Narrative:

I was flying the tandy STAR into sna with clearance to cross tandy at 14,000'. Autoplt was engaged with automatic leveloff function armed. We both acknowledged our 2000-1000 foot-to-go calls and the captain says he noted the 'altitude' light in the arm window at the 1000' call. I adjusted our heading inbound to tandy sometime between 15,000-14,000 and must have inadvertently touched the vertical speed wheel before altitude capture occurred. (The slightest adjustment of the vertical speed wheel will cnx the altitude capture function with no advisory, other than the 'altitude' light going out). The next thing we heard was the word 'altitude' from the awi as we passed 13,800'. I immediately disconnected the autoplt and leveled off at 14,000'. We bottomed out at about 13,700'. I have only been flying the medium large transport for 2 months and this is the 3RD or 4TH time this has occurred (and not just to me). We medium large transport pilots are all aware of this problem but the situation still occurs. The system was designed to allow more heads-up flying and ease the workload (especially in high density areas like sna). Thee needs to be some kind of advisory when the altitude capture function is disarmed by the vertical speed wheel. This is an incident or accident waiting to happen. The slightest touch of the vertical speed wheel cnx's the automatic level off!

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Original NASA ASRS Text

Title: ACR MLG ALT DEVIATION OVERSHOT DURING DESCENT USING AUTO LEVEL OFF.

Narrative: I WAS FLYING THE TANDY STAR INTO SNA WITH CLRNC TO CROSS TANDY AT 14,000'. AUTOPLT WAS ENGAGED WITH AUTO LEVELOFF FUNCTION ARMED. WE BOTH ACKNOWLEDGED OUR 2000-1000 FOOT-TO-GO CALLS AND THE CAPT SAYS HE NOTED THE 'ALT' LIGHT IN THE ARM WINDOW AT THE 1000' CALL. I ADJUSTED OUR HDG INBND TO TANDY SOMETIME BETWEEN 15,000-14,000 AND MUST HAVE INADVERTENTLY TOUCHED THE VERTICAL SPEED WHEEL BEFORE ALT CAPTURE OCCURRED. (THE SLIGHTEST ADJUSTMENT OF THE VERTICAL SPEED WHEEL WILL CNX THE ALT CAPTURE FUNCTION WITH NO ADVISORY, OTHER THAN THE 'ALT' LIGHT GOING OUT). THE NEXT THING WE HEARD WAS THE WORD 'ALTITUDE' FROM THE AWI AS WE PASSED 13,800'. I IMMEDIATELY DISCONNECTED THE AUTOPLT AND LEVELED OFF AT 14,000'. WE BOTTOMED OUT AT ABOUT 13,700'. I HAVE ONLY BEEN FLYING THE MLG FOR 2 MONTHS AND THIS IS THE 3RD OR 4TH TIME THIS HAS OCCURRED (AND NOT JUST TO ME). WE MLG PLTS ARE ALL AWARE OF THIS PROBLEM BUT THE SITUATION STILL OCCURS. THE SYSTEM WAS DESIGNED TO ALLOW MORE HEADS-UP FLYING AND EASE THE WORKLOAD (ESPECIALLY IN HIGH DENSITY AREAS LIKE SNA). THEE NEEDS TO BE SOME KIND OF ADVISORY WHEN THE ALT CAPTURE FUNCTION IS DISARMED BY THE VERTICAL SPEED WHEEL. THIS IS AN INCIDENT OR ACCIDENT WAITING TO HAPPEN. THE SLIGHTEST TOUCH OF THE VERTICAL SPEED WHEEL CNX'S THE AUTO LEVEL OFF!

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.