FMS fundamentals for millennials

Are you talking about putting a point in front of a point; i.e. descend to reach FLXXX 10 miles before WAYPT? Is that what you’re referring to?

If so, there’s probably a point in between you and that 10 miles before the fix.
Yes, in UNS speak it looks like
FIXXX -5 6000 on the vnav page for 6000’ at 5 miles prior to FIXXX. But sometimes it won’t take a negative number in the offset. In the case I was having trouble with there was no other fix between us and the one I wanted to vnav to. It would take 0 offset and a positive offset (past the fix) but not prior to. Usually it works just fine but some scenarios it just won’t do it.

I know some FMSs will create a new waypoint (labeled something like XPD001 or something) for the VNAV offset that then appears in the flight plan, as far as I can tell that’s not the way the UNS handles it (or if it is, it hides that logic away from the pilots and it doesn’t show in the flight plan).

I’m suspecting now that it has something to do with selecting an earlier fix than what auto populates on the vnav page. Say for example your flight plan is currently -D-> FIXXX FIXXY FIXXZ, for some reason that I don’t know when you go to the vnav page often the active vnav fix will default to FIXXY or FIXXZ. When you change it to FIXXX, seems to be when the offset gets jammed up.
 
Yes, in UNS speak it looks like
FIXXX -5 6000 on the vnav page for 6000’ at 5 miles prior to FIXXX. But sometimes it won’t take a negative number in the offset. In the case I was having trouble with there was no other fix between us and the one I wanted to vnav to. It would take 0 offset and a positive offset (past the fix) but not prior to. Usually it works just fine but some scenarios it just won’t do it.

I know some FMSs will create a new waypoint (labeled something like XPD001 or something) for the VNAV offset that then appears in the flight plan, as far as I can tell that’s not the way the UNS handles it (or if it is, it hides that logic away from the pilots and it doesn’t show in the flight plan).

I’m suspecting now that it has something to do with selecting an earlier fix than what auto populates on the vnav page. Say for example your flight plan is currently -D-> FIXXX FIXXY FIXXZ, for some reason that I don’t know when you go to the vnav page often the active vnav fix will default to FIXXY or FIXXZ. When you change it to FIXXX, seems to be when the offset gets jammed up.
Ah ok. Yeah it’s been a while since I’ve dealt with UNS. I’ll defer to the experts.
 
Are you inputting on the vnav page, or building a waypoint in the flight plan that you'll then assign an altitude?
 
Are you inputting on the vnav page, or building a waypoint in the flight plan that you'll then assign an altitude?
Inputting on the vnav page. As stated above, entering
FIXXX -5 6000
On VNAV page one, but any time I was putting a negative offset it would just blink the offset box at me and reset to 0.
 
sounds like I have 3 more legs to fly tonight, I’ll see if I can replicate the problem and provide further information.
 
Alright, here’s what triggers it. If you enter your flight plan like:
KABC
KDEF
And then enter your approach and transition (FIXXX.RNAV 08), it won’t let you enter a negative vnav offset from FIXXX. In addition, say the routing of the IAP is
FIXXX
FIXXY
FIXXZ
it won’t let you enter a negative offset from FIXXY or FIXXZ that extends prior to FIXXX.

If you enter your flight plan like this:
KABC
FIXXX
KDEF
and THEN add the approach and transition (FIXXX.RNAV 08) it works fine. Or even no approach and transition.

I’m guessing it’s because when you build it the first way you’ve created, according to the system’s logic, an unlinked flight plan, and even once you delete any no links there is some residual weirdness with the way it handles that.
 
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