735+Smiths FMC Update 5

[SR71]

Normally PROG PG 1 indicates the distance to go in track miles to the destination...the length of the magenta line.

The figure would be applicable if you were flying in LNAV (assuming that you're actually ON the track and haven't engaged LNAV on an intercept heading to the track from miles away).

On the base leg to a 12NM final (say), you choose to take an extended centre line.

How does the FMC then compute the track miles to go to destination for subsequent display on PROG PG 1?

Does it:

1) Compute a perpendicular line to the extended centre line from your instantaneous position and add this distance to 12? Not very helpful unless you happen to be flying a "base" leg so to speak.

2) Does it compute your instantaneous track at the time you select the extended centre line, extrapolate this line until it intercepts the extended centre line and then add the two distances together?

Anyone with any comment on the algorithm?

Thanks.

:ok:

[SR71]

And the answer to my question is 1).

Add X-TRK error on PROG PG 3 to the appropriate distance of the intercept on final to the field to get total track miles to go in worst case.

Should be of some use and agree with PROG PG 1 track miles to go figure.