I've had the JS41 for a week and after much frustration I've found that 'direct to' with NAV engaged does not track the correct path if the first waypoint is greater than 180 deg from your initial runway heading. 

 

An example flightplan is FAPE-OKSET-UQ48-ITMAM-UQ31-GE-FACT. Departing rwy08 from FAPE and using HDG select to circle 180 deg to OKSET and then using 'direct to' and engaging NAV results in a random turn to 360 degrees away from desired track. I've tried this at different airports with the same effect.

 

I'm surprised no-one else has discovered this yet unless it's explained somewhere in the manual (which I haven't found)? 

 

There are probably at least two good reasons for this. The first being the obvious one that a way point can not be more the 180 degrees from your heading since by definition, it would be less then 180 degrees in the other direction. The second reason is also pretty obvious if you think about it; planning to track to a waypoint that is effectivly behind you is not actually planning, is it? 

 

I'm surprised no-one else has discovered this yet unless it's explained somewhere in the manual (which I haven't found)?

 

Not to be rude, but that's generally an indication that you're not doing something right, especially when the plane has been out for several years now.

 

Did the FMC even have an initial point when you took off?  If you go to the NAV page, and you see that it says FR (from) without a TO point immediately below it, then your plane doesn't exactly know where it's going.  Sure, it knows that there's a route out there, but computers aren't like your and my brain: they only know what they're taught (through programming) and what they're told, by you and me.  This FMC wasn't taught the same way Boeing's, Airbus', and MD's were.

 

Try this: http://library.avsim.net/download.php?DLID=156113

The first being the obvious one that a way point can not be more the 180 degrees from your heading since by definition, it would be less then 180 degrees in the other direction. The second reason is also pretty obvious if you think about it; planning to track to a waypoint that is effectivly behind you is not actually planning, is it? 

 

Actually there is nothing wrong with my planning and have been flying this departure for many years with other aircraft, but perhaps I should've been more clear. I'm flying a perfectly normal departure out of FAPE - the OKSET2A from rwy 08:

 

(http://www.caa.co.za/resource%20center/Charts/AERONAUTICAL%20CHARTS/P/FAPE_PORT%20ELIZABETH%20INTERNATIONAL/DEP_03/FAPE_DEP-03.pdf)

 

The fms scrambles the SID from the latest AIRAC so I have to fly the departure using VOR instruments, which is fine, but After circling right, the last part of the SID calls for direct to OKSET at 15nm from FAPE flying west (Cape Town is west of PE), and using 'Direct to' OKSET in the FMS and NAV does not track to OKSET. 

 

So I'm assuming the fms does not like your initial waypoint to be in the opposite direction of your initial rwy heading. I will have to try get the SID programmed correctly, maybe try another AIRAC cycle.

 

Did the FMC even have an initial point when you took off?  If you go to the NAV page, and you see that it says FR (from) without a TO point immediately below it, then your plane doesn't exactly know where it's going.

 

My FROM and TO are initialized in the NAV page as FAPE and OKSET. I've tried RWY08 and OKSET and that doesn't work either. 

Your first waypont is on runway heading at 3 DME from PEV with an altitude restriction of +500, but I get the impression that is not your problem. 

 

You have a route planned from OKSET to your destination with no discontinuities. You have left the runway and turned to HDG 176 until DME 9. From there, I am guessing you turned to 266, stabilised speed and engaged the AP with HDG and IAS modes?

 

At DME 15, you selected Direct_to and expected the heading to change to around 300? Can you detailed what happend next, and if you can remember, what modes were displayed?

 

If you are trying the flight again, at DME15, can you adjust heading until you are within 15degrees of OKSET and then engage direct-to and report what you see? (Screen shots help a lot!)

Be careful with the bug. If you spin it fast depending on the plane the aircraft will rotate as tight as possible to try and make the heading adjustment. IF you are carrying for passengers the unwanted g effect will be felt in there seat. Not say we don't put a little strain on passengers like that every know and again, I fly for a living and it happens to be my Step father who can complain about the smallest things, most likely being late for a Tee Time in FL.You need to set a course to intercept the radial of the VOR your trying to track. Not a steep angle very shallow, because the system will over correct when you reach your radial. The closer you are to a VOR the more sensitive the needles become. Your not flying a fighter so take things a little slower. VOR navigation still works or we wouldn't be using it. It is an art that learned over years of practice. Start with good habits and you will always fly true. Look on you tube for a tutorial on the FMS and VOR navigation. You should find one on VOR from Sportys. It is how it really works. By all means read the manual. I know it is long and you just want to fly. The realism of the J41 is so close it isn't funny. With VOR navigation, you shouldn't need an FMS other than for VNAV but hey it isn't real so use it. PMDG put alot of time into making there aircraft real, so it requires a little study to learn. Otherwise we would just have a ton of original aircraft that don't even come close to acting like the real ones. Good luck, Cpt out.

