Narrative:

Departed allentown en route to stewart (newburgh), ny. Departure clearance was to fly runway heading and maintain 3000 ft. After leveling off at 3000 ft, the autoplt was engaged. For some reason, the autoplt started a very gradual right turn. This was so slight that it did not appear on the ADI as a turn. The first officer was getting the ATIS and I was flying the aircraft. I did not notice the turn until asked by the center controller why we had turned off the runway heading. I replied truthfully that I did not know we had. The controller then gave us a turn back towards our original course. A further problem with this situation is that the airspeed indicator is difficult to see, as it is partially blocked by the control yoke. If the attitude direction indicator does not show the turn, as in this case, it might not be picked up by the crew. This was a night cargo flight that had been diverted to allentown and was running behind schedule. Having been up all night, fatigue certainly played a factor. Supplemental information from acn 561807: assigned a 060 degree heading by the tower and switched from approach to ZNY frequency, who informed us that we could expect higher momentarily. The captain, who was flying the aircraft on autoplt, then directed me to obtain the ATIS information from our destination (swf, ny) and advised me that he would monitor our primary radio frequency with ZNY while I obtained the ATIS. Our aircraft began a shallow r-hand turn/drift from our primary heading (still 060 degrees), which I assumed to be a new heading assigned by ZNY. ZNY asked us why we had deviated from our previous assigned heading. My compass read 085 degrees. ZNY then assigned us a new heading of about 010 degrees. The captain, who was still flying on autoplt, then made an abrupt turn to the left to comply, and apologized to ZNY citing an autoplt drift problem. In effect, our autoplt had malfunctioned and started a r-hand drift, even though all controls were centered and the aircraft was relatively in a trim condition. I incorrectly assumed that the captain was turning right to comply with a new ATC assigned heading. Overall, I learned that one has to be extremely vigilant when one pilot is flying the aircraft and complying with ATC instructions, when the other pilot is busy copying other information (ATIS).

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Original NASA ASRS Text

Title: HDG TRACK DEV BY A DC9 CARGO FLC WHEN THE CAPT FAILS TO NOTE A SLIGHT TURN TO THE R WHILE ON AUTOPLT DURING A RWY HDG DEP PROC FROM ABE, PA.

Narrative: DEPARTED ALLENTOWN ENRTE TO STEWART (NEWBURGH), NY. DEP CLRNC WAS TO FLY RWY HDG AND MAINTAIN 3000 FT. AFTER LEVELING OFF AT 3000 FT, THE AUTOPLT WAS ENGAGED. FOR SOME REASON, THE AUTOPLT STARTED A VERY GRADUAL R TURN. THIS WAS SO SLIGHT THAT IT DID NOT APPEAR ON THE ADI AS A TURN. THE FO WAS GETTING THE ATIS AND I WAS FLYING THE ACFT. I DID NOT NOTICE THE TURN UNTIL ASKED BY THE CTR CTLR WHY WE HAD TURNED OFF THE RWY HDG. I REPLIED TRUTHFULLY THAT I DID NOT KNOW WE HAD. THE CTLR THEN GAVE US A TURN BACK TOWARDS OUR ORIGINAL COURSE. A FURTHER PROB WITH THIS SIT IS THAT THE AIRSPD INDICATOR IS DIFFICULT TO SEE, AS IT IS PARTIALLY BLOCKED BY THE CTL YOKE. IF THE ATTITUDE DIRECTION INDICATOR DOES NOT SHOW THE TURN, AS IN THIS CASE, IT MIGHT NOT BE PICKED UP BY THE CREW. THIS WAS A NIGHT CARGO FLT THAT HAD BEEN DIVERTED TO ALLENTOWN AND WAS RUNNING BEHIND SCHEDULE. HAVING BEEN UP ALL NIGHT, FATIGUE CERTAINLY PLAYED A FACTOR. SUPPLEMENTAL INFO FROM ACN 561807: ASSIGNED A 060 DEG HDG BY THE TWR AND SWITCHED FROM APCH TO ZNY FREQ, WHO INFORMED US THAT WE COULD EXPECT HIGHER MOMENTARILY. THE CAPT, WHO WAS FLYING THE ACFT ON AUTOPLT, THEN DIRECTED ME TO OBTAIN THE ATIS INFO FROM OUR DEST (SWF, NY) AND ADVISED ME THAT HE WOULD MONITOR OUR PRIMARY RADIO FREQ WITH ZNY WHILE I OBTAINED THE ATIS. OUR ACFT BEGAN A SHALLOW R-HAND TURN/DRIFT FROM OUR PRIMARY HDG (STILL 060 DEGS), WHICH I ASSUMED TO BE A NEW HDG ASSIGNED BY ZNY. ZNY ASKED US WHY WE HAD DEVIATED FROM OUR PREVIOUS ASSIGNED HDG. MY COMPASS READ 085 DEGS. ZNY THEN ASSIGNED US A NEW HDG OF ABOUT 010 DEGS. THE CAPT, WHO WAS STILL FLYING ON AUTOPLT, THEN MADE AN ABRUPT TURN TO THE L TO COMPLY, AND APOLOGIZED TO ZNY CITING AN AUTOPLT DRIFT PROB. IN EFFECT, OUR AUTOPLT HAD MALFUNCTIONED AND STARTED A R-HAND DRIFT, EVEN THOUGH ALL CTLS WERE CTRED AND THE ACFT WAS RELATIVELY IN A TRIM CONDITION. I INCORRECTLY ASSUMED THAT THE CAPT WAS TURNING R TO COMPLY WITH A NEW ATC ASSIGNED HDG. OVERALL, I LEARNED THAT ONE HAS TO BE EXTREMELY VIGILANT WHEN ONE PLT IS FLYING THE ACFT AND COMPLYING WITH ATC INSTRUCTIONS, WHEN THE OTHER PLT IS BUSY COPYING OTHER INFO (ATIS).

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.