Narrative:

On descent I was the flying pilot(fp) sitting in the left seat. Weather conditions were showers and storms in the area. The cockpit workload was high due to high traffic volume; storms and WX radar scanning and having the runway assignment changed. The autopilot was operational and engaged. At 9000 feet MSL; approach issued 'descend to 6000 feet'. Using the altitude select knob; per the SOP; I selected 6000 feet and stated '6000'. The non flying pilot (nfp) acknowledged and pointed to his pfd; saying '6000'. At this time we were being assigned a different runway to expect; so the nfp was changing the FMS and pulling the appropriate chart. There was a lot of traffic so the approach controllers were constantly making calls to change speeds for many aircraft trying to get proper spacing. There was a lot of radio traffic. I had selected a 1500 feet per minute rate of descent for the descent from 9000 to 6000 feet. At 6000 feet; the aural warning tone indicating 1000 feet left to descend was heard. At this point I said; '6000 for 5000'. I said this because now the altitude selected indicated 5000. The fact that the aural warning of 1000 feet prior was heard at the assigned altitude did not register because I looked at the altitude of 5000 feet that now indicated in the selected window. Having set the altitude and confirming it with the nfp; it took a few seconds for it to register that the aircraft system had failed. By that time ATC called. At this point we were about 5500 MSL and I disengaged the autopilot; hand flew the aircraft to 6000 feet and then tried to determine what had just happened. I knew I had selected 6000 feet; but somehow the altitude showed 5000. Trying to briefly trouble shoot the system; I turned the altitude selector knob and noticed that it was not always changing a thousand feet with each click of the dial. That is not right. We continued the flight and landed. After taxiing in and discussing the event; I strongly felt there was a malfunction in the altitude select system. After shutting down the aircraft the nfp and captain of the flight checked the system. He was able to recreate the situation. After selecting an altitude and putting slight pressure; but not actually turning the knob; the altitude would 'drop down' one more thousand on its own. Also if the altitude was selected and a gentle tap to the glare shield occurred; it would drop a thousand feet. The light taps could easily have been similar to the light to moderate turbulence we were encountering descending through multiple layers and rain showers. The altitude select system was written up and sent to maintenance. I feel the nfp should select the altitude assignments on the altitude select system. They are making the radio calls and they should select the altitude so the fp has a secondary monitor. Having the aircraft system fail and not have an aural warning until the assigned altitude was being passed through; left little time for us as a crew to react. Other than that; I don't know what else can be done for an aircraft system failure

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Original NASA ASRS Text

Title: G200 flight crew reports altitude select window changing spontaneously during descent from 6000 to 5000; causing an altitude deviation. Crew was able to reproduce the discrepancy on the ground and the system was written up for repair.

Narrative: On descent I was the Flying Pilot(FP) sitting in the left seat. Weather conditions were showers and storms in the area. The cockpit workload was high due to high traffic volume; storms and WX RADAR scanning and having the runway assignment changed. The autopilot was operational and engaged. At 9000 feet MSL; Approach issued 'Descend to 6000 feet'. Using the Altitude Select Knob; per the SOP; I selected 6000 feet and stated '6000'. The Non Flying Pilot (NFP) acknowledged and pointed to his PFD; saying '6000'. At this time we were being assigned a different runway to expect; so the NFP was changing the FMS and pulling the appropriate chart. There was a lot of traffic so the Approach Controllers were constantly making calls to change speeds for many aircraft trying to get proper spacing. There was a lot of radio traffic. I had selected a 1500 feet per minute rate of descent for the descent from 9000 to 6000 feet. At 6000 feet; the aural warning tone indicating 1000 feet left to descend was heard. At this point I said; '6000 for 5000'. I said this because now the Altitude Selected indicated 5000. The fact that the aural warning of 1000 feet prior was heard at the assigned altitude did not register because I looked at the altitude of 5000 feet that now indicated in the selected window. Having set the altitude and confirming it with the NFP; it took a few seconds for it to register that the aircraft system had failed. By that time ATC called. At this point we were about 5500 MSL and I disengaged the autopilot; hand flew the aircraft to 6000 feet and then tried to determine what had just happened. I knew I had selected 6000 feet; but somehow the altitude showed 5000. Trying to briefly trouble shoot the system; I turned the Altitude Selector Knob and noticed that it was not always changing a thousand feet with each click of the dial. That is not right. We continued the flight and landed. After taxiing in and discussing the event; I strongly felt there was a malfunction in the Altitude Select System. After shutting down the aircraft the NFP and Captain of the flight checked the system. He was able to recreate the situation. After selecting an altitude and putting slight pressure; but not actually turning the knob; the altitude would 'drop down' one more thousand on its own. Also if the altitude was selected and a gentle tap to the glare shield occurred; it would drop a thousand feet. The light taps could easily have been similar to the light to moderate turbulence we were encountering descending through multiple layers and rain showers. The Altitude Select System was written up and sent to maintenance. I feel the NFP should select the altitude assignments on the Altitude Select System. They are making the radio calls and they should select the altitude so the FP has a secondary monitor. Having the aircraft system fail and not have an aural warning until the assigned altitude was being passed through; left little time for us as a crew to react. Other than that; I don't know what else can be done for an aircraft system failure

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.