FMC DISCONTINUITY problem

[Vauban 0]

Hi all,responding to a 'Proceed direct to XXX' request from ATC, I entered XXX in the FMC scratch area and then moved it to line 1 using 1L LSK.Line 1 showed the waypoint correctly and a distance to go of 45nm.A couple of seconds later, the line was replaced by THEN ... and the FMC displayed a DISCONTINUITY message.I repeated the action with the same result.Can anyone tell me what caused this? I was flying the B738.ThanksGeoff

[Tabs 2,790]

Geoff,Can you describe the exact scenario so I can try it here?

[Richard McDonald Woods 168]

Hi Geoff,By what you say, it appears to me that you were entering a waypoint other than one of those already remaining in your flight plan. If this is true, then the FMC is right to identify a discon. It is literally saying "if you send me there, then where to after that?" You need, therefore, to tell the FMC what is to be the next-but-one waypoint. If this is then a waypoint in your original plan, then just download the waypoint to your scratchpad and upload it to the discon boxes. Any intervening waypoints will then be disregarded. Don't forget to press the EXEC key to tell the FMC that this to to be the new, altered flight plan.If the next-but-one waypoint is again not in your exiting flight plan, then you will have to repeat the above until you can resume your original flight plan.I hope this helps,

[Vauban 0]

Hi Ryanwhat I was doing was>entering a>waypoint other than one of those already remaining in the>flight plan. So Richard is right on track - thanks for that Richard. I understand what you're saying, but what if I'm under ATC control and will probably not resume the original flight plan? Doesn't it happen a lot that you've got your route all set up, including the star and maybe right up to the runway, and then ATC pops up and asks you to fly direct XXX, where XXX isn't in your flight plan?In this case you don't need the rest of the route in the FMC because ATC has you under control and you can't anticipate the next wpt you'll be asked to fly to. All you want is a bearing to XXX, preferably without asking ATC.So is there a quick way to clear the remainder of the FMC waypoints and let it accept a new 'route'?ThanksGeoff

[Jeff Nielsen 1,385]

Geoff,Chances are, assuming your flying online and not using FS ATC, the shortcut you'd get will still be along your STAR or SID and would just be a matter of 'up-selecting' the next set of waypoints.You can actually do the same thing while in VNav for ALT restrictions.If you get a fix that isn't on your route of flight, then you'd have to input it manually, decide whether or not it's still on your route (in case of user or controller outdated navdata) by looking at the ND, then close all DISCO's as required.If you're close into the approach and you get that call from ATC, then chances are, after the fix given to you, you might simply be on vectors after that, so switching to HDG mode might be more appropriate.There are also, rare, but can happen instances where, Place-Bearing-Distance (PBD) instruction given by ATC in which you can also input those into the FMC as required. The manuals go over how to set those up pretty well.More often then not, if you've been given an instruction by ATC and no subsequent clearances after that, I guess you can just use the DEL function on the CDU to delete all other waypoints and DISCO's as needed. If you CAN NOT clear the DISCO, then chances are you have a duplciate waypoint or invalid one, and you'll have to clear those ahead of the DISCO before clearing the DISCO-blank line.If you're talking about a whole new route, I think you can do that also while airborne, I'm not at my home PC so I can't recall the exact CDU entries to do so.There's just to many situations we can invision, without knowing the exact detail(s) but I hope that helps a bit. It sounds to me you just need to 'up-select'Edit: And don't forget to 'EXEC' again so the FMC knows to start flying again. Double check your LNav wasn't knocked off also.

[Vauban 0]

Hi Jeff, thanks for all that.The bottom line seems to be that you can't enter a wpt that's not on your route and then get back to your original route again.In my situation, I was briefly taken under ATC control, asked to fly direct to xxx before being released again to land at an airport that was outside his area, so it would have been useful to keep the remainder of my original route.But I get your point - normally, any redirection would be to a wpt along the STAR, so it wouldn't be a problem. At least if it happens again I'll know what's causing it and be able to handle the situation better.Thanks again.Geoff

[Jeff Nielsen 1,385]

No problem, it's just a matter of learning the CDU and how it's oddities work, just like anything else.The more you use it, the more you'll be able to whizz right through anything anyone throws at you.

[Tabs 2,790]

Geoff,It shouldn't be erasing the rest of your route if you just input a direct to at LSK 1L. The only time it will erase things is if you upselect a waypoint that's already there further down the list on the LEGS page.

[Richard McDonald Woods 168]

I would like also to add that, in the real world, ATC are there to ensure that you and others have a safe and efficient routing to your destination. It is very unlikely that a professional ATC will feel the need to give you complex initial approach routings and then leave you in an unclear continuation situation. From a pilot's view, he/she must firstly ensure that the plane can be flown safely (aviate), and only then worry about unnecessarily complex reprogramming of the FMC (navigate) when requesting vectors may be the simplest way to proceed safely.Cheers,

[Vauban 0]

Hi Ryan,No, it's ok, it didn't erase the rest of the route for me.It's just that I understood from previous posts that the only way to get the FMC to accept a wpt not on the current route (without getting the discontinuity) was to manually erase the rest of the route yourself. So I was saying, if I did manage to get that wpt into the FMC, I would no longer have the rest of the route to go back to.It just occurred to me that that was a contributing factor to a crash in South America (not Sao Paolo). It was the one where ATC only had radio contact, because their radar had been destroyed earlier by terrorists.Anyway, Richard, thanks for your RW comments. It put things in perspective for me.CheersGeoff