Narrative:

The medium large transport flight guidance system has a design flaw that has trapped many of my peers and finally got me, too: the altitude alert is set by turning a knob on the glare shield, the set altitude is then 'armed' for capture by the autoplt and/or flight director by pulling on the same knob. If, in pulling the knob, you rotate the knob even slightly, the set altitude will change by 1000 ft in the direction of rotation and the new incorrect altitude will be armed. This flight departed den with the SID altitude of 10000 ft armed during preflight. My first officer was new to the aircraft and flying the leg. Immediately after contacting departure control, we were given a heading and cleared to FL200. The first officer dialed in the heading and reset the altitude alert. Departure control then noted a conflict with an inbound aircraft (descending to 11000 ft) and instructed us to level off at 10000. The first officer re-reset the altitude alert to 10000. The aircraft was on autoplt and the autoplt was climbing through 10000 ft. I said, 'it's not leveling at 10.' the first officer reached for the flight guidance panel (that's when I noticed that 11000 ft was set in the altitude alert) and I reached for the yoke. I noted 10300 on the digital altimeter as I pushed over. Departure control said, 'air carrier check altitude, I show 10400.' I clearly remember seeing 10000 ft in the altitude alert window on the re-reset. I obviously did not rechk the window after the altitude was armed. The automatics are great when they do what we expect. When they don't, it seems like eons before we can get back 'in the loop' and correct the problem. This problem, however, is set up by the design of the altitude alert knob. Knowing the potential exists doesn't make it any easier to double-check the altitude in the window after the system is armed. I will be much more diligent for a very long time, but this will happen again -- maybe not to me, but it will happen.

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Original NASA ASRS Text

Title: AN ACR MLG OVERSHOT ITS ASSIGNED ALT ON CLB.

Narrative: THE MLG FLT GUIDANCE SYS HAS A DESIGN FLAW THAT HAS TRAPPED MANY OF MY PEERS AND FINALLY GOT ME, TOO: THE ALT ALERT IS SET BY TURNING A KNOB ON THE GLARE SHIELD, THE SET ALT IS THEN 'ARMED' FOR CAPTURE BY THE AUTOPLT AND/OR FLT DIRECTOR BY PULLING ON THE SAME KNOB. IF, IN PULLING THE KNOB, YOU ROTATE THE KNOB EVEN SLIGHTLY, THE SET ALT WILL CHANGE BY 1000 FT IN THE DIRECTION OF ROTATION AND THE NEW INCORRECT ALT WILL BE ARMED. THIS FLT DEPARTED DEN WITH THE SID ALT OF 10000 FT ARMED DURING PREFLT. MY FO WAS NEW TO THE ACFT AND FLYING THE LEG. IMMEDIATELY AFTER CONTACTING DEP CTL, WE WERE GIVEN A HDG AND CLRED TO FL200. THE FO DIALED IN THE HDG AND RESET THE ALT ALERT. DEP CTL THEN NOTED A CONFLICT WITH AN INBOUND ACFT (DSNDING TO 11000 FT) AND INSTRUCTED US TO LEVEL OFF AT 10000. THE FO RE-RESET THE ALT ALERT TO 10000. THE ACFT WAS ON AUTOPLT AND THE AUTOPLT WAS CLBING THROUGH 10000 FT. I SAID, 'IT'S NOT LEVELING AT 10.' THE FO REACHED FOR THE FLT GUIDANCE PANEL (THAT'S WHEN I NOTICED THAT 11000 FT WAS SET IN THE ALT ALERT) AND I REACHED FOR THE YOKE. I NOTED 10300 ON THE DIGITAL ALTIMETER AS I PUSHED OVER. DEP CTL SAID, 'ACR CHK ALT, I SHOW 10400.' I CLRLY REMEMBER SEEING 10000 FT IN THE ALT ALERT WINDOW ON THE RE-RESET. I OBVIOUSLY DID NOT RECHK THE WINDOW AFTER THE ALT WAS ARMED. THE AUTOMATICS ARE GREAT WHEN THEY DO WHAT WE EXPECT. WHEN THEY DON'T, IT SEEMS LIKE EONS BEFORE WE CAN GET BACK 'IN THE LOOP' AND CORRECT THE PROB. THIS PROB, HOWEVER, IS SET UP BY THE DESIGN OF THE ALT ALERT KNOB. KNOWING THE POTENTIAL EXISTS DOESN'T MAKE IT ANY EASIER TO DOUBLE-CHK THE ALT IN THE WINDOW AFTER THE SYS IS ARMED. I WILL BE MUCH MORE DILIGENT FOR A VERY LONG TIME, BUT THIS WILL HAPPEN AGAIN -- MAYBE NOT TO ME, BUT IT WILL HAPPEN.

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.