IRS Initial Position
I searched for identical posts and didn't find any. Apologies if this has been covered elsewhere:
As some of you have indicated, there are companies that want you to put the airport in the IRS Initial Position Page--as opposed to entering the actual Gate lat/long coordinates--to minimize the possibility of a "typo." Since many FMCs update the aircraft's position to the beginning of the departure runway when takeoff thrust is set, entering the airport position might seem to be sufficient.
But now, let's take a case where we depart KDFW (Dallas, Texas, USA) and do an Atlantic crossing. The airport reference point may be 2 miles from your gate (KDFW is huge). If we are flying without radio updating (over the North Atlantic) on a non-GPS aircraft, when we see the "IRS Nav Only" message, will the airplane "jump" to being 2 miles off course--since that's what you "told it" by initializing it to the airport rather than the gate? This is my question.
In "75/767 Mysteries Revealed" by Tom Gooch--a great guide to the airplanes--he mentions an "error vector." That is, while still over land (and using radio updating), the FMC (not the IRS's) corrects the airplane's position. The FMC looks at the initial position, but corrects the 2 mile error at KDFW by referencing VOR/DME's along the route. I'm wondering if this Error Vector stays with the airplane during the IRS Nav Only portion of the flight? Then, the initial 2 mile error wouldn't seem to materialize. Without this error vector, I am of the humble opinion that the airplane will be 2 miles off course in the absence of radio updating--since you initialized the IRSs with a position that was "off" by 2 miles.