Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 200; Flight Crew Total 25000; Flight Crew Type 3500
Situational Awareness; Human-Machine Interface; Confusion; Troubleshooting
Aircraft Equipment Problem Critical; Conflict Airborne Conflict; Deviation - Track / Heading All Types
Air Traffic Control Provided Assistance; Air Traffic Control Separated Traffic; Aircraft Equipment Problem Dissipated; Flight Crew Took Evasive Action
Aircraft
During the SID climb departure it became immediately apparent that AHRS headings had both malfunctioned and were 30-45 degrees off from each other. NAV information was extremely unreliable. DG Fast Slave did not work on either AHRS and autopilot could not be used. Mag Compass was only immediate heading source. When ATC cleared us on a heading it was way off due to MFD map error. Down track VOR's were given but again displayed info was erroneous and direct course could not be flown precisely even with raw data green needles. It was during an attempted direct to a VOR that we began to deviate toward another aircraft in the vicinity of 34;000 FT. ATC Controller put us back on a divergent heading so conflict would not occur. We got no TCAS warning or loss of min separation indication from the Controller. Finally at higher altitudes and in smooth air; mag compass settled down enough to steer a reasonably accurate heading. Right AHRS also settled down enough at cruise to use in conjunction with autopilot.
The Reporter stated that the at no time during this event was a system warning or error displayed. Also; even though the left and right FMS information was divergent the two systems did not transition to independent operation. The C750 wingtips are only about 4 1/2 to 5 feet off the ground and so a magnetic anomaly near the end of LGA Runway 13 may affect the compass systems. At cruise altitude the navigation systems were not usable but after the reporter noticed the 240 kt crosswind in the left FMS he realized that was the offending system and put the two systems in independent operations. At that time the right FMS displayed correctly and was utilized for navigation. Eventually the left FMS also became usable.
Both of a CE750's AHRS malfunctioned after a LGA Runway 13 takeoff and required ATC vectors referencing the magnetic compass until the compass systems returned to normal at cruise operating independently.