporter revealed the following information: Reporter stated this was the second time in a week; on the same CL-604; that an Air Data Computer (ADC) had failed. The left ADC was previously changed after they encountered extremely heavy precipitation during climb out with a rising Indicated Airspeed (IAS) moving into red line and setting off the overspeed aural warning clacker. No circuit breaker exists in the cockpit to silence the overspeed clacker. The only inflight option to silence the clacker is to turn off one; or both; switches for the Data Concentrator Units (DCU's); located on the co-pilot's sidewall. But; doing so also silences other aural warning systems. The pitot heat seemed to be functioning. Reporter stated the contract maintenance Mechanic replaced the #2 ADC; performed a pitot system test and believes the pitot static drains were also opened and cleared. No further problems have occurred since.

Data retrieved from NASA's ASRS site as of May 2009 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.