Narrative:

The captain was flying the leg. The large transport had a block 5 autoplt and leveled itself at FL370. The captain was using the heading mode. Mia ATC issued a heading of 020 degrees, captain complies. Approximately 2 mins pass while captain inquires about the heading issued. The reply was 020 degrees. The first officer's heading read 025 degrees, while the captain replied his heading was 040 degrees, a 15 degree difference between the 2. The compass on a check read 040 degrees, the aircraft had drifted to the right of assigned heading. First officer reset his compass 15 degree correction. The captain started to correct back to 020 degrees when the autoplt responds with a 20 degree bank to the right with full scale deflection with turn knob to the left. The captain disengaged the autoplt and flew it manually back towards the assigned heading. The captain disengaged the autoplt, tried a, B, and ab mode to reengage the autoplt. Upon reaching the 020 degree heading, ATC assigned a new heading of 010 degrees. ATC inquired as to our heading. The first officer responded passing through 015 degrees towards 010 degrees. Then ATC issued a new heading of 360 degrees. Upon reaching 360 degree heading, captain tried again to engage in a, B, and ab mode. All resulted in an immediate right bank. Upon reengaging the autoplt the roll is not as severe and captain controled the wings level by using slight left turn knob, and altitude hold in ab mode. After a short period of time, the autoplt eventually responded normally. The so recycled every circuit breaker relating to the autoplt. Later, while in jacksonville airspace, we were given a phone number to call ZMA upon our arrival in boston. Captain later briefed me that our 360 degree vector would keep us clear of an medium large transport that was climbing to FL410. We were TCASII equipped and at no time did a conflict appear to be imminent, nor was there any hint of urgency with ZMA. The controller said that the spacing was 4.8 mi and 800 ft vertical separation behind us. Upon arrival in boston, maintenance replaced the roll computer and autoplt performed normally on return trip south. Supplemental information from acn 233661: flight director showing fly left, heading bug on 020 degrees, heading mode annunciator light on. 15 degree differences in our compasses. So we cycle all the autoplt circuit breakers and try everything again.

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Original NASA ASRS Text

Title: AN ACR LGT HAD LTSS WITH A CPR MLG.

Narrative: THE CAPT WAS FLYING THE LEG. THE LGT HAD A BLOCK 5 AUTOPLT AND LEVELED ITSELF AT FL370. THE CAPT WAS USING THE HDG MODE. MIA ATC ISSUED A HDG OF 020 DEGS, CAPT COMPLIES. APPROX 2 MINS PASS WHILE CAPT INQUIRES ABOUT THE HDG ISSUED. THE REPLY WAS 020 DEGS. THE FO'S HDG READ 025 DEGS, WHILE THE CAPT REPLIED HIS HDG WAS 040 DEGS, A 15 DEG DIFFERENCE BTWN THE 2. THE COMPASS ON A CHK READ 040 DEGS, THE ACFT HAD DRIFTED TO THE R OF ASSIGNED HDG. FO RESET HIS COMPASS 15 DEG CORRECTION. THE CAPT STARTED TO CORRECT BACK TO 020 DEGS WHEN THE AUTOPLT RESPONDS WITH A 20 DEG BANK TO THE R WITH FULL SCALE DEFLECTION WITH TURN KNOB TO THE L. THE CAPT DISENGAGED THE AUTOPLT AND FLEW IT MANUALLY BACK TOWARDS THE ASSIGNED HDG. THE CAPT DISENGAGED THE AUTOPLT, TRIED A, B, AND AB MODE TO REENGAGE THE AUTOPLT. UPON REACHING THE 020 DEG HDG, ATC ASSIGNED A NEW HDG OF 010 DEGS. ATC INQUIRED AS TO OUR HDG. THE FO RESPONDED PASSING THROUGH 015 DEGS TOWARDS 010 DEGS. THEN ATC ISSUED A NEW HDG OF 360 DEGS. UPON REACHING 360 DEG HDG, CAPT TRIED AGAIN TO ENGAGE IN A, B, AND AB MODE. ALL RESULTED IN AN IMMEDIATE R BANK. UPON REENGAGING THE AUTOPLT THE ROLL IS NOT AS SEVERE AND CAPT CTLED THE WINGS LEVEL BY USING SLIGHT L TURN KNOB, AND ALT HOLD IN AB MODE. AFTER A SHORT PERIOD OF TIME, THE AUTOPLT EVENTUALLY RESPONDED NORMALLY. THE SO RECYCLED EVERY CIRCUIT BREAKER RELATING TO THE AUTOPLT. LATER, WHILE IN JACKSONVILLE AIRSPACE, WE WERE GIVEN A PHONE NUMBER TO CALL ZMA UPON OUR ARR IN BOSTON. CAPT LATER BRIEFED ME THAT OUR 360 DEG VECTOR WOULD KEEP US CLR OF AN MLG THAT WAS CLBING TO FL410. WE WERE TCASII EQUIPPED AND AT NO TIME DID A CONFLICT APPEAR TO BE IMMINENT, NOR WAS THERE ANY HINT OF URGENCY WITH ZMA. THE CTLR SAID THAT THE SPACING WAS 4.8 MI AND 800 FT VERT SEPARATION BEHIND US. UPON ARR IN BOSTON, MAINT REPLACED THE ROLL COMPUTER AND AUTOPLT PERFORMED NORMALLY ON RETURN TRIP S. SUPPLEMENTAL INFO FROM ACN 233661: FLT DIRECTOR SHOWING FLY L, HDG BUG ON 020 DEGS, HDG MODE ANNUNCIATOR LIGHT ON. 15 DEG DIFFERENCES IN OUR COMPASSES. SO WE CYCLE ALL THE AUTOPLT CIRCUIT BREAKERS AND TRY EVERYTHING AGAIN.

Data retrieved from NASA's ASRS site as of July 2007 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.