B777-Track miles after centerline extension!

[A380 Jockey]

Ridiclous question here.
Humour it..
But say you are on a downwind heading (777 or 380) to a runway on a radar vectors,and you 'extend the centreline' to give you a decent situational awarness. In this case you will still get the 'Vnav path' indication or the VDI(Vertical displacement indicator)on the rhs of your ND. What distance finals is this VDI based upon and similarly what intercept to the final approach course is the progress page 'Distance to go' based upon.
Cheers ..

[Wizofoz]

ASAIK....

Vnav references present position direct to the active way-point (which wil be the IAF if you use the "Intercept" function on the Dep-Arr page) at clean config, idle power and generates a Vnav path based on that.

[Sunshine Express]

Depends on which direction you are pointing of course. Computer assumes the shortest distance apparently.
Nice intercept heading to the fix the VNAV scale may be of some use.
I find that it's best to ignore VNAV, once extended centerline is selected.

All IMHO of course.

[A380 Jockey]

Which is why I used the 'downwind heading' example. Present position to intermediate approach fix sounds about right. Don't know if the same holds true if you use another point along the extended centreline as the reference point for the extension. Say a point at 10 miles of finals for the same runway..

[a330pilotcanada]

I would also clear PPOS (sorry been a while as I am retired) to get lateral offset error to center line as well for situational awareness.

If I got the term wrong please let me know

[OPEN DES]

Can only speak about the airbus here. The distance assumed is the distance to go from PPOS to the TO-wpt + dist to the rwy. Normally you would extend the centreline from the IF or FA(F/P) not the IAF (IAF is typically located outside of the c/l).
DTG/track mileage is not reliable here, so will be the vertical deviation/VTK error. Also consider that the vertical deviation is not only based on a false DTG but also based on the planned (managed) descent speed, you will very likely be flying an ATC selected speed.
What helps for me is to use the 3 times table with reference to the TO-wpt, taking the present altitude minus the G/S intc alt, once turned on base. Allowing some extra miles for speed reduction/wind etc.

[OPEN DES]

I would also clear PPOS (sorry been a while as I am retired) to get lateral offset error to center line as well for situational awareness.

That's what we used to do on the airbus before the FM2 standard was introduced on the FMGS. Nowadays it's easier to make a direct to the FA(F/P) with a radial-in (reciprocal of the final app crs), and disarming the subsequent NAV mode. This gives you crosstrack error to the centreline and also a nice green centerline of the ND up to infinity for situational awareness.

[A380 Jockey]

So one can safely assume that when applied on the 777, your Ppos (at the time of extending the centreline),and your heading at that moment,dictates the vertical deviation indication on your ND.
And the VDI factors in the distance of the extended centerline fix from the runway threshold; all of the above at the planned speed on the Vnav descent page..

[OPEN DES]

Sorry to pollute your 777 thread with airbus stuff.

The vertical profile on the airbus after you've extended the centreline is dynamic. i.e. it assumes that you join the extented c/l in the shortest way possible which is a direct to the TO-wpt (FAP) in most cases e.g. when on downwind. The same would be the case for clearing the PPOS and having the flightplan sequenced with the FAP as the TO-wpt as described by A330pilotcanada (pre FM2 technique). That's why we consider it as a piece of non-information.
I would imagine something similar for the 777. Sorry I can't be of more help. No info in the FCOM or Bulfers' big boeing FMC guide?

[A380 Jockey]

@Open Des....I assumed it was similar for the 777 too. Dynamic to the Ppos v/s the Cf. Therefore the question.
And i'm not surprised by the large number of varied(and accurate)responses that i've been getting.