Narrative:

While climbing through FL300 we encountered an airspeed mis-compare error; followed shortly by an altitude mis-compare. We assessed which instrument set was accurate by using the stand-by instruments. Since the first officer's instrumentation was indicating correctly; I transferred control of the aircraft to the first officer and began the QRH. I then contacted dispatch and informed them of the situation and was then handed over to maintenance control. We then began explaining what indications we were getting; and while we were discussing this; the errors went away for about 30 seconds. Maintenance control informed me that if there are no present errors; we are okay with you continuing on to your destination. At that time the airspeed error came back and I then told maintenance control I was returning to departure airport and needed to talk to dispatch; we then did an air data computer reversion and returned which had significantly better weather than destination. Maintenance had changed out ahrs#1 in response to an earlier write up mirroring what we had experienced; so this was not a first time problem this day. With the error messages presented; which were: IAS mis-compare; and altitude mis-compare on both primary flight displays; sps advance; and ahrs 1 basic mode on EICAS; and ADC1 fail on the FMC; that combined with the weather at destination (forecast at our time of arrival to be winds out of south west at 20 gusting 35; overcast 500 ft heavy rain; visibility 1 SM strong thunderstorms) I felt that the safer course of action was to return where we had a relatively thin 2000 foot thick layer with bases around 4000 ft and clear skies directly overhead.

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Original NASA ASRS Text

Title: An EMB-145 Captain described experiencing Air Data Computer failure in flight resulting in Airspeed and Altitude Mis-Compare EICAS alerts. He elected to return to departure airport for maintenance.

Narrative: While climbing through FL300 we encountered an airspeed mis-compare error; followed shortly by an altitude mis-compare. We assessed which instrument set was accurate by using the stand-by instruments. Since the First Officer's instrumentation was indicating correctly; I transferred control of the aircraft to the First Officer and began the QRH. I then contacted Dispatch and informed them of the situation and was then handed over to Maintenance Control. We then began explaining what indications we were getting; and while we were discussing this; the errors went away for about 30 seconds. Maintenance Control informed me that if there are no present errors; we are okay with you continuing on to your destination. At that time the airspeed error came back and I then told Maintenance Control I was returning to departure airport and needed to talk to Dispatch; we then did an ADC reversion and returned which had significantly better weather than destination. Maintenance had changed out AHRS#1 in response to an earlier write up mirroring what we had experienced; so this was not a first time problem this day. With the error messages presented; which were: IAS Mis-Compare; and ALT Mis-Compare on both Primary Flight Displays; SPS Advance; and AHRS 1 Basic Mode on EICAS; and ADC1 fail on the FMC; that combined with the weather at destination (forecast at our time of arrival to be winds out of south west at 20 gusting 35; overcast 500 FT heavy rain; visibility 1 SM strong thunderstorms) I felt that the safer course of action was to return where we had a relatively thin 2000 foot thick layer with bases around 4000 FT and clear skies directly overhead.

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.