Track Miles for the 777
 

Say I have been vectored on an imaginary line downwind.

I then call for PNF to "extend the centre-line from the course fix".

From this postion, how does the FMC calculated the track miles to touch down? Is it a an imaginary 90 degree line for base from where I am? Or is it a gentle curve for the base turn to join the final leg? Or it is a direct distance from where I am to the runway threshold?

if i'm not mistaken it is the great-circle track from your PPOS to the next ACTIVE WPT then continuing on your LEGS to the runway.

SR

It doesn't matter what aircraft type it is, the FMC is simply a "calculator". It cannot possibly know how many "track" miles you have to touchdown, unless you have a programmed in actual route. Once you have selected an extended finals track, it only knows two things. Firstly the track miles from your present position to the final fix you selected directly and secondly the onward track miles to touchdown. Think of it as a piece of string from the runway to the selected final fix pivot and then to your present position. That is the distance it is measuring. Since you are unlikely to fly anything quite so radical ( especially if flying downwind at the time ), the actual track distance will be greater and the ideal profile initially higher than that displayed. Of course this would not be so if the actual track turned you inside of the selected final fix, or once established inbound towards the final intercept track.

It is good to know there is still a place for the mark one brain to sensibly interpret what the electronic abacus is showing.