FMC critical software behavior

[Guest]

Hallo!Some weeks ago I read one of these dramatic flight incident reports. It tells about a controlled flight into terrain. The pilot wants to continue the route with the FMC after flying few minutes by heading select mode. This was during approach with mountains in the vicinity. The pilot activates the route beginning with one waypoint ahead - but in fact the aircraft had already flown over the waypoint in the meantime. So the aircraft starts a left turn immediatly to catch the waypoint. The time to the impact was too short - the crew had no chance.Yesterday on my flight from Iraklion to Vienna when I was playing around with the holding page of the 767PIC FMC I experienced a bad surprise when I executed the changes. After pressing the exec key the pic starts to make a left turn. I left her go to watch what she wants to do. In the first moment I thought that the FMC is entering a holding pattern immediatly despite that I have set up a holding only for a waypoint 40 miles away.But then I realized that the pic is steering to a waypoint I have flown over few minutes before! dejavu - my version of the disasterOK, later I found out the following: if the pilot makes changes to the active route the PIC FMC put them in the well known temporary buffer and displays all changes by white lines. In the meantime the currently active route (in green) will be normaly processed. Now if the aircraft passes a waypoint it will be deleted in the active route and the FMC proceeds with the next waypoint. But in the temporary route the waypoint remains! And after pressing exec when the temp route is moved to the active route the waypoint is back again!So for me I have learned "checking the changes before making them active" means also to check which waypoints are already passed and remove them as required.Is this the same way of working like the real B767 FMC?Or is this a software bug of the PIC?happy landings,Rainer MeiselAustrian Virtual Airlineshttp://www.aua.com/virtual

[Steve Dra 7,588]

That is an excellent question.I would hope it would give the "Not on an intercept heading" message, arm the LNAV mode and switch to hdg sel or hdg hold until the pilot turned the aircraft towards the waypoint. If the waypoint was in front (or near the current heading), then LNAV would activate and fly the plane. If the waypoint was behind or way off the current course, the AP would switch to (or stay in) the other mode until the pilot turned the aircraft, and give the message on the FMC.This way the aircraft wouldn't make any unexpected turns. :)Regards,Steve Dra