Narrative:

After leveling off at FL370, the captain (PNF) needed to use the restroom. Sometime during the process of putting on my oxygen and the captain leaving the flight deck, we were cleared direct to the oak VOR. I remember doing the direct intercept on the FMC and executing the command and noting the aircraft making a slight right turn. Seeing the aircraft turn, I assumed the intercept was complete and directed my attention to the cockpit door and the captain exiting and the flight attendant entering the cockpit. After returning to the cockpit and pinning the door and putting his head set on, center called with a frequency change and acted as though they had been calling and we missed the call. Upon switching to the new center frequency, we were asked if we were going direct to beaty. After checking the FMC and MCP, I noted that we were in control wheel steering on the autoplt and not navigating to any fix, but our heading was close to a heading that was direct beaty. The captain told center no, but we could go direct beaty. Center cleared us direct and noted that we were off course to oal. Looking back on the event, the captain and I concluded that we could have been in control wheel steering for 1 or 2 reasons. The first was the LNAV system kicked off by itself, which is unlikely. The most likely way was that when given the direct to oal, I entered the intercept and executed the command and while the aircraft started its turn I reached up and pushed the LNAV button on the MCP, not seeing the light on and disengaged LNAV and put us in control wheel steering. I must have noted the turn in direction from the initial execution of the original execution of the intercept and because of the distraction of the captain leaving the cockpit did not notice that we were in control wheel steering. Whenever the cockpit door is opened, I turn in my seat and focus my total attention to the door until it is secure again. The mistake I made was not going back and ensuring that the intercept was done correctly. Any time changes in navigation or planning take place when a secondary distraction is diverting our attention, we need to rechk our work after the secondary distraction is over. Don't assume that everything is ok because the aircraft initially responds to an input in a manner you expect. Supplemental information from acn 564075: after returning to the cockpit and getting settled into my seat, I must have missed a radio call about the time the first officer was putting his oxygen mask away. The next call was to switch to ZOA. That controller advised us we were off course to oal and wanted to know if we were given direct to bty. We now believe what happened was a series of small errors that led to the course deviation. When we were given the clearance, the PF executed it in the CDU and we think that out of habit also pressed the LNAV button on the MCP. This happened as I distraction him with me leaving the cockpit and him having to put his oxygen mask on, then the flight attendant entering the cockpit with these distrs he didn't realize at the time that by pressing the LNAV button he actually disengaged the LNAV. This we believe is how we ended up in control wheel steering. After I returned to the cockpit I then missed a radio call. By the time I heard the next call we obviously had been off course for a little while. I also failed to notice the control wheel steering annunciator on the AFDS.

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Original NASA ASRS Text

Title: A B737-300 CREW, IN CRUISE AT FL370, DISCOVERED THE AUTOPLT WAS IN CTL WHEEL STEERING INSTEAD OF IN LNAV, AS ASSUMED.

