Citationjet (C525/C526) - CJ I / II / III / IV
Flight Crew Private; Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 30; Flight Crew Total 1300; Flight Crew Type 6
Training / Qualification; Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance
Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem
Aircraft; Human Factors
I was the pilot flying in the left seat receiving dual instruction from the type rated ATP pilot in the right seat. Approach cleared us down to 4;000 ft and was vectoring us for the visual approach. Approach assigned a speed of 200 kts and an altitude of 4;000. I was at 200 kts and then noticed that the indicated speed started climbing; so I started pulling back on the throttles. As I kept pulling back on the throttles; the pilot in the other seat then noticed that both our speed and altitude were off of the assignments. Just as he asked me 'What are you doing?' and I was answering 'Maintaining 200 kts and...' The warning annunciations started going off like crazy. We both looked at my screen and then his and quickly realized that everything I was seeing on my #1 screen was wrong because the #1 ADC had failed. He then directed me to flip the 'reversion' switch to revert the pilots instruments to the #2 ADC. As soon as I did that; the indications came back correctly. Right then ATC inquired as to our altitude; which was now showing 4;300. We told them that we were having indication problems with a failed air data computer. They asked if we needed further assistance or a vector; which we didn't at that point; because it was working fine after reverting to the non-failed ADC. The really scary part of this incident is that the annunciator panel (and my screen) didn't know that the ADC had failed for some unknown period of time. It wasn't until there was a very significant difference between reality and what was being shown on the screen (and what was being shown on the copilots instruments) that it finally recognized the failure and annunciated it. I still do not know why it took the plane so long to recognize the failure and tell the pilots.
C525A pilot reported altitude and airspeed deviations resulted from the failure of the #1 ADC.