744 Issue

[Guest 13579]

Greetings everyone,I'm having issues on intercepting waypoints in a normal flight plan. Whenever I exceed a particular speed - very often above 200 Knots - the LNAV fails to function completely and very rarely does it re-intercept wayspoints back again. This is very common when the flight plan includes normal turns (not even close to sharp turns) that deviate away from a normal line.Any suggestions?

[downscc 1,327]

Are you referring to crossing a waypoint? The term 'intercept' usually refers to courses or radials, not points or fixes. Anyway, most fixes are 'flyby' rather than 'overfly.' The FMC will calculate a 'lead distance' before the fix based on degrees of turn and speed where it starts the turn to end on on the desired track or course leaving the fix. This means the aircraft flys-by the fix. But this might be the wrong answer if I didn't get your question right.

[Guest 13579]

If 'crossing a waypoint' you state it is, then I don't understand why the FMC displays a "not on intercept" notice. And no, it is not referring to ILS or LOC or even APPR intercepts. This notice is usually displayed on the FMC as soon as the LNAV stops 'crossing a waypoint'.In simple terms, if I was to implement a test plan where the aircraft would take-off from any airport, and follow the plan until approaching a nearby airport, near the one I took off from (for testing purposes), the LNAV simply FAILS to follow the plan prior to reaching the approach when I apply a speed of over 200 KNOTS. This frequently happens when there are turns - especially sharp turns - which deviate away from a normal straight line.

[downscc 1,327]

Now I think I understand... the "not on intercept" message is the clue. What is probably happening here is your initial departure heading (or current heading) is greater than some value (I think 40-deg) from the desired course. The FMC doesn't attempt to intercept a course if the required heading change is too great. This is a faithful reproduction of how the real systems work. I hope this helps.

[Guest 13579]

Indeed. This is now starting to make sense. However, I did think the FMC would monitor and correct the HDG automatically... This is the case in the PSS 777 for instance. Do I have to manually correct the HDG all the time? How can I re-intercept the course if it's disrupted? Because I've never being able to...Thank you very much for you help.