Narrative:

This FMS aircraft flew a flawless VNAV descent from FL370 non-stop to idle to 11000 ft. I had programmed the FMS to cross zzzzz at 11000 ft and 310 KTS. The aircraft captured 11000 ft and leveled off. This was followed immediately by the speed being driven to 224 KTS with the throttles retarding. In the initial confusion I selected speed select and rotated the speed back to 310 KTS. A very brief discussion ensued as to why it did that. First officer pulled up the descent page and verified that it said zzzzz at 11000 ft and 310 KTS. My first officer then called out 'altitude!' we had descended to 10700 ft. I disengaged the autoplt and climbed back to 11000 ft. ATC did not mention it. All I can think that happened was that when I selected speed select we still had some residual down vvi and were not completely in FMS altitude hold that created the insidious descent. Had I selected altitude hold prior to speed select; as we do exiting performance in any other airplanes; this wouldn't have happened. However; I don't understand why it flew such a perfect descent; leveled off with instructions to cross zzzzz at 11000 ft and 310 KTS then slowed the speed to 224 KTS. Why not 240 KTS? And why would it do that without captured at the 11000 ft altitude? From a human factors point of view; it was very late (for me); dark; and the 'surprise' distraction of an uncommanded speed reduction to unique speed (224 KTS versus 240 KTS) were just enough to keep me from catching the solid hard 11000 ft leveloff. Supplemental information from acn 778175: reflecting back on the incident; I would venture to guess that the captain might have pressed 'speed select' as the autoplt was completing its altitude capture at 11000 ft; thus putting the aircraft into a vertical speed descent just as it approached 11000 ft. We both were taken by surprise as to how it got into vertical speed but; regardless of the cause; the most important lesson learned is that you simply cannot turn your back on the aircraft and the operation of its automation even for a brief moment.

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Original NASA ASRS Text

Title: AN MD80 AUTOPILOT SWITCHED TO THE 'VERT SPD' MODE AND DSNDED BELOW THE CLRED ALT WHEN THE CAPT SELECTED 'SPD' ON THE PFD AS THE ACFT WAS LEVELING.

Narrative: THIS FMS ACFT FLEW A FLAWLESS VNAV DSCNT FROM FL370 NON-STOP TO IDLE TO 11000 FT. I HAD PROGRAMMED THE FMS TO CROSS ZZZZZ AT 11000 FT AND 310 KTS. THE ACFT CAPTURED 11000 FT AND LEVELED OFF. THIS WAS FOLLOWED IMMEDIATELY BY THE SPD BEING DRIVEN TO 224 KTS WITH THE THROTTLES RETARDING. IN THE INITIAL CONFUSION I SELECTED SPD SELECT AND ROTATED THE SPD BACK TO 310 KTS. A VERY BRIEF DISCUSSION ENSUED AS TO WHY IT DID THAT. FO PULLED UP THE DSCNT PAGE AND VERIFIED THAT IT SAID ZZZZZ AT 11000 FT AND 310 KTS. MY FO THEN CALLED OUT 'ALT!' WE HAD DSNDED TO 10700 FT. I DISENGAGED THE AUTOPLT AND CLBED BACK TO 11000 FT. ATC DID NOT MENTION IT. ALL I CAN THINK THAT HAPPENED WAS THAT WHEN I SELECTED SPD SELECT WE STILL HAD SOME RESIDUAL DOWN VVI AND WERE NOT COMPLETELY IN FMS ALT HOLD THAT CREATED THE INSIDIOUS DSCNT. HAD I SELECTED ALT HOLD PRIOR TO SPD SELECT; AS WE DO EXITING PERFORMANCE IN ANY OTHER AIRPLANES; THIS WOULDN'T HAVE HAPPENED. HOWEVER; I DON'T UNDERSTAND WHY IT FLEW SUCH A PERFECT DSCNT; LEVELED OFF WITH INSTRUCTIONS TO CROSS ZZZZZ AT 11000 FT AND 310 KTS THEN SLOWED THE SPD TO 224 KTS. WHY NOT 240 KTS? AND WHY WOULD IT DO THAT WITHOUT CAPTURED AT THE 11000 FT ALT? FROM A HUMAN FACTORS POINT OF VIEW; IT WAS VERY LATE (FOR ME); DARK; AND THE 'SURPRISE' DISTR OF AN UNCOMMANDED SPD REDUCTION TO UNIQUE SPD (224 KTS VERSUS 240 KTS) WERE JUST ENOUGH TO KEEP ME FROM CATCHING THE SOLID HARD 11000 FT LEVELOFF. SUPPLEMENTAL INFO FROM ACN 778175: REFLECTING BACK ON THE INCIDENT; I WOULD VENTURE TO GUESS THAT THE CAPT MIGHT HAVE PRESSED 'SPD SELECT' AS THE AUTOPLT WAS COMPLETING ITS ALT CAPTURE AT 11000 FT; THUS PUTTING THE ACFT INTO A VERT SPD DSCNT JUST AS IT APCHED 11000 FT. WE BOTH WERE TAKEN BY SURPRISE AS TO HOW IT GOT INTO VERT SPD BUT; REGARDLESS OF THE CAUSE; THE MOST IMPORTANT LESSON LEARNED IS THAT YOU SIMPLY CANNOT TURN YOUR BACK ON THE ACFT AND THE OP OF ITS AUTOMATION EVEN FOR A BRIEF MOMENT.

Data retrieved from NASA's ASRS site as of May 2009 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.