Narrative:

Autoplt disconnect accompanied by cruise altitude deviation. Autoplt #1 disconnected without the autoplt warning light illuminating to give warning of the disconnect. First indication of disconnect was the altitude aural warning followed by a TCASII alert for traffic at FL310 at the 11 O'clock position, 10 mi ring. Incident occurred in an md-80 at approximately XA05Z at FL330 along J58 just prior to center handoff from ZDV to ZAB. Autoplt had been engaged for cruise flight for 1 hour at this point and was in the performance cruise mode. Altitude loss in deviation was 800 ft. At the time of deviation, copilot was reading an en route information PA he had written out. The captain had just pulled out the arrival charts for dfw and had begun to review them in between periodic monitoring of instruments. Autoplt disconnect was insidious in that there was no autoplt warning light and the altitude aural warning was the first indication of problem. Captain pressed altitude hold to affect a smooth correction, looked for adjustment on vsi and altimeter. Steady increase of left angle of bank and continued descent directed his attention back to autoplt engage switch on flight director panel which was then noted to be in the off position. Captain re-engaged #1 autoplt after manually flying and establishing a slight climb. Autoplt #1 did not capture rearmed altitude of FL330. Manually flew aircraft back down to FL330 and engaged autoplt #2. Discrepancy entered in maintenance log.

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Original NASA ASRS Text

Title: 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.

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.