Gulfstream Jet Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Aircraft Equipment Problem Critical; Deviation - Speed All Types
Flight Crew Became Reoriented; Flight Crew Requested ATC Assistance / Clarification
Aircraft; Human Factors; Weather
Cruising at FL410 we became aware that ADC 1 and ADC 2 we're giving erroneous airspeed indications. After discussion with company and maintenance we decided ADC 3 was providing accurate information. During descent in the mid 30's the yaw damp began applying left and right inputs causing oscillations and both engines went into ALT CONT.  We selected the yaw damp off causing the autopilot to disengage and the flight director cues disappeared.  We tracked slightly off course and corrected back onto the arrival.  We asked Brasilia and Rio control for relief from crossing restrictions and continued on the approval path to an uneventful landing.
Gulfstream Captain experiences the loss of ADC 1&2 in cruise at FL410. ADC 3 is determined to be reliable and the flight continues to destination.