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744 taking shortcuts instead of flying LEGS as programm...

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Sometimes I have an issue with the 744 taking shortcuts, instead of flying the legs as programmed in the FMC. This is while flying in LNAV mode. A recent example:I am approaching Quito SEQU on the Descent 6 arrival for ILS runway 35, using VNAV and LNAV. This is a circling approach, using the QMS VOR as entry point for a standard (=right hand) hold pattern. It goes something like this:- Fly HDG 352 to QMS, cross QMS at 16,000- At QMS, enter right hand hold and descend to 12,000. Outbound leg HDG is 172- On the inbound part at D6 QMS, level at 12,000 and continue to D3.5 QMS- At D3.5 QMS continue descent to cross QMS at 10,900- When reaching QMS, exit the hold and continue straight to runwayIn other words, laterally this is a standard hold pattern, but vertically it is a bit tricky as one needs to level at 12,000 for portions of the hold pattern and only then continue descending to the target altitude of 10,900.Thus, entering HOLD at QMS with a target altitude of 10,900 into the FMC isn't really an option as the plane would merely go down to 10,900 at a constant V/S, instead of going down to 12,000 first, level at 12,000 for some time, and only then continue to 10,900.I therefore programmed the different waypoints of the hold myself, using P/B/D references and the corresponding SPD and ALT constraints. All is well until some nm before reaching the entry point of the hold which is QMS. The plane suddenly discovers that there is a waypoint to its right which it needs to go to later on anyway. So instead of flying to QMS, entering the hold, and flying the programmed waypoints to effectively fly a hold, it simply jumps a couple of waypoints and turns right immediately. Of course it is way to high to respect the ALT constraint programmed for that waypoint, because it didn't go all the lateral way as it was supposed to do.How can I fix that? For me the only way around that was to do lateral navigation with the HDG function, but that is quite cumbersome in a situation like an approach into Quito where difficult terrain, lots of ATC interaction, difficult weather already requires all the attention. Anyone knows how to avoid this issue of "lateral shortcuts"? On a wider note, can the PMDG planes (744 and 737NG) fly RNAV approaches?`ThanksChris

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The FMC typically "smooths" the desired track, I suspect this is what is happening to you when you add the additional waypoints.Descent in holding is pretty common, but as you have discovered the FMC isn't capable of anything other than a hold at an altitude and even that seems to be designed for enroute or missed approaches.I believe the only way to get the desired result is to try your hand at modifying the sidstar file. Instead of using the "HOLD AT FIX" keywords you could use the waypoint strategy by putting fixes where you need them using the "COLOCATED" keyword since everything is pretty much referenced to QMS. The tricky part is figuring out where to put the fix on the outbound leg such that you fly something that resembles the "racetrack" but that's not too hard.If you've never tried modifying sidstars before, don't be afraid of trying. The syntax is very easy to learn and everything is in plain text. Terry's got an excellent tutorial available at his site.Alternative, you can always use the A/P MCP controls HDG and ALT to get the result you want. I suspect that's how it's done real world.


Dan Downs KCRP

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>The FMC typically "smooths" the desired track, I suspect this>is what is happening to you when you add the additional>waypoints.>>Descent in holding is pretty common, but as you have>discovered the FMC isn't capable of anything other than a hold>at an altitude and even that seems to be designed for enroute>or missed approaches.>>I believe the only way to get the desired result is to try>your hand at modifying the sidstar file. Instead of using the>"HOLD AT FIX" keywords you could use the waypoint strategy by>putting fixes where you need them using the "COLOCATED">keyword since everything is pretty much referenced to QMS. The>tricky part is figuring out where to put the fix on the>outbound leg such that you fly something that resembles the>"racetrack" but that's not too hard.>>If you've never tried modifying sidstars before, don't be>afraid of trying. The syntax is very easy to learn and>everything is in plain text. Terry's got an excellent tutorial>available at his site.>>Alternative, you can always use the A/P MCP controls HDG and>ALT to get the result you want. I suspect that's how it's done>real world.>Dan, thanks a lot.I'll try doing a sidstar myself. Where can I find that tutorial? I have indeed never done it before.ThanksChris

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Chris-When you are reading the tutorial- pay close attention to the difference between FLY PAST and FLY OVER waypoints...It can create just the problem you describe vis a vis bypassing wapoints in a turn.


Robert S. Randazzo coolcap.gif

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