Narrative:

Copilot was PF, I was PNF. As we were climbing through FL290 to the assigned FL310, I turned off the seat belt sign and proceeded to perform the associated public announcements. I told the PNF I would be 'off the air' during this time, during my announcement. The PNF leveled the aircraft at FL310 and engaged the 'B' autoplt. I finished my announcement, turned my VHF-1 volume back up, looked up and saw we were climbing through FL314 in a right turn (approximately 20 degree angle of bank). The PF took control and made the wings level, then corrected our climb by leveling off at about FL317 -- then began a descent back to FL310 while turning back to course. There was initial confusion as to what was happening. We both thought at first that the FMC had malfunctioned, or that the computer was giving erroneous instruction to the autoplt. However, the autoplt was not engaged. I did not see the PF engage the autoplt, but he said he did, and that it properly engaged and tracked the LNAV-VNAV functions. We had an omc observing, and he confirmed this. The omc was a company new hire recently qualified in our aircraft as first officer. As we descended through FL313, ZLA queried our altitude. I told them we were through FL312, that our autoplt had disengaged, and we were correcting the course as well. He said 'no problem - - just keeping you honest.' we then tried to diagnose our problem. We tried to re-engage the 'B' autoplt. The button lit -- then went dark. I tried 'a' autoplt -- it did not light. I tried the 'cws' mode to no avail. At this point, we realized that there never was an autoplt disconnect horn. I asked the omc to check and rechk all circuit breakers. I then became PF -- and asked the copilot to review our flight manual. All circuit breakers were in, there was no flight manual guidance on this problem. The first officer then noticed the autoplt disengage bar was slightly downward. He pushed it back up. I engaged the 'B' autoplt and it worked normally. The bar was down just slightly, not enough to prominently reveal the yellow area behind it, or illuminate that area, as it does when fully disengaged. This was our second leg in this plane, and it worked fine to this point. No other traffic was visible or appeared on TCASII. The flight continued normally, and normally for the next 3 legs we flew it. However, we did have questions about this occurrence: 1) why there was no autoplt disconnect warning horn. Both the PF and omc believed the autoplt did engage, which should have produced the horn for a disengagement. 2) we heard no altitude warning horn through FL313, as we should have. It may have sounded, but nobody recalled it. 3) how did the autoplt disengage bar get to the position it was in. How long was it like that? Because of these questions, we tested these system on this leg. They all worked normally. The autoplt horn and altitude warning horn both sounded in cases they should have. As a precaution, I reported the problem to our company maintenance for their review of these system. The autoplt disconnect bar was not fully down -- in a disconnect position. On a flight the following day, I discovered (in another plane) that the 'B' autoplt engage button will illuminate for a brief time (1 second) and then extinguish with no horn or 'command' annunciation. The 'a' will stay dark if pushed. I conclude that since we got no horn, and the plane was not trimmed for level flight, the autoplt did not engage. Callback conversation with reporter revealed the following information: the reporter was flying a B737-300 at the time and since his discovery he has been able to duplicate the almost-engaging of the autoplt on several B737S. He said that since this event he always checks the engage paddles during his preflight procedure and if he can he demonstrates the phenomenon to his first officer. He stated that he was never trained to completely seat the paddle in the 'cmd' (command) position during his initial training.

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Original NASA ASRS Text

Title: AN ACR B737 FLC FINDS THAT THE AUTOPLT ENGAGEMENT PADDLES COULD BE ALMOST IN THE 'CMD' POS AND REMAIN THERE AND YET NOT BE ACTUALLY ENGAGED. THE ACFT CLBED ABOVE ITS ASSIGNED ALT AND TURNED OFF ITS COURSE SINCE THE AUTOPLT WAS NOT, IN FACT, ENGAGED.

