Narrative:

We had descended from FL340 to FL240 for en route chop. Aircraft operation on FMC autoplt, not in performance. FMA indicated proper parameters. We had been level for at least 5 mins. I visually noticed an aircraft at 11 O'clock high. It looked close and descending. I called the traffic to first officer and jump seater, mr. X, of FAA headquarters. The potential conflict was confirmed by TCASII as TA symbol. We were surprised of no traffic notice from the controller, frequency 124.87 ZFW. The autoplt had been operating normally but for some reason it had begun a slow climb while we were judging the possible conflict. The autoplt altitude alert sounded. I immediately disengaged the autoplt and descended back to FL240. At time of disconnect, altitude was 24350 ft with 100 FPM climb ivsi. Controller inquired our altitude, first officer responded FL240 and correcting. Controller informed we were cited with altitude violation. The crew had been performing normal cockpit duties. Autoplt had functioned properly in performance but had been switched to FMC at FL240 for speed restr. All appeared normal. Cannot explain why autoplt entered climb at this unfortunate time that every eye was judging possible traffic conflict. Note: several other aircraft in the control of 124.87 inquired about conflicting traffic that was unrpted by controller. A saab responded to an RA. Mr. X witnessed all of the above and intends to call the manager at ZFW. He seemed concerned for the performance of the controller and recognized the reason that the altitude deviation went unnoticed. Supplemental information from acn 306035: on at least 2 other occasions, I have noted the autoplt in mach select enter a slight climb or descent, plus or minus 100 ft. On the other occasions, this was caught and corrected before exceeding 300 ft. I have no doubt that we would have caught the deviation as well had we not been distracted by the other aircraft in close proximity.

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Original NASA ASRS Text

Title: ACFT EQUIP PROB AUTOPLT ALT HOLD CAUSES ALTDEV AND LTSS.

Narrative: WE HAD DSNDED FROM FL340 TO FL240 FOR ENRTE CHOP. ACFT OP ON FMC AUTOPLT, NOT IN PERFORMANCE. FMA INDICATED PROPER PARAMETERS. WE HAD BEEN LEVEL FOR AT LEAST 5 MINS. I VISUALLY NOTICED AN ACFT AT 11 O'CLOCK HIGH. IT LOOKED CLOSE AND DSNDING. I CALLED THE TFC TO FO AND JUMP SEATER, MR. X, OF FAA HEADQUARTERS. THE POTENTIAL CONFLICT WAS CONFIRMED BY TCASII AS TA SYMBOL. WE WERE SURPRISED OF NO TFC NOTICE FROM THE CTLR, FREQ 124.87 ZFW. THE AUTOPLT HAD BEEN OPERATING NORMALLY BUT FOR SOME REASON IT HAD BEGUN A SLOW CLB WHILE WE WERE JUDGING THE POSSIBLE CONFLICT. THE AUTOPLT ALT ALERT SOUNDED. I IMMEDIATELY DISENGAGED THE AUTOPLT AND DSNDED BACK TO FL240. AT TIME OF DISCONNECT, ALT WAS 24350 FT WITH 100 FPM CLB IVSI. CTLR INQUIRED OUR ALT, FO RESPONDED FL240 AND CORRECTING. CTLR INFORMED WE WERE CITED WITH ALT VIOLATION. THE CREW HAD BEEN PERFORMING NORMAL COCKPIT DUTIES. AUTOPLT HAD FUNCTIONED PROPERLY IN PERFORMANCE BUT HAD BEEN SWITCHED TO FMC AT FL240 FOR SPD RESTR. ALL APPEARED NORMAL. CANNOT EXPLAIN WHY AUTOPLT ENTERED CLB AT THIS UNFORTUNATE TIME THAT EVERY EYE WAS JUDGING POSSIBLE TFC CONFLICT. NOTE: SEVERAL OTHER ACFT IN THE CTL OF 124.87 INQUIRED ABOUT CONFLICTING TFC THAT WAS UNRPTED BY CTLR. A SAAB RESPONDED TO AN RA. MR. X WITNESSED ALL OF THE ABOVE AND INTENDS TO CALL THE MGR AT ZFW. HE SEEMED CONCERNED FOR THE PERFORMANCE OF THE CTLR AND RECOGNIZED THE REASON THAT THE ALTDEV WENT UNNOTICED. SUPPLEMENTAL INFO FROM ACN 306035: ON AT LEAST 2 OTHER OCCASIONS, I HAVE NOTED THE AUTOPLT IN MACH SELECT ENTER A SLIGHT CLB OR DSCNT, PLUS OR MINUS 100 FT. ON THE OTHER OCCASIONS, THIS WAS CAUGHT AND CORRECTED BEFORE EXCEEDING 300 FT. I HAVE NO DOUBT THAT WE WOULD HAVE CAUGHT THE DEV AS WELL HAD WE NOT BEEN DISTRACTED BY THE OTHER ACFT IN CLOSE PROX.

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.