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Guest Douglas Thompson

FMCs and DP checkpoints

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To all,Be aware of something - your FMCs won't necessarily get you out of Dept's airspace. I learned this the hard way the other day, glad I did. I'll know to monitor things more closely, get my head out of those checklists and... actually pay attention.PMDG b737ng - Flying out of KSFO to the east coast. The DP's first checkpoint is NORMM, the SFO 281^r/13m, then back to OAK and eastbound from there. Taking off rwy 28R I thought myself in good shape to engage LNAV.The FMC turned the aircraft too soon. I didn't notice the error (I happened to glance at the AdvDisplay noticing NORMM to be 4m away), until I was well into a right turn back towards OAK, like almost northbound. I certainly would've missed NORMM. I MIGHT'VE gotten as close as 3m. Not good enough! RC demands 2m for DPs. That won't change.What I did to remidy this, there ought to be a critique for it but we won't discuss this :-lol Just be aware ;-)

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Many guidance systems rely on anticipated turns. The Reality-XP Apollo GPS and the PMDG 737NGs FMC. I am always ready to take over with HDG on the MCP where I know a sharp turn is coming up. I keep a sharp eye on the ND (EHSI) to see if the path is close enough to the waypoint on an FMC driven display.Also pay attention to your bank limit selection on the main panel of the 737NG. I do not know if the FMC predicts the the path from the turn rate and start of turn based on this. I'll ask on their forum.It appears from cockpit videos I have of Zurich departures south that yaw is hand flown followed by heading select until the 90 degree change at BREGO is accomplished. The other item is to keep your speed low enough to negotiate the turn as close as possible to a departure sharp course direction change.

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I suspect the problem is related to the FMC anticipating the turn, compounded by the big turn you had to make (~180 degrees).I have learned to use HDG throughout departure. I find that you're safe to turn on LNAV after you cross your first waypoint or are cleared to an new altitue from your initial. Meaning, if my clearance is "fly runway heading, maintain 6000, expect 15,000 after 10 minutes", I will fly with HDG until I hit the first waypoint or until I get clearance to 15,000. After that LNAV seems to be a champ.Bob

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>Many guidance systems rely on anticipated turns. It also depends on the type of waypoint: fly-over or fly-by. If it is fly-over than the turn is not initiated until after crossing the waypoint. I know that PMDG, LDS, Project Magenta provide the means to make that distinction ... but I have not seen that distinction made in any charts. I would make the conservative assumption that if a waypoint is depicted with no annotation, then it is flyover waypoint.-michael

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