Narrative: COPLT WAS PF, I WAS PNF. AS WE WERE CLBING THROUGH FL290 TO THE ASSIGNED FL310, I TURNED OFF THE SEAT BELT SIGN AND PROCEEDED TO PERFORM THE ASSOCIATED PUBLIC ANNOUNCEMENTS. I TOLD THE PNF I WOULD BE 'OFF THE AIR' DURING THIS TIME, DURING MY ANNOUNCEMENT. THE PNF LEVELED THE ACFT AT FL310 AND ENGAGED THE 'B' AUTOPLT. I FINISHED MY ANNOUNCEMENT, TURNED MY VHF-1 VOLUME BACK UP, LOOKED UP AND SAW WE WERE CLBING THROUGH FL314 IN A R TURN (APPROX 20 DEG ANGLE OF BANK). THE PF TOOK CTL AND MADE THE WINGS LEVEL, THEN CORRECTED OUR CLB BY LEVELING OFF AT ABOUT FL317 -- THEN BEGAN A DSCNT BACK TO FL310 WHILE TURNING BACK TO COURSE. THERE WAS INITIAL CONFUSION AS TO WHAT WAS HAPPENING. WE BOTH THOUGHT AT FIRST THAT THE FMC HAD MALFUNCTIONED, OR THAT THE COMPUTER WAS GIVING ERRONEOUS INSTRUCTION TO THE AUTOPLT. HOWEVER, THE AUTOPLT WAS NOT ENGAGED. I DID NOT SEE THE PF ENGAGE THE AUTOPLT, BUT HE SAID HE DID, AND THAT IT PROPERLY ENGAGED AND TRACKED THE LNAV-VNAV FUNCTIONS. WE HAD AN OMC OBSERVING, AND HE CONFIRMED THIS. THE OMC WAS A COMPANY NEW HIRE RECENTLY QUALIFIED IN OUR ACFT AS FO. AS WE DSNDED THROUGH FL313, ZLA QUERIED OUR ALT. I TOLD THEM WE WERE THROUGH FL312, THAT OUR AUTOPLT HAD DISENGAGED, AND WE WERE CORRECTING THE COURSE AS WELL. HE SAID 'NO PROB - - JUST KEEPING YOU HONEST.' WE THEN TRIED TO DIAGNOSE OUR PROB. WE TRIED TO RE-ENGAGE THE 'B' AUTOPLT. THE BUTTON LIT -- THEN WENT DARK. I TRIED 'A' AUTOPLT -- IT DID NOT LIGHT. I TRIED THE 'CWS' MODE TO NO AVAIL. AT THIS POINT, WE REALIZED THAT THERE NEVER WAS AN AUTOPLT DISCONNECT HORN. I ASKED THE OMC TO CHK AND RECHK ALL CIRCUIT BREAKERS. I THEN BECAME PF -- AND ASKED THE COPLT TO REVIEW OUR FLT MANUAL. ALL CIRCUIT BREAKERS WERE IN, THERE WAS NO FLT MANUAL GUIDANCE ON THIS PROB. THE FO THEN NOTICED THE AUTOPLT DISENGAGE BAR WAS SLIGHTLY DOWNWARD. HE PUSHED IT BACK UP. I ENGAGED THE 'B' AUTOPLT AND IT WORKED NORMALLY. THE BAR WAS DOWN JUST SLIGHTLY, NOT ENOUGH TO PROMINENTLY REVEAL THE YELLOW AREA BEHIND IT, OR ILLUMINATE THAT AREA, AS IT DOES WHEN FULLY DISENGAGED. THIS WAS OUR SECOND LEG IN THIS PLANE, AND IT WORKED FINE TO THIS POINT. NO OTHER TFC WAS VISIBLE OR APPEARED ON TCASII. THE FLT CONTINUED NORMALLY, AND NORMALLY FOR THE NEXT 3 LEGS WE FLEW IT. HOWEVER, WE DID HAVE QUESTIONS ABOUT THIS OCCURRENCE: 1) WHY THERE WAS NO AUTOPLT DISCONNECT WARNING HORN. BOTH THE PF AND OMC BELIEVED THE AUTOPLT DID ENGAGE, WHICH SHOULD HAVE PRODUCED THE HORN FOR A DISENGAGEMENT. 2) WE HEARD NO ALT WARNING HORN THROUGH FL313, AS WE SHOULD HAVE. IT MAY HAVE SOUNDED, BUT NOBODY RECALLED IT. 3) HOW DID THE AUTOPLT DISENGAGE BAR GET TO THE POS IT WAS IN. HOW LONG WAS IT LIKE THAT? BECAUSE OF THESE QUESTIONS, WE TESTED THESE SYS ON THIS LEG. THEY ALL WORKED NORMALLY. THE AUTOPLT HORN AND ALT WARNING HORN BOTH SOUNDED IN CASES THEY SHOULD HAVE. AS A PRECAUTION, I RPTED THE PROB TO OUR COMPANY MAINT FOR THEIR REVIEW OF THESE SYS. THE AUTOPLT DISCONNECT BAR WAS NOT FULLY DOWN -- IN A DISCONNECT POS. ON A FLT THE FOLLOWING DAY, I DISCOVERED (IN ANOTHER PLANE) THAT THE 'B' AUTOPLT ENGAGE BUTTON WILL ILLUMINATE FOR A BRIEF TIME (1 SECOND) AND THEN EXTINGUISH WITH NO HORN OR 'COMMAND' ANNUNCIATION. THE 'A' WILL STAY DARK IF PUSHED. I CONCLUDE THAT SINCE WE GOT NO HORN, AND THE PLANE WAS NOT TRIMMED FOR LEVEL FLT, THE AUTOPLT DID NOT ENGAGE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR WAS FLYING A B737-300 AT THE TIME AND SINCE HIS DISCOVERY HE HAS BEEN ABLE TO DUPLICATE THE ALMOST-ENGAGING OF THE AUTOPLT ON SEVERAL B737S. HE SAID THAT SINCE THIS EVENT HE ALWAYS CHKS THE ENGAGE PADDLES DURING HIS PREFLT PROC AND IF HE CAN HE DEMONSTRATES THE PHENOMENON TO HIS FO. HE STATED THAT HE WAS NEVER TRAINED TO COMPLETELY SEAT THE PADDLE IN THE 'CMD' (COMMAND) POS DURING HIS INITIAL TRAINING.

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.