Narrative:

At altitude of 11000 ft MSL with autoplt engaged, I selected a speed of 250 KTS from cruise airspeed of about 300 KTS in preparation for descent. No descent was selected nor issued, and autoplt was on with 11000 ft indicated in altitude window. The aircraft began its speed reduction as commanded, but the aircraft began descending below the selected altitude of 11000 ft MSL. I observed the aircraft descend about 150 ft then I disconnected the autoplt and regained 11000 ft. I then re-engaged autoplt and it operated normally the rest of flight. No conflict was involved and none indicated by approach control. I just wanted other FMS aircraft pilots to realize that constant vigilance is required. It would also be helpful if the altitude alert 'horn' would engage sooner than its preselected altitude deviation of 250 ft to alert crew prior to that time.

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Original NASA ASRS Text

Title: AN ACR FLC FLYING AN F100 INTO SAV FAILS TO MAINTAIN ALT AS ASSIGNED BY ATC.

Narrative: AT ALT OF 11000 FT MSL WITH AUTOPLT ENGAGED, I SELECTED A SPD OF 250 KTS FROM CRUISE AIRSPD OF ABOUT 300 KTS IN PREPARATION FOR DSCNT. NO DSCNT WAS SELECTED NOR ISSUED, AND AUTOPLT WAS ON WITH 11000 FT INDICATED IN ALT WINDOW. THE ACFT BEGAN ITS SPD REDUCTION AS COMMANDED, BUT THE ACFT BEGAN DSNDING BELOW THE SELECTED ALT OF 11000 FT MSL. I OBSERVED THE ACFT DSND ABOUT 150 FT THEN I DISCONNECTED THE AUTOPLT AND REGAINED 11000 FT. I THEN RE-ENGAGED AUTOPLT AND IT OPERATED NORMALLY THE REST OF FLT. NO CONFLICT WAS INVOLVED AND NONE INDICATED BY APCH CTL. I JUST WANTED OTHER FMS ACFT PLTS TO REALIZE THAT CONSTANT VIGILANCE IS REQUIRED. IT WOULD ALSO BE HELPFUL IF THE ALT ALERT 'HORN' WOULD ENGAGE SOONER THAN ITS PRESELECTED ALTDEV OF 250 FT TO ALERT CREW PRIOR TO THAT TIME.

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.