Narrative:

About 70-100 mi from fam, we were given a clearance direct fam. I moved the dfgc switch from 1 to 2, selected autoplt and autothrottles on and pushed altitude hold. I then checked the FMA to verify altitude hold. I then selected navigation and a moment or so later selected performance. I now reached down to pick up my navigation chart to verify frequency and coordinates for fam. Within a very short time ATC asked us to verify FL270. Looking up I noticed the aircraft had begun an unselected climb. I disconnected the autoplt and returned to FL270 post haste. ATC informed no conflict. Not sure of what happened, as I know we were in altitude hold when performance was selected, I conducted an experiment and found that if a higher altitude is inadvertently selected performance would begin a climb without selecting it on pms. We tried this 5 times and found that with a higher altitude selected moving from altitude hold to performance causes an unselected climb. Vertical speed at the time was not a factor as even if aircraft was in a descent it would change to climb. It should be added that it sometimes would momentarily go to cruise then to climb. As to how a higher altitude got selected after leveloff unknown. I may have bumped it, it may have changed on its own, or it may not have been in the detent. This is an anomaly I was unaware of. Sorry for the deviation. Supplemental information from acn 304948: if the altitude set knob is accidentally hit to a higher altitude and you go from altitude hold to performance the aircraft will start an unselected climb. The aircraft did this 3 out 5 times. I can find nothing in the book or through my experience that shows this anomaly.

Related reports:

Google
 

Original NASA ASRS Text

Title: ALTDEV ALT OVERSHOT IN CLB.

Narrative: ABOUT 70-100 MI FROM FAM, WE WERE GIVEN A CLRNC DIRECT FAM. I MOVED THE DFGC SWITCH FROM 1 TO 2, SELECTED AUTOPLT AND AUTOTHROTTLES ON AND PUSHED ALT HOLD. I THEN CHKED THE FMA TO VERIFY ALT HOLD. I THEN SELECTED NAV AND A MOMENT OR SO LATER SELECTED PERFORMANCE. I NOW REACHED DOWN TO PICK UP MY NAV CHART TO VERIFY FREQ AND COORDINATES FOR FAM. WITHIN A VERY SHORT TIME ATC ASKED US TO VERIFY FL270. LOOKING UP I NOTICED THE ACFT HAD BEGUN AN UNSELECTED CLB. I DISCONNECTED THE AUTOPLT AND RETURNED TO FL270 POST HASTE. ATC INFORMED NO CONFLICT. NOT SURE OF WHAT HAPPENED, AS I KNOW WE WERE IN ALT HOLD WHEN PERFORMANCE WAS SELECTED, I CONDUCTED AN EXPERIMENT AND FOUND THAT IF A HIGHER ALT IS INADVERTENTLY SELECTED PERFORMANCE WOULD BEGIN A CLB WITHOUT SELECTING IT ON PMS. WE TRIED THIS 5 TIMES AND FOUND THAT WITH A HIGHER ALT SELECTED MOVING FROM ALT HOLD TO PERFORMANCE CAUSES AN UNSELECTED CLB. VERT SPD AT THE TIME WAS NOT A FACTOR AS EVEN IF ACFT WAS IN A DSCNT IT WOULD CHANGE TO CLB. IT SHOULD BE ADDED THAT IT SOMETIMES WOULD MOMENTARILY GO TO CRUISE THEN TO CLB. AS TO HOW A HIGHER ALT GOT SELECTED AFTER LEVELOFF UNKNOWN. I MAY HAVE BUMPED IT, IT MAY HAVE CHANGED ON ITS OWN, OR IT MAY NOT HAVE BEEN IN THE DETENT. THIS IS AN ANOMALY I WAS UNAWARE OF. SORRY FOR THE DEV. SUPPLEMENTAL INFO FROM ACN 304948: IF THE ALT SET KNOB IS ACCIDENTALLY HIT TO A HIGHER ALT AND YOU GO FROM ALT HOLD TO PERFORMANCE THE ACFT WILL START AN UNSELECTED CLB. THE ACFT DID THIS 3 OUT 5 TIMES. I CAN FIND NOTHING IN THE BOOK OR THROUGH MY EXPERIENCE THAT SHOWS THIS ANOMALY.

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.