FMC Error message

[martinlest2 58]

I can't find any reference to this in any of the PMDG PDF manulas - and "Googling" didn't help either. Perhaps someone here can enlighten me?Why do I sometimes get an error showing in the PMDG (737) FMC, usually I think on descent/approach: "PTH DES NOT AVAILABLE". What am I omitting to do?Thanks.Martin

[Mats_J 2]

Martin,This is what Cpt. Bill Bulfer says about this message in his excellent book FMC User's Guide B737:"MESSAGE:PATH DES NOT AVAILABLECAUSING CONDITION:Within 5 nm of T/D, a path descent is planned, and a computed path is not available.CORRECT ACTION:Execute a speed descent if desired, or construct an acceptable path, or use MCP for descent."Hope it helps,

[martinlest2 58]

Thanks. What would be really good is to know how to "construct an acceptable path" though..M. :-)

[downscc 1,327]

May have something to do with the final segments of the route you are flying. Do you have a STAR and IAP selected? What are they?

[martinlest2 58]

I'll check next time. The last few flights I have made have been to airports with no ILS facilities..M.

[downscc 1,327]

Perhaps no approach or star selected is the problem? The FMC doesn't have altitude constraints for which to create a descent path.

[Jeff Nielsen 1,385]

You can select another approach from the list if one is available. If there is no ILS then a GPS approach is usually what I select.As far as constructing the whole thing...just make sure if you're using a STAR that your beginning approach segment starts with the end of the last waypoint of the STAR (or similar/equivalent).Some STAR's are RNAV, others end with vectors to the approach, etc. Some STAR's are also RWY specific, especially if overseas...that kind of thing.Some airports may not have any approaches in the 'Nav Data' base, but you can always make one up if you need to using place bearing distance (PBD) waypoints from the RWY or a VOR.Hope that's not too confusing...lol :-)

[martinlest2 58]

Sort of OK, but will make more sense when I try it out during a flight, FMC in front of me! Thanks for the replies,M.

[onurksn 1]

>Thanks. What would be really good is to know how to>"construct an acceptable path" though..Hello,If you already don't know this, you can plan your descent using the 3.3 rule (I don't actually know if this is a "rule"). Multiply your cruise altitude by 3.3 to get a TOD distance.For example if your CRZ altitude is FL300 you can start your descent at about 100nm from your destination. Bearing in mind that you can descend at a faster rate at higher altitudes, try to be at 10000 feet roughly 30nm from your IAF. These are of course approximate figures but they do work out for me every time.If you stick to this when entering altitude constraints in your legs page, your VNAV descent should work perfectly. (If you have a tail wing start your descent earlier)Hope I didn't repeat anything you're already aware of.RegardsOnurLTBA

[martinlest2 58]

Yes, I knew that, but thanks anyway!!M. :-)