Narrative:

We were air carrier XXX. Shortly after turning right at ZZZZ intersection; we received a 290 degree vector. We were at FL340; with traffic at FL320 and FL300 slightly ahead and left of us in sight. The captain was flying with autoplt #1 engaged; using INS #1 for navigation guidance; he had true heading reference; while I had magnetic heading so I could monitor VOR airway guidance. The captain switched his heading reference to magnetic and started to turn left; showing a need to turn several degrees. I showed a 280 degree heading before he started turning; and told him I showed a need to come right to get to 290 degrees. He stopped his turn; cross checked the standby magnetic compass and ascertained my compass was correct (I had reslaved my compass; and it didn't change more than 2 degrees). He then turned right to satisfy my compass. Once I was satisfied we were on 290 degree magnetic heading; I turned my attention to my chart and looking out my side window for traffic since we were in busy airspace. Perhaps a minute later I detected a flashing light in my left peripheral vision; and looked at my instrument panel. The amber altitude alerter light on my altimeter was flashing; and I saw that the red autoplt #1 fail light was flashing too. The #1 autoplt lever was disengaged; and our altitude was FL337 and slowly decreasing. I called out to the captain that the autoplt was disengaged and we were descending; and began corrective action immediately. Right after my call to the captain; washington center queried us on our altitude. I replied we were 300 or 400 ft low (I don't recall which I said) and correcting; and that we'd had an autoplt malfunction. The captain returned to FL340; reengaged autoplt #1; and monitored it for proper operation. We discussed the incident and the captain said he thought the autoplt dropped off when; after he had turned to 290 degree heading; he had used the #1 compass set heading knob to try to resynchronize his magnetic compass to the correct heading. He then turned the heading set knob; and verified that it made the #1 autoplt drop off. There was not aural warning when it dropped off; and we determined that bright sunlight entering the cockpit over his left shoulder had washed out the amber and red warning lights on his instrument panel so he didn't see them. The click the autoplt lever made when it dropped to 'off' wasn't loud enough for any of the three of us to hear and the altitude change was slow enough to not give a 'seat of the pants' feel. Adding an aural tone anytime the autoplt disengages (presently occurs in land mode) would probably prevent similar events.

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Original NASA ASRS Text

Title: A DC10-30 CREW UNKNOWINGLY DSCNDED 300 TO 400 FT AFTER AN INADVERTENT AUTOPLT DISCONNECT WHILE TROUBLESHOOTING A COMPASS PROB. THE AURAL AUTOPLT DISCONNECT ALARM OPERATES ONLY IN SINGLE OR DUAL AUTOPLT AUTOLAND MODE.

Narrative: WE WERE ACR XXX. SHORTLY AFTER TURNING R AT ZZZZ INTERSECTION; WE RECEIVED A 290 DEG VECTOR. WE WERE AT FL340; WITH TFC AT FL320 AND FL300 SLIGHTLY AHEAD AND L OF US IN SIGHT. THE CAPT WAS FLYING WITH AUTOPLT #1 ENGAGED; USING INS #1 FOR NAV GUIDANCE; HE HAD TRUE HDG REFERENCE; WHILE I HAD MAGNETIC HDG SO I COULD MONITOR VOR AIRWAY GUIDANCE. THE CAPT SWITCHED HIS HDG REFERENCE TO MAGNETIC AND STARTED TO TURN L; SHOWING A NEED TO TURN SEVERAL DEGS. I SHOWED A 280 DEG HDG BEFORE HE STARTED TURNING; AND TOLD HIM I SHOWED A NEED TO COME R TO GET TO 290 DEGS. HE STOPPED HIS TURN; CROSS CHKED THE STANDBY MAGNETIC COMPASS AND ASCERTAINED MY COMPASS WAS CORRECT (I HAD RESLAVED MY COMPASS; AND IT DIDN'T CHANGE MORE THAN 2 DEGS). HE THEN TURNED R TO SATISFY MY COMPASS. ONCE I WAS SATISFIED WE WERE ON 290 DEG MAGNETIC HDG; I TURNED MY ATTENTION TO MY CHART AND LOOKING OUT MY SIDE WINDOW FOR TFC SINCE WE WERE IN BUSY AIRSPACE. PERHAPS A MINUTE LATER I DETECTED A FLASHING LIGHT IN MY L PERIPHERAL VISION; AND LOOKED AT MY INSTRUMENT PANEL. THE AMBER ALT ALERTER LIGHT ON MY ALTIMETER WAS FLASHING; AND I SAW THAT THE RED AUTOPLT #1 FAIL LIGHT WAS FLASHING TOO. THE #1 AUTOPLT LEVER WAS DISENGAGED; AND OUR ALT WAS FL337 AND SLOWLY DECREASING. I CALLED OUT TO THE CAPT THAT THE AUTOPLT WAS DISENGAGED AND WE WERE DSNDING; AND BEGAN CORRECTIVE ACTION IMMEDIATELY. RIGHT AFTER MY CALL TO THE CAPT; WASHINGTON CTR QUERIED US ON OUR ALT. I REPLIED WE WERE 300 OR 400 FT LOW (I DON'T RECALL WHICH I SAID) AND CORRECTING; AND THAT WE'D HAD AN AUTOPLT MALFUNCTION. THE CAPT RETURNED TO FL340; REENGAGED AUTOPLT #1; AND MONITORED IT FOR PROPER OPERATION. WE DISCUSSED THE INCIDENT AND THE CAPT SAID HE THOUGHT THE AUTOPLT DROPPED OFF WHEN; AFTER HE HAD TURNED TO 290 DEG HDG; HE HAD USED THE #1 COMPASS SET HDG KNOB TO TRY TO RESYNCHRONIZE HIS MAGNETIC COMPASS TO THE CORRECT HDG. HE THEN TURNED THE HDG SET KNOB; AND VERIFIED THAT IT MADE THE #1 AUTOPLT DROP OFF. THERE WAS NOT AURAL WARNING WHEN IT DROPPED OFF; AND WE DETERMINED THAT BRIGHT SUNLIGHT ENTERING THE COCKPIT OVER HIS L SHOULDER HAD WASHED OUT THE AMBER AND RED WARNING LIGHTS ON HIS INSTRUMENT PANEL SO HE DIDN'T SEE THEM. THE CLICK THE AUTOPLT LEVER MADE WHEN IT DROPPED TO 'OFF' WASN'T LOUD ENOUGH FOR ANY OF THE THREE OF US TO HEAR AND THE ALT CHANGE WAS SLOW ENOUGH TO NOT GIVE A 'SEAT OF THE PANTS' FEEL. ADDING AN AURAL TONE ANYTIME THE AUTOPLT DISENGAGES (PRESENTLY OCCURS IN LAND MODE) WOULD PROBABLY PREVENT SIMILAR EVENTS.

Data retrieved from NASA's ASRS site as of January 2009 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.