Boeing extended centreline
Hi guys,
On our B757 fleet, pilots often select, as I'm sure lots of others do, direct to the centre fix on the legs page of the CDU and then type in the inbound course to the runway as an intercept. To those familiar with this Boeing FMC, you'll know that the result is a magenta line extending out from the threshold on the HSI. My question regards the profile deviation info the box now provides on PROGRESS PAGE 2 and the HSI.
The above procedure is often done on the equivalent to a base leg and the profile deviation (vertically) info given on PROG page 2 always seems very accurate. My specific question is - the vertical profile info given always seems pretty spot so what exactly is the box using as it's "track miles to run"? Does it look at current aircraft position (continuously) and then work out the track miles to the threshold if the aircraft is turned directly on a base leg at any given moment?
I can't find anything in the Boeing manual and a couple of training captains weren't sure either.
I use it (along with other indications and my three times table) to make CDAs as it seems to to invariably give good profile deviation info even if it's activated on headings which aren't very base-leg-ish.
Any thoughts appreciated.
B&S