FLC DISTR WITH GENERATOR OFF PROC DIDN'T NOTICE AUTOPLT DISCONNECT WITH SUBSEQUENT ALTDEV.

Narrative

LEVEL CRUISE FLT FL290 #1 AUTOPLT ENGAGED CAPT FLYING. #3 GENERATOR TRIPPED OFF LINE CAPT (ME) ASKED FE TO PULL OUT THE PROC BOOK AND WE WENT THROUGH 'GENERATOR OFF' PROC. I DID NOT ASSIGN THE ACFT CTL TO THE COPLT BECAUSE WE WERE IN CRUISE AND THIS GENERATOR PROB SHOULD NOT HAVE AFFECTED OUR LEVEL CRUISE FLT. ABOUT 3 MIN INTO THE 'GENERATOR OFF' PROC WHILE RESETTING AND DISCUSSING THE GENERATOR. THE FO STATED 'HEY HOW COME WE ARE CLBING' FOR SOME REASON THE #1 AUTOPLT HAD DISCONNECTED AND THE ACFT HAD CLBED TO FL300 -- ABOUT THAT TIME ZID CALLED US -- WE IMMEDIATELY DSNDED TO FL290 AND STATED LEVEL FL290. WE HAD TCASII OPERATIONAL ON BOARD AND NO OTHER FLTS WERE NEAR US. THERE WAS NO REASON IN THIS CASE FOR THE AUTOPLT TO DISCONNECT -- COINCIDENCE; HOWEVER IN ALL CASES OF 'ABNORMALS' THE FLYING SHOULD BE ASSIGNED TO THE COPLT WHILE THE CAPT AND SO ADDRESS THE ABNORMAL -- EVEN IN CRUISE LEVEL FLT AT ALT. A LESSON WELL LEARNED. UNLESS FLYING IS TRANSFERRED AND ASSIGNED TO COPLT ALL 3 PLTS WILL CONTINUE TO GET INVOLVED IN THE ABNORMAL!