Narrative:

Aircraft was a B727 equipped with a block V autoplt which has automatic altitude select, capture, and hold ability. Aircraft logbook had maintenance open item 'autoplt altitude select/altitude hold inoperative.' I interpreted this to mean that either the arming switch did not work or the automatic capture and hold functions did not work. I hand flew the aircraft up to FL310 and manually engaged the autoplt and made sure the vertical speed knob was centered in the altitude hold position and that the altitude hold function was illuminated on the annunciator panel. The aircraft was stable and trimmed, all seemed normal. I diverted my attention to my airway chart to determine our next VOR and radial, when center called us and asked what altitude we were climbing to. I saw that the altimeter showed FL314 and a slow climb. I immediately disconnected the autoplt and flew back down to FL310. Further testing showed that the autoplt would not maintain altitude in any mode, nor could we use altitude select knob to maintain altitude as it was not sensitive enough. I hand flew the aircraft for the rest of the flight. It did not appear that there were any aircraft near us, and no loss of separation occurred. In retrospect, I believe the autoplt system should have been labeled inoperative in its entirety, not just the select/hold function. The autoplt was in fact unusable in cruise, and the way it was written up led to my confusion as to believing only the automatic capture and hold function was inoperative. In fact, the entire pitch hold didn't work even though the annunciators showed it was working. Supplemental information from acn 497883: accepted aircraft with autoplt altitude capture discrepancy. Proceeding direct mem, captain leveled off at FL310 and engaged autoplt and manually ensured vertical speed wheel was in center detent. Altitude green annunciator light illuminated, which indicated altitude hold was engaged on autoplt. Both captain and I checked to make sure aircraft maintained altitude. Satisfied, we both turned to put away charts and check data on others. Few moments later, approximately 15-20 seconds, I saw altimeter read FL314 and heard controller ask where we were going. Captain immediately disconnected autoplt and descended aircraft to FL310. No aural sound was emitted from the altitude alerter, nor did the autoplt warning sound to indicate that a failure had occurred in the altitude (vertical) mode. Aircraft was hand flown for remainder of flight without incident. It was our collective assertion initially that only the altitude capture feature was inoperative, not the altitude hold mode. This was arrived at after reading the aircraft MEL.

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Original NASA ASRS Text

Title: B727-200 CREW HAD ALT EXCURSION IN ZME CLASS A AIRSPACE.

Narrative: ACFT WAS A B727 EQUIPPED WITH A BLOCK V AUTOPLT WHICH HAS AUTOMATIC ALT SELECT, CAPTURE, AND HOLD ABILITY. ACFT LOGBOOK HAD MAINT OPEN ITEM 'AUTOPLT ALT SELECT/ALT HOLD INOP.' I INTERPED THIS TO MEAN THAT EITHER THE ARMING SWITCH DID NOT WORK OR THE AUTOMATIC CAPTURE AND HOLD FUNCTIONS DID NOT WORK. I HAND FLEW THE ACFT UP TO FL310 AND MANUALLY ENGAGED THE AUTOPLT AND MADE SURE THE VERT SPD KNOB WAS CTRED IN THE ALT HOLD POS AND THAT THE ALT HOLD FUNCTION WAS ILLUMINATED ON THE ANNUNCIATOR PANEL. THE ACFT WAS STABLE AND TRIMMED, ALL SEEMED NORMAL. I DIVERTED MY ATTN TO MY AIRWAY CHART TO DETERMINE OUR NEXT VOR AND RADIAL, WHEN CTR CALLED US AND ASKED WHAT ALT WE WERE CLBING TO. I SAW THAT THE ALTIMETER SHOWED FL314 AND A SLOW CLB. I IMMEDIATELY DISCONNECTED THE AUTOPLT AND FLEW BACK DOWN TO FL310. FURTHER TESTING SHOWED THAT THE AUTOPLT WOULD NOT MAINTAIN ALT IN ANY MODE, NOR COULD WE USE ALT SELECT KNOB TO MAINTAIN ALT AS IT WAS NOT SENSITIVE ENOUGH. I HAND FLEW THE ACFT FOR THE REST OF THE FLT. IT DID NOT APPEAR THAT THERE WERE ANY ACFT NEAR US, AND NO LOSS OF SEPARATION OCCURRED. IN RETROSPECT, I BELIEVE THE AUTOPLT SYS SHOULD HAVE BEEN LABELED INOP IN ITS ENTIRETY, NOT JUST THE SELECT/HOLD FUNCTION. THE AUTOPLT WAS IN FACT UNUSABLE IN CRUISE, AND THE WAY IT WAS WRITTEN UP LED TO MY CONFUSION AS TO BELIEVING ONLY THE AUTOMATIC CAPTURE AND HOLD FUNCTION WAS INOP. IN FACT, THE ENTIRE PITCH HOLD DIDN'T WORK EVEN THOUGH THE ANNUNCIATORS SHOWED IT WAS WORKING. SUPPLEMENTAL INFO FROM ACN 497883: ACCEPTED ACFT WITH AUTOPLT ALT CAPTURE DISCREPANCY. PROCEEDING DIRECT MEM, CAPT LEVELED OFF AT FL310 AND ENGAGED AUTOPLT AND MANUALLY ENSURED VERT SPD WHEEL WAS IN CTR DETENT. ALT GREEN ANNUNCIATOR LIGHT ILLUMINATED, WHICH INDICATED ALT HOLD WAS ENGAGED ON AUTOPLT. BOTH CAPT AND I CHKED TO MAKE SURE ACFT MAINTAINED ALT. SATISFIED, WE BOTH TURNED TO PUT AWAY CHARTS AND CHK DATA ON OTHERS. FEW MOMENTS LATER, APPROX 15-20 SECONDS, I SAW ALTIMETER READ FL314 AND HEARD CTLR ASK WHERE WE WERE GOING. CAPT IMMEDIATELY DISCONNECTED AUTOPLT AND DSNDED ACFT TO FL310. NO AURAL SOUND WAS EMITTED FROM THE ALT ALERTER, NOR DID THE AUTOPLT WARNING SOUND TO INDICATE THAT A FAILURE HAD OCCURRED IN THE ALT (VERT) MODE. ACFT WAS HAND FLOWN FOR REMAINDER OF FLT WITHOUT INCIDENT. IT WAS OUR COLLECTIVE ASSERTION INITIALLY THAT ONLY THE ALT CAPTURE FEATURE WAS INOP, NOT THE ALT HOLD MODE. THIS WAS ARRIVED AT AFTER READING THE ACFT MEL.

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.