 

PS. One more thing to check, are you tracking the FMS or VOR when you look below your horizon ind. The row of buttons at the top decide what is being tracked, again read the manual or at least You Tube it.

 

Be careful with the bug. If you spin it fast depending on the plane the aircraft will rotate as tight as possible

I know you think you are trying to help, but seriously? "If you spin it fast..."

Hi Paul,

 

You have a route planned from OKSET to your destination with no discontinuities. You have left the runway and turned to HDG 176 until DME 9. From there, I am guessing you turned to 266, stabilised speed and engaged the AP with HDG and IAS modes?

Yes, exactly right. 

 

At DME 15, you selected Direct_to and expected the heading to change to around 300? Can you detailed what happend next, and if you can remember, what modes were displayed?

At DME 15 I set a heading direct to OKSET using HDG bug, and then select 'Direct to' on the FMS, and change to NAV lateral mode. The aircraft then turns right off track to 360 degrees and maintains that course. This behaviour is repeatable, and I've tried this departure 3 times with the same effect.

 

I have the same issue at FAJS when flying the APDAK departure, so my conclusion is that NAV with 'direct to' will not work if your first hard fix is >90 degrees from your initial departure heading. As I said before I'm surprised no-one else has picked up on this, but perhaps just because South African airports have some unorthodox departure procedures. Although I did see some posts with similar NAV problems that were unresolved, so perhaps this is the cause.

 

What I eventually did was modify the AIRAC OKSET SID for FAPE by replacing the 'Heading to DME' procedures with hard fixes (i.e. PEV9, PEV15 etc). Now when stabilized on the climb out to PEV9 I can hit NAV and everything works fine. 

 

I'll attach a screen shot later if that helps. But it does seem like a bug that should be fixed.

 

Other than that little quirk, this aircraft is just such a joy to fly! 

I just tried it and it worked.

 

I couldn't set your exact route as UQ48 didn't offer me the choice of ending at ITMAM but perhaps my nav data is out of date. Can I suggest you try again while following the tutorial to see if maybe there is a step you are missing?

Mmm.. interesting. Can you tell the FMS waypoints you used until OKSET? Or did you simply use FAPE-OKSET as your first nav waypoint?

FAPE - OKSET - ITMAM - GE - FACT

I was able to enter UQ31 but I didn't bother since that wasn't where the problem was showing. It has been a while since I flew the JS41 so I did need to follow the tutorial when setting up the rest of the FMC (it is not the most intuitive unit I have used). I set up the FMS as described between pages 39 and 49, but using your route instead of theirs, then skipped to the takeoff on p77. The screen shot is just after I selected Direct_to Okset and then selected NAV mode.

 

 

Okay thanks for that, there must be something weird going on then because I am also following the tutorial line-by-line, including the FMS programming. I will post my screenshots later tonight when I have more time.

I have had the FMC do that turn to the north once in a while when engaging the LNAV.  Open your default GPS (shift + 1) after you engage the LNAV.  If there is no nav info on the GPS - like showing you are direct to OKSET, for example, program the GPS to go direct to the waypoint you need.  The LNAV goes back to normal and follows the remaining waypoints in the FMC - you do not have to mess with the GPS after that.

 

I  do not know why this happens.

 

I was attempting a reference to the heading bug. I hold a current real world ATP Ticket and A First Class Medical which is also current. I will refrain from offering help so the pros can do it. I am only type rated in 5 real world aircraft. If you crank a heading bug around too quickly you can get unwanted results. Flying requires some skill and one of those being VOR tracking a radial, or finding an airway or simply following ATC instructions. Constantly reminded why I don't offer help. Sorry guys and gals. Cpt out.

 Flying requires some skill and one of those being VOR tracking a radial

The reason I didn't reply to your post is because my problem had nothing to do with navigation skills, i.e. tracking a VOR radial. Perhaps you should have read my problem a little closer. Thanks for sharing your knowledge anyways. 

 

Paul I set up the OKSET departure again exactly as I did before and this time NAV worked! So I'm a little confused with this, it seem to occur randomly. 

 

Frank thanks, I will try the GPS trick the next time it happens!