Narrative:

During descent into cle, descent was made below assigned altitude of 7000' MSL. Type of aircraft was adv medium large transport. Copilot was flying aircraft during descent to 7000'. Captain went off frequency to notify F/a's of reported moderate turbulence on approach. 7000' was noted to be set in altitude alerter prior to leaving frequency. Upon return to approach frequency, 5000' was noted to be in the altitude alerter and descending through 7000' captain thought lower altitude had been assigned by approach control. At about 6500' MSL approach asked to verify assigned altitude. Thinking 5000' was assigned, captain responded accordingly. Controller responded that assigned altitude was 7000'. At this time further descent to 3000' was assigned by ATC. Normal approach and landing was made with no conflict noted. Copilot stated that he had not reset alerter to 5000'. He thought the captain had set lower altitude into alerter. Captain had not reset alerter since 7000' had been previously assigned. We have had reports of several of the altitude alerters on adv medium large transport aircraft changing altitude settings on their own west/O pilot input. In my opinion that is what happened on the event in question, since neither pilot touched altitude alerter after 7000' had been set. I think this type of alerter is of very poor design. Aside from the possibility of an internal malfunction, the instrument is difficult to read in less than ideal lighting conditions. The calibration is also too sensitive since one 'click' of rotation is equal to 1000' of altitude. The older mechanical alerter seems to be a more user friendly device. Newer is not always better. Supplemental information from acn 100831: approach gave us a heading change which I responded to and complied with while I was flying the aircraft manually. The captain finished the passenger announcement and we continued descending. At 8000' I noticed 5000' set in the mode control panel altitude window. At this time the captain talking to operations increasing our fuel load for the return flight to clt. I thought he had received clearance from approach control to descend further to 5000' during another heading change that we received and the captain responded to and I had missed the new altitude assignment. Neither one of us questioned the other. Descending through approximately 6000' cle approach asked our last altitude assignment and the captain responded '5000'.' I firmly believe this was another incident of the altitude digital readout on the mode control panel changing west/O any pilot input.

Related reports:

Google
 

Original NASA ASRS Text

Title: ACR MLG ALT OVERSHOT ON DESCENT. ALT ALERTER DESIGN QUESTIONED BY FLT CREW.

Narrative: DURING DSCNT INTO CLE, DSCNT WAS MADE BELOW ASSIGNED ALT OF 7000' MSL. TYPE OF ACFT WAS ADV MLG. COPLT WAS FLYING ACFT DURING DSCNT TO 7000'. CAPT WENT OFF FREQ TO NOTIFY F/A'S OF RPTED MODERATE TURB ON APCH. 7000' WAS NOTED TO BE SET IN ALT ALERTER PRIOR TO LEAVING FREQ. UPON RETURN TO APCH FREQ, 5000' WAS NOTED TO BE IN THE ALT ALERTER AND DESCENDING THROUGH 7000' CAPT THOUGHT LOWER ALT HAD BEEN ASSIGNED BY APCH CTL. AT ABOUT 6500' MSL APCH ASKED TO VERIFY ASSIGNED ALT. THINKING 5000' WAS ASSIGNED, CAPT RESPONDED ACCORDINGLY. CTLR RESPONDED THAT ASSIGNED ALT WAS 7000'. AT THIS TIME FURTHER DSCNT TO 3000' WAS ASSIGNED BY ATC. NORMAL APCH AND LNDG WAS MADE WITH NO CONFLICT NOTED. COPLT STATED THAT HE HAD NOT RESET ALERTER TO 5000'. HE THOUGHT THE CAPT HAD SET LOWER ALT INTO ALERTER. CAPT HAD NOT RESET ALERTER SINCE 7000' HAD BEEN PREVIOUSLY ASSIGNED. WE HAVE HAD RPTS OF SEVERAL OF THE ALT ALERTERS ON ADV MLG ACFT CHANGING ALT SETTINGS ON THEIR OWN W/O PLT INPUT. IN MY OPINION THAT IS WHAT HAPPENED ON THE EVENT IN QUESTION, SINCE NEITHER PLT TOUCHED ALT ALERTER AFTER 7000' HAD BEEN SET. I THINK THIS TYPE OF ALERTER IS OF VERY POOR DESIGN. ASIDE FROM THE POSSIBILITY OF AN INTERNAL MALFUNCTION, THE INSTRUMENT IS DIFFICULT TO READ IN LESS THAN IDEAL LIGHTING CONDITIONS. THE CALIBRATION IS ALSO TOO SENSITIVE SINCE ONE 'CLICK' OF ROTATION IS EQUAL TO 1000' OF ALT. THE OLDER MECHANICAL ALERTER SEEMS TO BE A MORE USER FRIENDLY DEVICE. NEWER IS NOT ALWAYS BETTER. SUPPLEMENTAL INFO FROM ACN 100831: APCH GAVE US A HDG CHANGE WHICH I RESPONDED TO AND COMPLIED WITH WHILE I WAS FLYING THE ACFT MANUALLY. THE CAPT FINISHED THE PAX ANNOUNCEMENT AND WE CONTINUED DSNDING. AT 8000' I NOTICED 5000' SET IN THE MODE CTL PANEL ALT WINDOW. AT THIS TIME THE CAPT TALKING TO OPS INCREASING OUR FUEL LOAD FOR THE RETURN FLT TO CLT. I THOUGHT HE HAD RECEIVED CLRNC FROM APCH CTL TO DSND FURTHER TO 5000' DURING ANOTHER HDG CHANGE THAT WE RECEIVED AND THE CAPT RESPONDED TO AND I HAD MISSED THE NEW ALT ASSIGNMENT. NEITHER ONE OF US QUESTIONED THE OTHER. DSNDING THROUGH APPROX 6000' CLE APCH ASKED OUR LAST ALT ASSIGNMENT AND THE CAPT RESPONDED '5000'.' I FIRMLY BELIEVE THIS WAS ANOTHER INCIDENT OF THE ALT DIGITAL READOUT ON THE MODE CONTROL PANEL CHANGING W/O ANY PLT INPUT.

Data retrieved from NASA's ASRS site as of August 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.