Narrative:

I was the pilot monitoring. We were leveling at four thousand feet and about 12-15 miles out on the ILS xxr. I was in green needles. ATC assigned a speed reduction of 210 to 170 knots and had cleared us for the visual approach. The pilot flying had the autopilot engaged. The aircraft suddenly began a descent and increased speed. I called for the pilot flying to stop descent. I felt automation was incorrect and stated 'my aircraft' and disconnected the autopilot and corrected for 300-400 foot altitude loss. At this point I transferred controls back to the original pilot flying and re-engaged the autopilot. Moments later the aircraft started a descent. Again I called 'my aircraft' disconnected autopilot and corrected back onto the glideslope and speed correction to 170 knots. ATC had advised us for a 2nd or 3rd time they needed speed of 170. We were at 170 knots at his last request; but speed management was not my first priority when detecting and correcting for the unexpected descent. After stabilizing the aircraft I noted the original pilot flying side altimeter read 31.22 and it was determined that the original pilot flying was turning the baro knob instead of the navigation select knob when transferring to green needles. In doing so; the aircraft thought we were higher and needed to descend to 4000 feet. There were no traffic conflicts and we were cleared on a visual approach.the pilot flying had the correct altimeter settings prior to the approach. The baro knob was inadvertently turned instead of the nav select knob. The location and size of the knobs are similar with the baro knob located just below. The only indication we had which was at the initiation of descent was an altitude chord sound which still took a moment to detect and react to the aircraft descending. Perhaps a message anytime there is a discrepancy between the altimeter settings set on the pfds to caution the pilots.

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Original NASA ASRS Text

Title: CRJ-200 flight crew reported an altitude excursion occurred when the altimeter baro knob was spun instead of the nav source knob.

Narrative: I was the pilot monitoring. We were leveling at four thousand feet and about 12-15 miles out on the ILS XXR. I was in green needles. ATC assigned a speed reduction of 210 to 170 knots and had cleared us for the visual approach. The pilot flying had the autopilot engaged. The aircraft suddenly began a descent and increased speed. I called for the pilot flying to stop descent. I felt automation was incorrect and stated 'my aircraft' and disconnected the autopilot and corrected for 300-400 foot altitude loss. At this point I transferred controls back to the original pilot flying and re-engaged the autopilot. Moments later the aircraft started a descent. Again I called 'my aircraft' disconnected autopilot and corrected back onto the glideslope and speed correction to 170 knots. ATC had advised us for a 2nd or 3rd time they needed speed of 170. We were at 170 knots at his last request; but speed management was not my first priority when detecting and correcting for the unexpected descent. After stabilizing the aircraft I noted the original pilot flying side altimeter read 31.22 and it was determined that the original pilot flying was turning the baro knob instead of the NAV select knob when transferring to green needles. In doing so; the aircraft thought we were higher and needed to descend to 4000 feet. There were no traffic conflicts and we were cleared on a visual approach.The pilot flying had the correct altimeter settings prior to the approach. The baro knob was inadvertently turned instead of the nav select knob. The location and size of the knobs are similar with the baro knob located just below. The only indication we had which was at the initiation of descent was an altitude chord sound which still took a moment to detect and react to the aircraft descending. Perhaps a message anytime there is a discrepancy between the altimeter settings set on the PFDs to caution the pilots.

Data retrieved from NASA's ASRS site 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.