ALTDEV ALT EXCURSION DURING A NIGHT OP.

Narrative

AUTOPLT DISCONNECT ACCOMPANIED BY CRUISE ALTDEV. AUTOPLT #1 DISCONNECTED WITHOUT THE AUTOPLT WARNING LIGHT ILLUMINATING TO GIVE WARNING OF THE DISCONNECT. FIRST INDICATION OF DISCONNECT WAS THE ALT AURAL WARNING FOLLOWED BY A TCASII ALERT FOR TFC AT FL310 AT THE 11 O'CLOCK POS; 10 MI RING. INCIDENT OCCURRED IN AN MD-80 AT APPROX XA05Z AT FL330 ALONG J58 JUST PRIOR TO CTR HDOF FROM ZDV TO ZAB. AUTOPLT HAD BEEN ENGAGED FOR CRUISE FLT FOR 1 HR AT THIS POINT AND WAS IN THE PERFORMANCE CRUISE MODE. ALT LOSS IN DEV WAS 800 FT. AT THE TIME OF DEV; COPLT WAS READING AN ENRTE INFO PA HE HAD WRITTEN OUT. THE CAPT HAD JUST PULLED OUT THE ARR CHARTS FOR DFW AND HAD BEGUN TO REVIEW THEM IN BTWN PERIODIC MONITORING OF INSTS. AUTOPLT DISCONNECT WAS INSIDIOUS IN THAT THERE WAS NO AUTOPLT WARNING LIGHT AND THE ALT AURAL WARNING WAS THE FIRST INDICATION OF PROB. CAPT PRESSED ALT HOLD TO AFFECT A SMOOTH CORRECTION; LOOKED FOR ADJUSTMENT ON VSI AND ALTIMETER. STEADY INCREASE OF L ANGLE OF BANK AND CONTINUED DSCNT DIRECTED HIS ATTN BACK TO AUTOPLT ENGAGE SWITCH ON FLT DIRECTOR PANEL WHICH WAS THEN NOTED TO BE IN THE OFF POS. CAPT RE-ENGAGED #1 AUTOPLT AFTER MANUALLY FLYING AND ESTABLISHING A SLIGHT CLB. AUTOPLT #1 DID NOT CAPTURE REARMED ALT OF FL330. MANUALLY FLEW ACFT BACK DOWN TO FL330 AND ENGAGED AUTOPLT #2. DISCREPANCY ENTERED IN MAINT LOG.