Narrative:

RNAV departure off runway 19L las. Engaged LNAV at 400 ft. Engaged autoplt after fixix in turn to roppr passing about 4000 ft MSL. Announced, 'LNAV and autoplt engaged.' noticed autoplt was taking airplane to about a 30 degree intercept to get to roppr (heading of about 265 degrees). We both commented on it and as I was turning aircraft back to the left, towards roppr, I noticed LNAV was not engaged anymore. Departure called us and said we were going the wrong way and to turn left immediately to heading of 180 degrees and gave us a phone number to call. Arriving in destination, I called our ATC specialist who called me back and said 'not to worry.' whoever he talked to said it was not a problem. I don't know why the LNAV disengaged itself, but I can see that being a huge problem on a single engine departure out of rno. We were reconfiguring the bleeds, after a bleeds-off takeoff, so I wasn't staring at the LNAV annunciator -- maybe I should now. Also, the reason we didn't stop the turn sooner is that when you take off on runway 25R in LNAV on the cowboy departure, the airplane 'overcorrects' the turn to roppr and we were thinking maybe that's what was happening. It was a disconcerting experience to say the least, but the bottom line is that I will watch LNAV very closely now to make sure it's still working and will let ATC know immediately when it disengaged. P.south. Obviously this would have been noticed on a -700 with the map display a whole lot sooner! Callback conversation with reporter revealed the following information: the pilot stated that the airplane transitioned from LNAV to control wheel steering/control wheel pitch shortly after LNAV was engaged. I questioned whether, at the position they were at when LNAV was engaged, they were also capturing the 7000 ft restr at roppr. She stated that it was possible but not sure that was occurring at that moment. The transition happened for unknown reasons. Both the first officer and she saw LNAV. Supplemental information from acn 632972: when I noticed we were turning off course, I looked at the scoreboard and noticed that we were in control wheel steering and control wheel pitch, LNAV was not selected anymore. As we started the turn back to course, las approach called and told us we were going the wrong way and to turn to a heading of 180 degrees and maintain FL190. I'm not sure when the LNAV became unselected, but I remember checking that it was selected prior to reconfiguring the packs.

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Original NASA ASRS Text

Title: A B737-300 WITH AN FMC BUT NO GLASS DEVIATED FROM THE RNAV DEP ROUTING WHEN AN LNAV DISCONNECT WENT UNNOTICED AS THE CREW RECONFIGURED FROM A BLEEDS OFF TKOF.

Narrative: RNAV DEP OFF RWY 19L LAS. ENGAGED LNAV AT 400 FT. ENGAGED AUTOPLT AFTER FIXIX IN TURN TO ROPPR PASSING ABOUT 4000 FT MSL. ANNOUNCED, 'LNAV AND AUTOPLT ENGAGED.' NOTICED AUTOPLT WAS TAKING AIRPLANE TO ABOUT A 30 DEG INTERCEPT TO GET TO ROPPR (HDG OF ABOUT 265 DEGS). WE BOTH COMMENTED ON IT AND AS I WAS TURNING ACFT BACK TO THE L, TOWARDS ROPPR, I NOTICED LNAV WAS NOT ENGAGED ANYMORE. DEP CALLED US AND SAID WE WERE GOING THE WRONG WAY AND TO TURN L IMMEDIATELY TO HDG OF 180 DEGS AND GAVE US A PHONE NUMBER TO CALL. ARRIVING IN DEST, I CALLED OUR ATC SPECIALIST WHO CALLED ME BACK AND SAID 'NOT TO WORRY.' WHOEVER HE TALKED TO SAID IT WAS NOT A PROB. I DON'T KNOW WHY THE LNAV DISENGAGED ITSELF, BUT I CAN SEE THAT BEING A HUGE PROB ON A SINGLE ENG DEP OUT OF RNO. WE WERE RECONFIGURING THE BLEEDS, AFTER A BLEEDS-OFF TKOF, SO I WASN'T STARING AT THE LNAV ANNUNCIATOR -- MAYBE I SHOULD NOW. ALSO, THE REASON WE DIDN'T STOP THE TURN SOONER IS THAT WHEN YOU TAKE OFF ON RWY 25R IN LNAV ON THE COWBOY DEP, THE AIRPLANE 'OVERCORRECTS' THE TURN TO ROPPR AND WE WERE THINKING MAYBE THAT'S WHAT WAS HAPPENING. IT WAS A DISCONCERTING EXPERIENCE TO SAY THE LEAST, BUT THE BOTTOM LINE IS THAT I WILL WATCH LNAV VERY CLOSELY NOW TO MAKE SURE IT'S STILL WORKING AND WILL LET ATC KNOW IMMEDIATELY WHEN IT DISENGAGED. P.S. OBVIOUSLY THIS WOULD HAVE BEEN NOTICED ON A -700 WITH THE MAP DISPLAY A WHOLE LOT SOONER! CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE PLT STATED THAT THE AIRPLANE TRANSITIONED FROM LNAV TO CTL WHEEL STEERING/CTL WHEEL PITCH SHORTLY AFTER LNAV WAS ENGAGED. I QUESTIONED WHETHER, AT THE POS THEY WERE AT WHEN LNAV WAS ENGAGED, THEY WERE ALSO CAPTURING THE 7000 FT RESTR AT ROPPR. SHE STATED THAT IT WAS POSSIBLE BUT NOT SURE THAT WAS OCCURRING AT THAT MOMENT. THE TRANSITION HAPPENED FOR UNKNOWN REASONS. BOTH THE FO AND SHE SAW LNAV. SUPPLEMENTAL INFO FROM ACN 632972: WHEN I NOTICED WE WERE TURNING OFF COURSE, I LOOKED AT THE SCOREBOARD AND NOTICED THAT WE WERE IN CTL WHEEL STEERING AND CTL WHEEL PITCH, LNAV WAS NOT SELECTED ANYMORE. AS WE STARTED THE TURN BACK TO COURSE, LAS APCH CALLED AND TOLD US WE WERE GOING THE WRONG WAY AND TO TURN TO A HDG OF 180 DEGS AND MAINTAIN FL190. I'M NOT SURE WHEN THE LNAV BECAME UNSELECTED, BUT I REMEMBER CHKING THAT IT WAS SELECTED PRIOR TO RECONFIGURING THE PACKS.

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.