B737-300
Training / Qualification; Distraction; Human-Machine Interface; Confusion
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Aircraft
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.
[This Report contains no additional information.]
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.