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.