Narrative:

We were descending and just leveled off. Altitude was 13000' MSL. Cleared by tampa approach to deviation as necessary around thunderstorms. During turns around build ups, the aircraft slowly descended (autoplt engaged, but altitude hold switch inadvertently not engaged). At 12700' MSL, where the altitude alert warning should have sounded an alert, the PNF reached over to reset the new setting to a lower altitude. This was corrected immediately, but the off altitude alert was already cancelled by this inadvertent change of altitude. Therefore, there was not an audible or visibility (light) signal to alert the crew of the deviation. Aircraft descended to 12300' MSL before immediate return to 13000'. We asked approach control if there was a conflict. They replied that there was not and assigned us a lower altitude to continue the approach. The autoplt altitude hold switch not being engaged after level off was an obvious oversight. What was very unusual and/or coincidental was that as the aircraft slowly descended to 300' below assigned altitude, the altitude alert setting was lowered in error as its knob was moved instead of the altimeter setting knob that need adjustment. I'm not sure that I will ever see this exact chain of events to facilitate an altitude bust, but it all starts somewhere. In this case it was using the autoplt in level flight west/O altitude hold engaged and head outside cockpit. Perhaps being occupied by WX avoidance precluded an otherwise routine use of the autoplt function switches.

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Original NASA ASRS Text

Title: MLG DESCENDS BELOW ASSIGNED ALT.

Narrative: WE WERE DSNDING AND JUST LEVELED OFF. ALT WAS 13000' MSL. CLRED BY TAMPA APCH TO DEV AS NECESSARY AROUND TSTMS. DURING TURNS AROUND BUILD UPS, THE ACFT SLOWLY DSNDED (AUTOPLT ENGAGED, BUT ALT HOLD SWITCH INADVERTENTLY NOT ENGAGED). AT 12700' MSL, WHERE THE ALT ALERT WARNING SHOULD HAVE SOUNDED AN ALERT, THE PNF REACHED OVER TO RESET THE NEW SETTING TO A LOWER ALT. THIS WAS CORRECTED IMMEDIATELY, BUT THE OFF ALT ALERT WAS ALREADY CANCELLED BY THIS INADVERTENT CHANGE OF ALT. THEREFORE, THERE WAS NOT AN AUDIBLE OR VIS (LIGHT) SIGNAL TO ALERT THE CREW OF THE DEV. ACFT DSNDED TO 12300' MSL BEFORE IMMEDIATE RETURN TO 13000'. WE ASKED APCH CTL IF THERE WAS A CONFLICT. THEY REPLIED THAT THERE WAS NOT AND ASSIGNED US A LOWER ALT TO CONTINUE THE APCH. THE AUTOPLT ALT HOLD SWITCH NOT BEING ENGAGED AFTER LEVEL OFF WAS AN OBVIOUS OVERSIGHT. WHAT WAS VERY UNUSUAL AND/OR COINCIDENTAL WAS THAT AS THE ACFT SLOWLY DSNDED TO 300' BELOW ASSIGNED ALT, THE ALT ALERT SETTING WAS LOWERED IN ERROR AS ITS KNOB WAS MOVED INSTEAD OF THE ALTIMETER SETTING KNOB THAT NEED ADJUSTMENT. I'M NOT SURE THAT I WILL EVER SEE THIS EXACT CHAIN OF EVENTS TO FACILITATE AN ALT BUST, BUT IT ALL STARTS SOMEWHERE. IN THIS CASE IT WAS USING THE AUTOPLT IN LEVEL FLT W/O ALT HOLD ENGAGED AND HEAD OUTSIDE COCKPIT. PERHAPS BEING OCCUPIED BY WX AVOIDANCE PRECLUDED AN OTHERWISE ROUTINE USE OF THE AUTOPLT FUNCTION SWITCHES.

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.