Narrative:

On descent into ZZZ; [we] had leveled off at 11;000. [We] noticed traffic ahead and to the left on TCAS at -1;000 ft. We were also at 300 or greater assigned on the speed; and were flying at 320 KTS. ATC said to descend down to 10;000. I mentioned to wait just a second to first officer to see if traffic was descending as well. Once confirmed that traffic was descending as well (I was surprised ATC made it so close with little traffic in the area); I confirmed 10;000 set in the altitude select. First officer began descent in vertical speed (vs) just as ATC said to slow to 210 KTS. I was just finishing a previously started approach check; when I looked up as I heard 500 ft and noticed aircraft descending 300 ft to level at 10;000; but at a rate that appeared excessive. I told first officer to watch the level off - that I did not think it would do it on its own and just as we were at 10;000 and still descending; he clicked off the autopilot and pulled the aircraft up to return to 10;000 ft. ATC said to maintain 10;000; and we acknowledged that we would. The momentary altitude deviation was 200 ft. The closest the other aircraft got was 500 ft and about a mile with no ACARS RA or TA. The first officer said the vs selection had reverted to pitch mode; which may have caused the high descent rate; and he also said it happened again on the next selection of vs. Not sure of cause; but suspect a switch to speed mode after the selection initially to vs mode may have caused the switch to pitch mode; but with first officer's testimony to it happening a second time - decided to write it up. I suggest speed mode seems to work well below 10;000 at 250; but may cause excessive descents at 320 KTS above 10;000. Also; slight distraction with uncompleted approach check may have prevented me from seeing the error prior to 300 ft.

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Original NASA ASRS Text

Title: a CRJ900 autopilot switched from the Vertical Speed mode to PITCH in order to maintain a speed and failed to level off the altitude in the ALTITUDE SEL window.

Narrative: On descent into ZZZ; [we] had leveled off at 11;000. [We] noticed traffic ahead and to the left on TCAS at -1;000 FT. We were also at 300 or greater assigned on the speed; and were flying at 320 KTS. ATC said to descend down to 10;000. I mentioned to wait just a second to First Officer to see if traffic was descending as well. Once confirmed that traffic was descending as well (I was surprised ATC made it so close with little traffic in the area); I confirmed 10;000 set in the altitude select. First Officer began descent in VERTICAL SPEED (VS) just as ATC said to slow to 210 KTS. I was just finishing a previously started Approach check; when I looked up as I heard 500 FT and noticed aircraft descending 300 FT to level at 10;000; but at a rate that appeared excessive. I told First Officer to watch the level off - that I did not think it would do it on its own and just as we were at 10;000 and still descending; he clicked off the autopilot and pulled the aircraft up to return to 10;000 FT. ATC said to maintain 10;000; and we acknowledged that we would. The momentary altitude deviation was 200 FT. The closest the other aircraft got was 500 FT and about a mile with no ACARS RA or TA. The First Officer said the VS selection had reverted to PITCH mode; which may have caused the high descent rate; and he also said it happened again on the next selection of VS. Not sure of cause; but suspect a switch to SPEED mode after the selection initially to VS mode may have caused the switch to PITCH mode; but with First Officer's testimony to it happening a second time - decided to write it up. I suggest speed mode seems to work well below 10;000 at 250; but may cause excessive descents at 320 KTS above 10;000. Also; slight distraction with uncompleted Approach check may have prevented me from seeing the error prior to 300 FT.

Data retrieved from NASA's ASRS site as of April 2012 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.