Narrative:

On a recent flight, the altitude alerter changed by itself. While we were climbing to 11000 ft, we were reclred to 17000 ft. I cranked the altitude knob clockwise to 16000 ft and then turned it slowly 1 click. The window should have displayed 17000 ft but it remained on 16000 ft. I then clicked it slowly 1 more click and it changed to the correct setting of 17000 ft. The first officer verified the altitude. I also checked my FMA to verify 17000 ft was armed. We had only climbed a few thousand ft when I noticed that 18000 ft was set in both the FMA and altitude set window on the dfgp. During all this, we were in a constant airspeed climb on a set heading. Neither the first officer nor I touched anything on the flight guidance control panel. It is not unusual, when changing the course selector on the navigation receiver, to have to turn the knob more than 1 click to change the setting 1 degree. When this happens, you quite often find that later the course selected has changed 1 degree left or right. I believe a similar thing occurred when it required 2 clicks of the knob to change the altitude 1000 ft. This incorrect altitude was relatively easy to spot because you are almost never cleared to climb to FL180 during a climb out to a high altitude sector. There was no altitude bust in this case. The md-80 altitude alert system seems to be in need of a fix judging from all the as soon as possible reports filed on altitude busts in the aircraft.

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Original NASA ASRS Text

Title: THE ALT ALERTER TENDS TO 'SKIP' FROM ONE SET ALT TO ANOTHER.

Narrative: ON A RECENT FLT, THE ALT ALERTER CHANGED BY ITSELF. WHILE WE WERE CLBING TO 11000 FT, WE WERE RECLRED TO 17000 FT. I CRANKED THE ALT KNOB CLOCKWISE TO 16000 FT AND THEN TURNED IT SLOWLY 1 CLICK. THE WINDOW SHOULD HAVE DISPLAYED 17000 FT BUT IT REMAINED ON 16000 FT. I THEN CLICKED IT SLOWLY 1 MORE CLICK AND IT CHANGED TO THE CORRECT SETTING OF 17000 FT. THE FO VERIFIED THE ALT. I ALSO CHKED MY FMA TO VERIFY 17000 FT WAS ARMED. WE HAD ONLY CLBED A FEW THOUSAND FT WHEN I NOTICED THAT 18000 FT WAS SET IN BOTH THE FMA AND ALT SET WINDOW ON THE DFGP. DURING ALL THIS, WE WERE IN A CONSTANT AIRSPD CLB ON A SET HDG. NEITHER THE FO NOR I TOUCHED ANYTHING ON THE FLT GUIDANCE CTL PANEL. IT IS NOT UNUSUAL, WHEN CHANGING THE COURSE SELECTOR ON THE NAV RECEIVER, TO HAVE TO TURN THE KNOB MORE THAN 1 CLICK TO CHANGE THE SETTING 1 DEG. WHEN THIS HAPPENS, YOU QUITE OFTEN FIND THAT LATER THE COURSE SELECTED HAS CHANGED 1 DEG L OR R. I BELIEVE A SIMILAR THING OCCURRED WHEN IT REQUIRED 2 CLICKS OF THE KNOB TO CHANGE THE ALT 1000 FT. THIS INCORRECT ALT WAS RELATIVELY EASY TO SPOT BECAUSE YOU ARE ALMOST NEVER CLRED TO CLB TO FL180 DURING A CLBOUT TO A HIGH ALT SECTOR. THERE WAS NO ALT BUST IN THIS CASE. THE MD-80 ALT ALERT SYS SEEMS TO BE IN NEED OF A FIX JUDGING FROM ALL THE ASAP RPTS FILED ON ALT BUSTS IN THE ACFT.

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