Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification
Aircraft; Airport
Our aircraft was equipped with AHRS and per company policy we are required to take off in DG mode; with no flags; heading slewed to the known magnetic heading on runways with known magnetic anomalies. Runway 13 at LGA is one such runway and we followed this procedure prior to taking the active runway. During climbout I believe we had at least one EFIS COMP MON master caution while on an assigned heading. Shortly thereafter; and still on climbout; we were assigned direct to a fix. The heading flags and EFIS COMP MON messages had disappeared. When we selected NAV mode we were still climbing and the aircraft appeared to be going to the fix. When the aircraft continued to turn past the fix I asked ATC for a heading instead and we used our standby compass for guidance. We briefly leveled off and; with no flags present; I compared our PFD compass headings to the standby compass and switched us back to MAG mode one at a time. When I reached over to switch First Officer (FO) switch to back to MAG mode it was already in that position. I asked ATC to reclear us to the fix. When we went back to NAV mode the course depiction started turning quickly which; to me; indicated an equipment malfunction. I then instructed my FO to go back to heading mode. We were also back in the climb and I asked for another heading to the fix. I also mentioned to the controller that I would not be able to work on the issue until we were straight and level. He handed me off to the next controller; who was aware of the issue but queried for further details and I told him we were having a problem with our gyros and asked him to remain on a heading on the course. Once we were leveled off again; I consulted the QRH even though we had no flags or master caution at the time. At this time; our standby compass and PFD headings were all indicating the same course. I asked the controller to give us a heading to the next fix and when NAV mode was selected the navigation system was on course and we had no further issues. I should have asked the first controller for a level off but he seemed stressed and anxious to get us out of his airspace so I asked him for a heading instead; thinking that would temporarily solve the problem even though I know that a standby compass is not correct until we are in straight; level; unaccelerated flight. I am used to this procedure but the AHRS system has often caused complications in cruise flight when departing from runways with known magnetic anomalies; regular EFIS COMP MON caution messages; further distraction for the crew during high workload periods; aborted takeoffs. It would be extremely helpful to have these systems replaced with IRS equipped aircraft.
When taking off we were in DG mode and both DG were aligned with the runway prior to take off. At around 8;000 while in level flight the captain (pilot monitoring) switched both DGS to MAG mode. As this was done both heading indicators began to spin rapidly; at the same time ATC assigned us direct to a fix and a climb. We put the changes in the box and began the turn. It was about 10 sec later the captain and I began to sense something was wrong. Our heading indicators were displaying heading 90 degrees apart and a red MAG flag then appeared. At that same moment ATC reminded us to go direct a fix. Captain switch back to DG mode and tried to align the DG with the stand by compass. This was taking too long because the heading were now more the 90 different. We asked ATC for a heading to the fix. They then tried to give us different fixes thinking it was a problem with our database. We said the were flying headings only (from standby compose) until we could get this sorted out. ATC assigned a heading that took us 30 sec to get to because of the nature of the stand by compass. After 5 to 6 heading changes form ATC we were finally paralleling our course by about 2 miles. there was a 150kt cross wind that was making it very difficult for ATC to assign good headings to fixes. I kept the controls and took the radios and the captain got out the QRH. The Checklist required level flight so we flew headings off the standby until reaching our cruising altitude. After leveling we ran the checklist and were able to get the heading indicators align in MAG mode. We requested a fix and the FMS navigated the rest of the rough correctly. This is the 3rd time something like this has happed off [this runway]. This was by far the worst occurrence. The other 2 times when switch to MAG mode the heading indicators spun 360 degrees. We were able to switch back to DG mode with no problems and continue with no ATC involvement. I don't know what we can do to solve the hardware problem but if it has happened to me 3 times in the last year it must be happening to others.
CRJ-900 flight crew reported a heading problem with the AHRS system on climb out after a night departure.