Narrative:

Given 210 heading out of our departure airport. Then given direct a route fix. I put fix in top; executed; went into LNAV mode; moved both switches to LNAV and confirmed LNAV on scoreboard. Aircraft started turn and I started collecting my approach plates and getting out the destination approach plates because there was weather in that area and I was uncomfortable with having to do a localizer approach since the aircraft was not capable of doing the RNAV. I wanted some time to review the procedures and the plates. Center called and asked if we were direct the cleared fix at which time I looked at the box and it said not on intercept heading. Box was locked up and airplane had gone into heading mode. I was finally able to bring the fix on top and without abeam points it would take direct. I executed went back to LNAV and it turned to the fix with no further problems. Better software and modernization of the fleet. And as I have mentioned on previous reports; the system needs to tell you if it kicks out of the mode you have selected. A simple warning light on the glareshield would be perfect. We spend copious amounts of time on minuscule details that make no difference yet for years we have turned a blind eye to a glaring safety of flight issue that has a very simple and easy fix. There will be a time when an aircraft kicks out of VNAV into vertical speed with 0 set and the holes in swiss cheese line up or the airplane gets to a point and kicks out of LNAV and goes into heading select putting itself into danger. A simple flashing yellow light could save the day.

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Original NASA ASRS Text

Title: A B737-300 FMC transitioned from LNAV to HDG SEL after a down track fix was selected and activated but the pilot did not see the error until ATC questioned the aircraft's track.

Narrative: Given 210 heading out of our departure airport. Then given direct a route fix. I put fix in top; executed; went into LNAV mode; moved both switches to LNAV and confirmed LNAV on scoreboard. Aircraft started turn and I started collecting my approach plates and getting out the destination approach plates because there was weather in that area and I was uncomfortable with having to do a LOC approach since the aircraft was not capable of doing the RNAV. I wanted some time to review the procedures and the plates. Center called and asked if we were direct the cleared fix at which time I looked at the box and it said not on intercept heading. Box was locked up and airplane had gone into HDG mode. I was finally able to bring the fix on top and without abeam points it would take direct. I executed went back to LNAV and it turned to the fix with no further problems. Better software and modernization of the fleet. And as I have mentioned on previous reports; the system needs to tell you if it kicks out of the mode you have selected. A simple warning light on the glareshield would be perfect. We spend copious amounts of time on minuscule details that make no difference yet for years we have turned a blind eye to a glaring safety of flight issue that has a very simple and easy fix. There will be a time when an aircraft kicks out of VNAV into vertical speed with 0 set and the holes in Swiss cheese line up or the airplane gets to a point and kicks out of LNAV and goes into heading select putting itself into danger. A simple flashing yellow light could save the day.

Data retrieved from NASA's ASRS site as of July 2013 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.