15 years ago we used to have the coordinates of every gate in the FMS database. Not any more. Just one position for each airport. The pilot can choose to enter this position, or type in the actual coordinates of the gate. Now that IRS position is only a back up, and not the main nav position source it doesn't really matter. In your example I expect this is what happened. Even though the IRS is showing a position to the left of the runway, the FMS which will have been updated with DME (no GPS fitted) will show the actual aircraft position on its display, and on the EHSI.
By the way, I use the term IRS on purpose. B742 and L1011 era had INS fitted. FMS did not appear until the late 1970s. So the INS was the main navigation system. All modern aircraft are navigated by the FMS using inputs from various sources. You cannot navigate with a modern IRS (Inertial Reference System) unless you have a map to plot on. The FMS does the navigation, the IRS only tells it where it is.
Last edited by Swedish Steve; 25th Sep 2008 at 08:20.