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FMC - Way Point Programming

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On all routes that I have input to date, the FMC usually programmes a turn before the next way point so that - unless no hdg change is required - the aircraft never actually overflies the way point. This is very noticeable with a large hdg change, say 120 degrees plus.Does anyone know if it is possible to define way points so that the FMC will fly to overhead before commencing the turn to the next point - cannot find anything relating to this in the manuals.Many thanksJohn Rooum

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Guest Ruahrc

The early turns are done on purpose and is behavior that you should see in any FMC aircraft- even real life ones. Basically the FMC is plotting a route that will result in the smallest deviation from the "ideal" point-to-point (connect the dots) route, taking into account the fact that a jet at cruise speed takes a pretty wide turn. It does this in order to prevent the aircraft overshooting the route when it bends and having to waste fuel and time getting back on track. Starting the turn a little early reduces that. Also it would make for a smoother ride since you can make a gradual turn with a small bank instead of having to bank sharply at the last minute in order to make it around the bend.And yes, if the bend in the course is large the earlier the turn will be initiated, because it takes longer to turn 120 degrees instead of 20. (as an aside, what route has you turning 120 degrees? that's pretty extreme for any route)If you are worried about flyover- the FMC will automatically cycle to the next w/p when it gets near (and eventually passes by) the previous w/p. You don't need to be directly over it within 0.01 feet to count as "passing" the waypoint. What is your concern? Or just curiosity?Lastly- if the automatically generated curves look too large to you on the ND maybe it is that the range you have selected for the ND is low i.e. 5 miles or something. At scales like that the curves seem pretty big and look like they take you way off course, but if you zoom out to 40 or 80 miles you will see that in actuality these "rounded corners" are very small compared to the overall route.Ruahrc

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Guest Ruahrc

accidental double-post

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RuahrcVery many thanks for your very comprehensive response - yes it was mainly interest as the aircraft I am used to [30 years on jet fighters] rarely had effective autopilots let alone FMC. Also I appreciate your point on planning the most effective route through several turning points [and the ND display range setting]. I have to say, this is a tremendous programme - a great pity about the problems PSS are having with others [see "whats wrong with you PSS"] - they might be a commercial organisation but they have my sympathy!John

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Guest dt1951

The nautical terms for you describe are advance and displacement. When you turn an aircraft (or ship) it doesn't turn on a point. Rather, the vectors for the turn will have hoizontal displacement (hence, displacment) in the original direction, And vectors to the intended direction (advance). As the aircraft (ship) approaches the new course, the displacment vectors decrease and the advance vectors increase. The FMC calculates how much advance and displacement are required to make the turn to land on the new course. Gets even more fun when you throw in wind (current) and increase the new course angle. When you wait until you reach the turn point, you will overshoot the turn and end up steering back to intercept the new course (which is what happens when you are very heavy).

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