Narrative:

Aircraft was coupled to autoplt and autoplt was armed for the ILS (8L at atlanta). Aircraft intercepted and captured localizer at approximately 15 NM from airfield, aircraft at 5000 ft. I identified localizer. As per company procedure captain rotated heading select knob to 340 degree for missed approach heading, but unknown to either of us this multifunction knob was pushed in far enough to activate 'heading hold'. I did not notice the flight mode annunciator window change from 'localizer trk' to 'heading hld'. Of course, the ADI (flight director) display remained as before with the pitch bar giving altitude hold at 5000 ft and the bank bar still centered but centered because we were on heading not localizer. Obviously we gradually started to drift right. The HSI (navigation display) was selected on missed approach mode (20 mi scale). On this scale a small deviation off localizer is too small to detect. I monitored the glide slope (raw data display) and saw it descend through the flight director pitch bar. I looked at the FMA and realized we were no longer armed for the ILS. I immediately announced to the captain and disconnected the autoplt to start descent and selected arc mode on the navigation display. I saw we were full scale localizer deflection so I put in about a 15 degree correction to course. At that moment atlanta approach called to tell us we were drifting into the parallel ILS course and he told us to maintain 4500 ft until established. (He also gave us a heading to correct). I leveled at 4700 ft and as I did the localizer centered up and the ILS was resumed uneventfully. Having missed approach mode in HSI instead of arc does not make a localizer deviation immediately obvious. Lack of continuous cross-check of FMA by pilots is a factor. Heading select knob doubles as heading hold button and an imperceptible extra push in on it activates heading hold. To correct the problem: fly ILS with arc (or rose) in missed approach to make deviations immediately obvious. Additionally, multifunction knobs should not be accepted on aircraft. It is simply too easy at night when you are tired or distracted to activate the wrong function. (In the medium large transport we have 3 multifunction knobs where different functions are activated depending on how far in you push the knob. It can be very tricky sometimes).

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Original NASA ASRS Text

Title: USE OF MISSED APCH DISPLAY NOT INCORPORATING RAW DATA INFORMATION AND PUSH PULL KNOB SELECTOR DESIGN CONTRIBUTE TO TRACK DEVIATION IN GLASS COCKPIT MLG.

Narrative: ACFT WAS COUPLED TO AUTOPLT AND AUTOPLT WAS ARMED FOR THE ILS (8L AT ATLANTA). ACFT INTERCEPTED AND CAPTURED LOC AT APPROX 15 NM FROM AIRFIELD, ACFT AT 5000 FT. I IDENTIFIED LOC. AS PER COMPANY PROC CAPT ROTATED HDG SELECT KNOB TO 340 DEG FOR MISSED APCH HDG, BUT UNKNOWN TO EITHER OF US THIS MULTIFUNCTION KNOB WAS PUSHED IN FAR ENOUGH TO ACTIVATE 'HDG HOLD'. I DID NOT NOTICE THE FLT MODE ANNUNCIATOR WINDOW CHANGE FROM 'LOC TRK' TO 'HDG HLD'. OF COURSE, THE ADI (FLT DIRECTOR) DISPLAY REMAINED AS BEFORE WITH THE PITCH BAR GIVING ALT HOLD AT 5000 FT AND THE BANK BAR STILL CENTERED BUT CENTERED BECAUSE WE WERE ON HDG NOT LOC. OBVIOUSLY WE GRADUALLY STARTED TO DRIFT RIGHT. THE HSI (NAV DISPLAY) WAS SELECTED ON MISSED APCH MODE (20 MI SCALE). ON THIS SCALE A SMALL DEVIATION OFF LOC IS TOO SMALL TO DETECT. I MONITORED THE GLIDE SLOPE (RAW DATA DISPLAY) AND SAW IT DSND THROUGH THE FLT DIRECTOR PITCH BAR. I LOOKED AT THE FMA AND REALIZED WE WERE NO LONGER ARMED FOR THE ILS. I IMMEDIATELY ANNOUNCED TO THE CAPT AND DISCONNECTED THE AUTOPLT TO START DSCNT AND SELECTED ARC MODE ON THE NAV DISPLAY. I SAW WE WERE FULL SCALE LOC DEFLECTION SO I PUT IN ABOUT A 15 DEG CORRECTION TO COURSE. AT THAT MOMENT ATLANTA APCH CALLED TO TELL US WE WERE DRIFTING INTO THE PARALLEL ILS COURSE AND HE TOLD US TO MAINTAIN 4500 FT UNTIL ESTABLISHED. (HE ALSO GAVE US A HDG TO CORRECT). I LEVELED AT 4700 FT AND AS I DID THE LOC CENTERED UP AND THE ILS WAS RESUMED UNEVENTFULLY. HAVING MISSED APCH MODE IN HSI INSTEAD OF ARC DOES NOT MAKE A LOC DEVIATION IMMEDIATELY OBVIOUS. LACK OF CONTINUOUS CROSS-CHECK OF FMA BY PLTS IS A FACTOR. HDG SELECT KNOB DOUBLES AS HDG HOLD BUTTON AND AN IMPERCEPTIBLE EXTRA PUSH IN ON IT ACTIVATES HDG HOLD. TO CORRECT THE PROBLEM: FLY ILS WITH ARC (OR ROSE) IN MISSED APCH TO MAKE DEVIATIONS IMMEDIATELY OBVIOUS. ADDITIONALLY, MULTIFUNCTION KNOBS SHOULD NOT BE ACCEPTED ON ACFT. IT IS SIMPLY TOO EASY AT NIGHT WHEN YOU ARE TIRED OR DISTRACTED TO ACTIVATE THE WRONG FUNCTION. (IN THE MLG WE HAVE 3 MULTIFUNCTION KNOBS WHERE DIFFERENT FUNCTIONS ARE ACTIVATED DEPENDING ON HOW FAR IN YOU PUSH THE KNOB. IT CAN BE VERY TRICKY SOMETIMES).

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.