Narrative:

While being stepped down in holding at willo, we were cleared to turn to 090 degree vector, and to descend to FL80. WX was cavu with several aircraft in sight. I was flying on autoplt and selected and engaged the heading and altitude on the flight control unit by pulling on the selector knob. Passing FL80 I checked the altitude alerter and it showed FL70. I got that ,'something isn't right' feeling, and asked the first officer if we had been cleared to FL70, as there was quite a bit of traffic on the radio. His first reaction was to check the alerter, and I could now see he was unsure. Before I could ask for verification from ATC the controller advised we had been cleared to FL80. I disconnected the autoplt and climbed immediately to FL80. We descended to about 74-75000 ft. We continued the approach with no further comment. I believe a major contributing factor is the over sensitivity of the altitude alert set knob. It takes very little rotational force to change the altitude setting. Sometimes just the pulling and releasing of the knob will change the setting. Which I think happened in this case. Most crews are aware of and watch for this occurrence. I was, I do and it still got me. I would recommend that the altitude alerter be modified, so that changing the altitude setting would require a more positive or definite move, (ie, increasing the friction or a definite detent). An instrument that could not be changed by mearly pulling on the knob to engage the level change. Other factors, 2 man, all night, transoceanic, and the fact that the WX was so good. That scan was both in and out.

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Original NASA ASRS Text

Title: ADV COCKPIT WDB ON ARR AT GATWICK DSNDS THROUGH 8000 FT CLRED ALT BY 500-600 FT WHEN ALT ALERT SETTING JUMPS TO NEXT VALUE DUE TO NOT BEING COMPLETELY IN ITS DETENT.

Narrative: WHILE BEING STEPPED DOWN IN HOLDING AT WILLO, WE WERE CLRED TO TURN TO 090 DEG VECTOR, AND TO DSND TO FL80. WX WAS CAVU WITH SEVERAL ACFT IN SIGHT. I WAS FLYING ON AUTOPLT AND SELECTED AND ENGAGED THE HDG AND ALT ON THE FLT CTL UNIT BY PULLING ON THE SELECTOR KNOB. PASSING FL80 I CHKED THE ALT ALERTER AND IT SHOWED FL70. I GOT THAT ,'SOMETHING ISN'T RIGHT' FEELING, AND ASKED THE FO IF WE HAD BEEN CLRED TO FL70, AS THERE WAS QUITE A BIT OF TFC ON THE RADIO. HIS FIRST REACTION WAS TO CHK THE ALERTER, AND I COULD NOW SEE HE WAS UNSURE. BEFORE I COULD ASK FOR VERIFICATION FROM ATC THE CTLR ADVISED WE HAD BEEN CLRED TO FL80. I DISCONNECTED THE AUTOPLT AND CLBED IMMEDIATELY TO FL80. WE DSNDED TO ABOUT 74-75000 FT. WE CONTINUED THE APCH WITH NO FURTHER COMMENT. I BELIEVE A MAJOR CONTRIBUTING FACTOR IS THE OVER SENSITIVITY OF THE ALT ALERT SET KNOB. IT TAKES VERY LITTLE ROTATIONAL FORCE TO CHANGE THE ALT SETTING. SOMETIMES JUST THE PULLING AND RELEASING OF THE KNOB WILL CHANGE THE SETTING. WHICH I THINK HAPPENED IN THIS CASE. MOST CREWS ARE AWARE OF AND WATCH FOR THIS OCCURRENCE. I WAS, I DO AND IT STILL GOT ME. I WOULD RECOMMEND THAT THE ALT ALERTER BE MODIFIED, SO THAT CHANGING THE ALT SETTING WOULD REQUIRE A MORE POSITIVE OR DEFINITE MOVE, (IE, INCREASING THE FRICTION OR A DEFINITE DETENT). AN INST THAT COULD NOT BE CHANGED BY MEARLY PULLING ON THE KNOB TO ENGAGE THE LEVEL CHANGE. OTHER FACTORS, 2 MAN, ALL NIGHT, TRANSOCEANIC, AND THE FACT THAT THE WX WAS SO GOOD. THAT SCAN WAS BOTH IN AND OUT.

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.