A CRJ 700 received an EFIS COMP MON while in position on LGA Runway 13 so; as per SOP; the crew switched to the DG Mode. After takeoff when the MAG Mode was selected the AHRS and ND displayed widely differing information and ATC issued vectors until the compass system was realigned in level flight.
Synopsis
A CRJ 700 received an EFIS COMP MON while in position on LGA Runway 13 so; as per SOP; the crew switched to the DG Mode. After takeoff when the MAG Mode was selected the AHRS and ND displayed widely differing information and ATC issued vectors until the compass system was realigned in level flight.
Narrative
[Assigned} Runway 13 LGA; Whitestone Climb. While entering the runway; we received an EFIS COMP MON Caution message. We selected DG mode on the compass panel and manually aligned the heading indicator to runway heading; per our procedures. After successfully completing the departure procedure; we were instructed by ATC to fly directly to ELIOT Intersection. While in a coordinated; stable; climbing turn; the pilot not flying reselected MAG mode in accordance with the After Take-off Checklist. Both heading indicators then swung wildly; and settled on a heading about 50 degrees different then what DG mode had indicated. Our FMS set a course to ELIOT; and we began to turn toward it. At this time; our moving map displays blanked and presented a red MAG flag. We rolled out on an approximate course to ELIOT and consulted the magnetic compass. It indicated that we were not flying in the right direction. Just as the pilot not flying was about to contact ATC to inform them of our issue; they called us and informed us we were not on a heading to ELIOT. We replied that we had lost our navigation system and they issued a vector; which I turned to using timed turns with the magnetic compass. After consulting the AOM Volume one; the pilot not flying again selected DG mode; realigned the heading indicators to the magnetic compass; and reselected MAG mode. The heading indicators aligned properly; and the moving map reappeared. We queried ATC; and they confirmed that we were in fact on course to ELIOT. No further defects were noted; and we elected to continue. At no time did we violate surrounding airspace; or conflict with other traffic. The AOM Volume one indicates that the compass mode should only be changed when the aircraft is stable. In fact; the mode should only be changed in straight and level flight. This should be an aircraft limitation.
Second reporter narrative
Taxied into position on Runway 13 at LGA. While holding; we had AHRS failures on both the Captain's and First Officer's EFIS. This is common for this runway because of the metal structures beneath the runway surface of this particular runway. We followed normal procedures for this occurrence and switched our AHRS from MAG mode to DG mode. We slewed the headings to runway heading. All was acceptable for takeoff. After takeoff we conducted the Whitestone Climb as published. After the initial climb we were given a turn to the southwest and a continued climb. At the same time; we did the after takeoff flow and checklist. This procedure requires that we switch back from DG mode to MAG mode on the AHRS. When we did this; the headings immediately slewed approximately 45 degrees to the left. We were then given a turn by ATC to go direct to ELIOT intersection. We turned direct to ELIOT according to our EFIS and FMS indications. After looking at our standby heading indicator we realized that our heading to ELIOT was not correct. At the same time; ATC told us that we were off course and not heading to ELIOT. Our EFIS still showed us going direct to ELIOT. I reported to ATC that we were having a problem with our compass system. ATC told us to head 300 degrees. The First Officer was the pilot flying. He was hand flying and began using the standby compass for heading guidance. We were level at 12;000 FT and holding a speed of 250 KTS. I went through the checklist which led us to switch the AHRS back to DG mode. After doing this; I slewed both heading indicators to match the standby compass. We were in stable flight. I then put the AHRS back into MAG mode and both heading indicators began indicating normal again. I told ATC that our AHRS appeared to be working normally again. They told us to go direct to ELIOT which we did. All indications still appeared to be working normal and ATC confirmed that we were direct ELIOT. We continued the flight and everything continued to work normally. This was a system malfunction when switching from DG mode to MAG mode on the AHRS.
NASA callback
The Reporter stated that the EFIS COMP MON alert has become normal on the LGA Runway 13 departure and that most crews select the MAG Mode during the climb transition even in a turn. This is the first time that the Reporter has seen a system not immediately switch to normal operation. He did go back and look in the Pilot Handbook and found that it does not state that selecting the MAG Mode in a turn is an acceptable procedure. Selecting the MAG Mode from the DG Mode is supposed be accomplished in level flight and at a constant acceleration. The Reporter believes that the manual and procedure should state a specific prohibition about selecting the MAG Mode during a turn.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.