Narrative: AFTER LEVELING OFF AT FL370, THE CAPT (PNF) NEEDED TO USE THE RESTROOM. SOMETIME DURING THE PROCESS OF PUTTING ON MY OXYGEN AND THE CAPT LEAVING THE FLT DECK, WE WERE CLRED DIRECT TO THE OAK VOR. I REMEMBER DOING THE DIRECT INTERCEPT ON THE FMC AND EXECUTING THE COMMAND AND NOTING THE ACFT MAKING A SLIGHT R TURN. SEEING THE ACFT TURN, I ASSUMED THE INTERCEPT WAS COMPLETE AND DIRECTED MY ATTN TO THE COCKPIT DOOR AND THE CAPT EXITING AND THE FLT ATTENDANT ENTERING THE COCKPIT. AFTER RETURNING TO THE COCKPIT AND PINNING THE DOOR AND PUTTING HIS HEAD SET ON, CTR CALLED WITH A FREQ CHANGE AND ACTED AS THOUGH THEY HAD BEEN CALLING AND WE MISSED THE CALL. UPON SWITCHING TO THE NEW CTR FREQ, WE WERE ASKED IF WE WERE GOING DIRECT TO BEATY. AFTER CHKING THE FMC AND MCP, I NOTED THAT WE WERE IN CTL WHEEL STEERING ON THE AUTOPLT AND NOT NAVING TO ANY FIX, BUT OUR HDG WAS CLOSE TO A HDG THAT WAS DIRECT BEATY. THE CAPT TOLD CTR NO, BUT WE COULD GO DIRECT BEATY. CTR CLRED US DIRECT AND NOTED THAT WE WERE OFF COURSE TO OAL. LOOKING BACK ON THE EVENT, THE CAPT AND I CONCLUDED THAT WE COULD HAVE BEEN IN CTL WHEEL STEERING FOR 1 OR 2 REASONS. THE FIRST WAS THE LNAV SYS KICKED OFF BY ITSELF, WHICH IS UNLIKELY. THE MOST LIKELY WAY WAS THAT WHEN GIVEN THE DIRECT TO OAL, I ENTERED THE INTERCEPT AND EXECUTED THE COMMAND AND WHILE THE ACFT STARTED ITS TURN I REACHED UP AND PUSHED THE LNAV BUTTON ON THE MCP, NOT SEEING THE LIGHT ON AND DISENGAGED LNAV AND PUT US IN CTL WHEEL STEERING. I MUST HAVE NOTED THE TURN IN DIRECTION FROM THE INITIAL EXECUTION OF THE ORIGINAL EXECUTION OF THE INTERCEPT AND BECAUSE OF THE DISTR OF THE CAPT LEAVING THE COCKPIT DID NOT NOTICE THAT WE WERE IN CTL WHEEL STEERING. WHENEVER THE COCKPIT DOOR IS OPENED, I TURN IN MY SEAT AND FOCUS MY TOTAL ATTN TO THE DOOR UNTIL IT IS SECURE AGAIN. THE MISTAKE I MADE WAS NOT GOING BACK AND ENSURING THAT THE INTERCEPT WAS DONE CORRECTLY. ANY TIME CHANGES IN NAV OR PLANNING TAKE PLACE WHEN A SECONDARY DISTR IS DIVERTING OUR ATTN, WE NEED TO RECHK OUR WORK AFTER THE SECONDARY DISTR IS OVER. DON'T ASSUME THAT EVERYTHING IS OK BECAUSE THE ACFT INITIALLY RESPONDS TO AN INPUT IN A MANNER YOU EXPECT. SUPPLEMENTAL INFO FROM ACN 564075: AFTER RETURNING TO THE COCKPIT AND GETTING SETTLED INTO MY SEAT, I MUST HAVE MISSED A RADIO CALL ABOUT THE TIME THE FO WAS PUTTING HIS OXYGEN MASK AWAY. THE NEXT CALL WAS TO SWITCH TO ZOA. THAT CTLR ADVISED US WE WERE OFF COURSE TO OAL AND WANTED TO KNOW IF WE WERE GIVEN DIRECT TO BTY. WE NOW BELIEVE WHAT HAPPENED WAS A SERIES OF SMALL ERRORS THAT LED TO THE COURSE DEV. WHEN WE WERE GIVEN THE CLRNC, THE PF EXECUTED IT IN THE CDU AND WE THINK THAT OUT OF HABIT ALSO PRESSED THE LNAV BUTTON ON THE MCP. THIS HAPPENED AS I DISTR HIM WITH ME LEAVING THE COCKPIT AND HIM HAVING TO PUT HIS OXYGEN MASK ON, THEN THE FLT ATTENDANT ENTERING THE COCKPIT WITH THESE DISTRS HE DIDN'T REALIZE AT THE TIME THAT BY PRESSING THE LNAV BUTTON HE ACTUALLY DISENGAGED THE LNAV. THIS WE BELIEVE IS HOW WE ENDED UP IN CTL WHEEL STEERING. AFTER I RETURNED TO THE COCKPIT I THEN MISSED A RADIO CALL. BY THE TIME I HEARD THE NEXT CALL WE OBVIOUSLY HAD BEEN OFF COURSE FOR A LITTLE WHILE. I ALSO FAILED TO NOTICE THE CTL WHEEL STEERING ANNUNCIATOR ON THE AFDS.

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.