Narrative:

During a routine flight to bwi we descended approximately 500 ft lower than our assigned altitude; due to an FMS/autopilot glitch; and an over-reliance on the automation.we were descending on the ravnn RNAV STAR and were between sacco and fimbo. They both have hard altitudes of 16;000. The captain was flying and I was pilot not flying. We crossed sacco at 16;000 and while the captain made a speed adjustment in the FMS; I did the in-range checklist. While scanning the overhead panel; I felt a sudden pull on the controls and glanced down to see our altimeter at 15;500 and climbing. The captain had quickly disconnected the autopilot and returned to 16;000 without any prompting from ATC. The mode control panel showed profile selected and had the hard altitude of 16;000 at fimbo clearly displayed. Even after the event; we were unable to determine what caused the automation to descend off-profile and without warning. We performed an uneventful approach and landing to runway 33L in baltimore; and debriefed the infraction on the ground.the airbus has an FMS that works great when descended from cruise; if no subsequent changes are made in the box. If a change is made after the descent has commenced; it will often calculate and re-calculate itself into trouble. A prime example is when you take it out of profile in order to get back on the path; and once the vertical deviation shows 0; i.e. You're back on the path; you select profile again; but instead it begins leveling off/climbing. This was a situation where an entry had been made in the box; and while it was calculating the new numbers; it began descending inexplicably. I suggest that the pilot not flying should monitor more closely when the FMS is calculating and hold all checklists until proper vertical path navigation is confirmed.

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Original NASA ASRS Text

Title: An A300; dispatched with autothrottles inoperative; failed to cross FIMBO at 16;000 as required by the RAVNN RNAV STAR to BWI.

Narrative: During a routine flight to BWI we descended approximately 500 FT lower than our assigned altitude; due to an FMS/autopilot glitch; and an over-reliance on the automation.We were descending on the RAVNN RNAV STAR and were between SACCO and FIMBO. They both have hard altitudes of 16;000. The Captain was flying and I was pilot not flying. We crossed SACCO at 16;000 and while the Captain made a speed adjustment in the FMS; I did the In-range checklist. While scanning the overhead panel; I felt a sudden pull on the controls and glanced down to see our altimeter at 15;500 and climbing. The Captain had quickly disconnected the autopilot and returned to 16;000 without any prompting from ATC. The Mode Control Panel showed Profile selected and had the hard altitude of 16;000 at FIMBO clearly displayed. Even after the event; we were unable to determine what caused the automation to descend off-profile and without warning. We performed an uneventful approach and landing to Runway 33L in Baltimore; and debriefed the infraction on the ground.The Airbus has an FMS that works great when descended from cruise; IF no subsequent changes are made in the box. If a change is made after the descent has commenced; it will often calculate and re-calculate itself into trouble. A prime example is when you take it out of Profile in order to get back on the path; and once the Vertical Deviation shows 0; i.e. you're back on the path; you select Profile again; but instead it begins leveling off/climbing. This was a situation where an entry had been made in the box; and while it was calculating the new numbers; it began descending inexplicably. I suggest that the pilot not flying should monitor more closely when the FMS is calculating and hold all checklists until proper vertical path navigation is confirmed.

Data retrieved from NASA's ASRS site as of July 2013 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.