Narrative:

We had a normal climb out and leveloff at FL310. After the cruise checklist was completed, I began my en route PA. We had been level for almost 10 mins when the first officer announced FL320 for FL330. I asked when we had been cleared to FL330, and we both thought uh-oh. I was about to tell ATC we were returning to altitude when he asked about our climb. He offered us FL350 if we wanted it, but we had already discussed with this controller about the rides and had previously stated we were going to stay at FL310. I asked if there had been any conflict, and he stated no problems at all. We then discussed what happened. Neither of us could remember having been anywhere near the altitude knob. We had been level for nearly 10 mins prior to the transgression and were mystified as to why the airplane climbed. However, I do have a working theory, although I cannot verify any of the scenario, as I did not see any of it. While level, the first officer was navigating via heading select to cty VOR. The VOR cap had been wandering as it normally does and he was making good manual corrections. I believe he may have inadvertently bumped the altitude knob, or even turned it thinking it was the heading knob. A common mistake. He had been in performance cruise. When the altitude got to FL330 in the window, the performance went to climb mode. We had a brief discussion as to the desired en route mach, and when he entered this, it was on the climb page with a valid altitude selected. The tri was in cruise, so the power application and pitch change were very slight and difficult to notice, especially in the choppy air we were encountering. We noticed the climb, but not soon enough. I believe we were as attentive as we could have been in that phase of flight. We were discussing the bumps and en route speed at the moment of the deviation. It was a VFR day and the outside view was nice. I would say it was less than 20 seconds of climb that took place before it was arrested. Chalk one up to experience. Read the pms closer. Watch your hands -- and his.

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Original NASA ASRS Text

Title: MD80 CREW HAD AN ALT EXCURSION WHEN THE AUTOPLT MADE AN UNCOMMANDED CLB.

Narrative: WE HAD A NORMAL CLBOUT AND LEVELOFF AT FL310. AFTER THE CRUISE CHKLIST WAS COMPLETED, I BEGAN MY ENRTE PA. WE HAD BEEN LEVEL FOR ALMOST 10 MINS WHEN THE FO ANNOUNCED FL320 FOR FL330. I ASKED WHEN WE HAD BEEN CLRED TO FL330, AND WE BOTH THOUGHT UH-OH. I WAS ABOUT TO TELL ATC WE WERE RETURNING TO ALT WHEN HE ASKED ABOUT OUR CLB. HE OFFERED US FL350 IF WE WANTED IT, BUT WE HAD ALREADY DISCUSSED WITH THIS CTLR ABOUT THE RIDES AND HAD PREVIOUSLY STATED WE WERE GOING TO STAY AT FL310. I ASKED IF THERE HAD BEEN ANY CONFLICT, AND HE STATED NO PROBS AT ALL. WE THEN DISCUSSED WHAT HAPPENED. NEITHER OF US COULD REMEMBER HAVING BEEN ANYWHERE NEAR THE ALT KNOB. WE HAD BEEN LEVEL FOR NEARLY 10 MINS PRIOR TO THE TRANSGRESSION AND WERE MYSTIFIED AS TO WHY THE AIRPLANE CLBED. HOWEVER, I DO HAVE A WORKING THEORY, ALTHOUGH I CANNOT VERIFY ANY OF THE SCENARIO, AS I DID NOT SEE ANY OF IT. WHILE LEVEL, THE FO WAS NAVING VIA HDG SELECT TO CTY VOR. THE VOR CAP HAD BEEN WANDERING AS IT NORMALLY DOES AND HE WAS MAKING GOOD MANUAL CORRECTIONS. I BELIEVE HE MAY HAVE INADVERTENTLY BUMPED THE ALT KNOB, OR EVEN TURNED IT THINKING IT WAS THE HDG KNOB. A COMMON MISTAKE. HE HAD BEEN IN PERFORMANCE CRUISE. WHEN THE ALT GOT TO FL330 IN THE WINDOW, THE PERFORMANCE WENT TO CLB MODE. WE HAD A BRIEF DISCUSSION AS TO THE DESIRED ENRTE MACH, AND WHEN HE ENTERED THIS, IT WAS ON THE CLB PAGE WITH A VALID ALT SELECTED. THE TRI WAS IN CRUISE, SO THE PWR APPLICATION AND PITCH CHANGE WERE VERY SLIGHT AND DIFFICULT TO NOTICE, ESPECIALLY IN THE CHOPPY AIR WE WERE ENCOUNTERING. WE NOTICED THE CLB, BUT NOT SOON ENOUGH. I BELIEVE WE WERE AS ATTENTIVE AS WE COULD HAVE BEEN IN THAT PHASE OF FLT. WE WERE DISCUSSING THE BUMPS AND ENRTE SPD AT THE MOMENT OF THE DEV. IT WAS A VFR DAY AND THE OUTSIDE VIEW WAS NICE. I WOULD SAY IT WAS LESS THAN 20 SECONDS OF CLB THAT TOOK PLACE BEFORE IT WAS ARRESTED. CHALK ONE UP TO EXPERIENCE. READ THE PMS CLOSER. WATCH YOUR HANDS -- AND HIS.

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.