ALTDEV ALT EXCURSION.

Narrative

WHILE AT CRUISE ALT OF FL280 WITH DISCRETION TO FL240; HDG W DIRECT VLA; 40 E OF VLA; THE AUTOPLT STARTED A GRADUAL CLB AND SLOWLY ACCELERATED THE CLB UNTIL THE ACFT APCHED FL300. THE FO FIRST NOTICED THE CLB AND IMMEDIATELY DISCONNECTED THE AUTOPLT AND RETURNED THE ACFT MANUALLY TO FL280. THE ALT ALERTER DID NOT GO OFF BECAUSE FL240 (THE PLT'S DISCRETION ALT) WAS SET IN ALT WINDOW AS IS NORMAL PROC. THE PLTS WERE BRIEFING THE APCH DURING THE INCIDENT. THE CLB BEGAN VERY INSIDIOUSLY BY THE AUTOPLT FOR UNKNOWN REASONS. THE TCASII SYS WAS ON AT THE TIME AND DID NOT ADVISE OF ANY CONFLICTS. THERE WERE NO TFC CONFLICTS VISIBLE TO EITHER PLT. CTR DID NOT ADVISE OF ANY CONFLICTS EITHER. POSSIBLY; IF SOMETHING WERE CHANGED ABOUT THE WAY THE ALT ALERT WORKS; THE DEV COULD HAVE BEEN MINIMIZED. FOR EXAMPLE; IF A LOWER ALT IS SET IN THE WINDOW; MAYBE THE ALERTER SHOULD GO OFF FOR DEVS ABOVE THE CURRENT ALT ONLY. ALSO; DURING APCH SETUP; PLTS MUST BE MORE VIGILANT THAN NORMAL TO AVOID SIMILAR SITS.