Narrative:

I (PIC) was flying along at FL350 when I noticed that my airspeed was down [to] about 220 KTS. ATC notified us of an [aircraft] that was 12 o'clock and at FL340 and asked us if we have traffic in sight. He then asked us [our] altitude. My side (captain's side) was reading about 34;600 ft; but I cross checked it with the standby and my first officer's and both read FL350. Then ATC said we show you at 34;600 ft and I changed the transponder from transponder 1 to transponder 2; which is why they saw a jump in the altitude. We notified dispatch through ACARS and they talked to moc as we ran the checklist. While trying to figure out what this air data computer was doing; at one point the airspeed was as low as 140 KTS. I think it was right above the white line; I remember that; but the other two airspeed indicators were reading around 250-260 KTS. Switched the air data computer on my side from 1 to 2 then back again a couple of times. While running the checklist and talking to dispatch; my air data computer starting working correctly all of a sudden. Told dispatch and was told to contact maintenance control when we land; which is what I did. I called them on my cell phone and told them what had happen and that the problem went away. They asked if I wrote it up and I told them no. They said [not to] worry about writing it up; it must have just been a 'gremlin' in the system. I told the other crew what had happened so they would keep and eye on it and then we went to the hotel. The altitude deviation was a faulty indication due to the problem with the #1 air data computer. At no time were we off on our altitude because the autopilot was flying in cruise flight. The air data computer was doing something stupid. I have been on this aircraft for about 9 years and have never seen anything like this. When I first noticed the air data computer #1 messing up should have changed to air data computer #2 when I noticed standby and first officer's were reading correctly; but I was evaluating the problem and looking for traffic while flying the airplane.

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Original NASA ASRS Text

Title: An E-140 flight crew was unaware of a faulty ADC which was providing inaccurate altitude information to ARTCC controllers.

Narrative: I (PIC) was flying along at FL350 when I noticed that my airspeed was down [to] about 220 KTS. ATC notified us of an [aircraft] that was 12 o'clock and at FL340 and asked us if we have traffic in sight. He then asked us [our] altitude. My side (Captain's side) was reading about 34;600 FT; but I cross checked it with the standby and my First Officer's and both read FL350. Then ATC said we show you at 34;600 FT and I changed the transponder from transponder 1 to transponder 2; which is why they saw a jump in the altitude. We notified Dispatch through ACARS and they talked to MOC as we ran the checklist. While trying to figure out what this ADC was doing; at one point the airspeed was as low as 140 KTS. I think it was right above the white line; I remember that; but the other two airspeed indicators were reading around 250-260 KTS. Switched the ADC on my side from 1 to 2 then back again a couple of times. While running the checklist and talking to Dispatch; my ADC starting working correctly all of a sudden. Told Dispatch and was told to contact Maintenance Control when we land; which is what I did. I called them on my cell phone and told them what had happen and that the problem went away. They asked if I wrote it up and I told them no. They said [not to] worry about writing it up; it must have just been a 'gremlin' in the system. I told the other crew what had happened so they would keep and eye on it and then we went to the hotel. The altitude deviation was a faulty indication due to the problem with the #1 ADC. At no time were we off on our altitude because the autopilot was flying in cruise flight. The ADC was doing something stupid. I have been on this aircraft for about 9 years and have never seen anything like this. When I first noticed the ADC #1 messing up should have changed to ADC #2 when I noticed standby and First Officer's were reading correctly; but I was evaluating the problem and looking for traffic while flying the airplane.

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.