Narrative:

We were on descent to lax on the 24/25 profile descent at the sadde intersection turn. We were descending to 10000' in the turn. Both pilots were scanning for traffic outside. As PF, I had the autoplt on with the altitude advisory system set at 10000'. I was controling the roll of the aircraft with the heading select knob. I took a glance at my altitude in the turn as I came through 10500' with the altitude advisory still armed at 10000'. Both pilots continued to scan outside and to complete the preliminary cockpit checklist. At 10000' the autoplt did not capture the altitude and we were alerted to the 250' deviation warning. I abruptly took manual control of the aircraft and returned to 10000'. We may have dipped to 9700' MSL before pullup. We continued the approach to a normal landing at lax. At time of discovery, the altitude alert system had come unarmed in the descent because we were scanning for traffic. Neither pilot picked it up. The altitude advisory system could have disengaged itself or, as PF, I could have brushed the spring-loaded altitude advisory knob and accidentally disengaged it. I would like to reiterate a design deficiency in the medium large transport flight guidance system. First, the altitude advisory set knob is spring-loaded. Second, the flight guidance control panel on the glareshield is mounted such that when the first officer is the PF and is controling roll function with the roll control knob, only a slight brush of the altitude set knob will disarm the system. In this instance, the altitude advisory system could have malfunctioned and disarmed the function, or the first officer could have brushed the altitude set knob and disarmed it. This is a dangerous situation that has probably caused the majority of altitude excursions in this aircraft. Recommendations: move the altitude set knob to the center instrument panel away from the roll control knob and change the design of the knob to a roller type with indentation at each altitude so that inadvertently brushing the knob with hand or clothing will not disarm it. There must be a definite roll of the knob to reset the altitude.

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Original NASA ASRS Text

Title: ACR MLG OVERSHOT ALT ON DESCENT. ALT CAPTURE BECAME DISARMED.

Narrative: WE WERE ON DSCNT TO LAX ON THE 24/25 PROFILE DSCNT AT THE SADDE INTXN TURN. WE WERE DSNDING TO 10000' IN THE TURN. BOTH PLTS WERE SCANNING FOR TFC OUTSIDE. AS PF, I HAD THE AUTOPLT ON WITH THE ALT ADVISORY SYS SET AT 10000'. I WAS CTLING THE ROLL OF THE ACFT WITH THE HDG SELECT KNOB. I TOOK A GLANCE AT MY ALT IN THE TURN AS I CAME THROUGH 10500' WITH THE ALT ADVISORY STILL ARMED AT 10000'. BOTH PLTS CONTINUED TO SCAN OUTSIDE AND TO COMPLETE THE PRELIMINARY COCKPIT CHKLIST. AT 10000' THE AUTOPLT DID NOT CAPTURE THE ALT AND WE WERE ALERTED TO THE 250' DEVIATION WARNING. I ABRUPTLY TOOK MANUAL CTL OF THE ACFT AND RETURNED TO 10000'. WE MAY HAVE DIPPED TO 9700' MSL BEFORE PULLUP. WE CONTINUED THE APCH TO A NORMAL LNDG AT LAX. AT TIME OF DISCOVERY, THE ALT ALERT SYS HAD COME UNARMED IN THE DSCNT BECAUSE WE WERE SCANNING FOR TFC. NEITHER PLT PICKED IT UP. THE ALT ADVISORY SYS COULD HAVE DISENGAGED ITSELF OR, AS PF, I COULD HAVE BRUSHED THE SPRING-LOADED ALT ADVISORY KNOB AND ACCIDENTALLY DISENGAGED IT. I WOULD LIKE TO REITERATE A DESIGN DEFICIENCY IN THE MLG FLT GUIDANCE SYS. FIRST, THE ALT ADVISORY SET KNOB IS SPRING-LOADED. SECOND, THE FLT GUIDANCE CTL PANEL ON THE GLARESHIELD IS MOUNTED SUCH THAT WHEN THE F/O IS THE PF AND IS CTLING ROLL FUNCTION WITH THE ROLL CTL KNOB, ONLY A SLIGHT BRUSH OF THE ALT SET KNOB WILL DISARM THE SYS. IN THIS INSTANCE, THE ALT ADVISORY SYS COULD HAVE MALFUNCTIONED AND DISARMED THE FUNCTION, OR THE F/O COULD HAVE BRUSHED THE ALT SET KNOB AND DISARMED IT. THIS IS A DANGEROUS SITUATION THAT HAS PROBABLY CAUSED THE MAJORITY OF ALT EXCURSIONS IN THIS ACFT. RECOMMENDATIONS: MOVE THE ALT SET KNOB TO THE CENTER INSTRUMENT PANEL AWAY FROM THE ROLL CTL KNOB AND CHANGE THE DESIGN OF THE KNOB TO A ROLLER TYPE WITH INDENTATION AT EACH ALT SO THAT INADVERTENTLY BRUSHING THE KNOB WITH HAND OR CLOTHING WILL NOT DISARM IT. THERE MUST BE A DEFINITE ROLL OF THE KNOB TO RESET THE ALT.

Data retrieved from NASA's ASRS site as of August 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.